Pre-cast RC Structures - Notes

Prepared By: Ajay.N, Ashwin.M.Joshi and Arvind Sagar Assistant Professors, RASTA-Center for Road Technology, Bangalore

PRE-ENGINEERED STRUCTURES NOTES

Module 1 Types of RC Prefabricated Structures, Long wall and cross wall large panel buildings, One way and two way prefabricated slabs, Framed buildings with partial and curtain walls, single storey industrial buildings with trusses and shells, Crane, Gantry systems. Module 2 Functional Design Principles: Modular coordination – Standardization - Disuniting, Diversity of prefabricates – Production – Transportation – Erection - Stages of loading,codal provisions- Safety factors - Material properties - Deflection control Lateral load resistance - Location and types of shear walls. Module 3 Floors, Stairs and Roofs: Types of floor slabs – Methods of Analysis and design example of cored and panel types and two-way systems - Staircase slab design - Types of roof slabs and insulation requirements - Description of joints, behaviour and requirements - Deflection control for short term and long term loads - Ultimate strength calculations in shear and flexure. Module 4 Walls: Types of wall panels - Blocks of large panels – Curtain partition and load bearing walls Load transfer from floor to wall panels – Vertical loads Eccentricity and stability of wall panels –Use of Design curves -Types of wall joints, their behaviour and design – Leak prevention, Joint sealents, sandwich wall panels. Module 5 Industrial Buildings: Components of single storey industrial sheds with crane gantry systems - Design aspects of R.C. Roof Trusses - Roof panels R.C. Crane - Gantry Girders - Corbels and columns and Wind bracing.

MODULE-1 1.1 Prefabrication-General Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site and transporting complete assembles to the construction site where the structure is to be located. Prefabricated building is the completely assembled and erected building of which the structural parts consist of prefabricated individual units or assemblies using ordinary or controlled materials. Prefabricated construction is a new technique and is desirable for large scale housing programmes. 1.2 Principles 1) To effect economy in cost 2) To improve in quality as the components can be manufactured under controlled conditions. 3) To speed up construction since no curing is necessary. 4) To use locally available materials with required characteristics. 5) To use the materials which possess their innate characteristics like light weight, easy workability, thermal insulation and combustibility etc. 1.3 Need for Prefabrication  Prefabricated structures are used for sites which are not suitable for normal construction method such as hilly region and also when normal construction materials are not easily available.  PFS facilities can also be created at near a site as is done to make concrete blocks used in plane of conventional knick.  Structures which are used repeatedly and can be standardized such as mass housing storage sheds, godowns, shelter, bus stand security cabins, site offices, fool over bridges road bridges. Tubular structures, concrete building blocks etc., are prefabricated structures 1.4 Advantages  Speed of construction, owing to the ability to begin casting components for the superstructure while foundation work is in progress. Precast concrete components can also be cast and erected year-round, without delays caused by harsh weather;  Aesthetic flexibility, due to the variety of textures, colors, finishes and inset options that can be provided. Precast is extremely plastic and can mimic granite, limestone, brick, and other masonry products. This allows it to blend economically with nearby buildings finished with more expensive materials;  Design flexibility, resulting from the long-span capabilities to provide open interiors;

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Durability, which allows the material to show minimal wear over time and resist impacts of all types without indicating stress; Energy efficiency, due to the material’s high thermal mass. This is enhanced by the use of insulated panels, which include an insulated core; Environmental friendliness, as seen in its contributions to achieving certification in the Leadership in Energy & Environmental Design (LEED) program from the U.S. Green Building Council (USGBC); and High quality, resulting from the quality control achieved by casting the products in the plant. Plants certified by PCI undergo stringent audits of their quality procedures, ensuring the quality of fabrication in these facilities.

1.5 Disadvantages  Careful handling of prefabricated components such as concrete panels or steel and glass panels is required.  Attention has to be paid to the strength and corrosion-resistance of the joining of prefabricated sections to avoid failure of the joint.  Similarly, leaks can form at joints in prefabricated components.  Transportation costs may be higher for Voluminous. Prefabricated sections than for the materials of which they are made, which can often be packed more efficiently.  Large Prefabricated Structures require heavy-duty cranes & Precision measurement and handling to place in position. 1.6 Precast Concrete Construction Precast concrete consists of concrete that is cast into a specific shape at a location other than its in service position. The concrete is placed into a form, typically wood or steel, and cured before being stripped from the form, usually the following day. These components are then transported to the construction site for erection into place. Precast concrete can be plant-cast or site-cast. Precast concrete components are reinforced with either conventional reinforcing bars, strands with high tensile strength, or a combination of both. The strands are pretensioned in the form before the concrete is poured. Once the concrete has cured to a specific strength, the strands are cut (detensioned). As the strands, having bonded to the concrete, attempt to regain their original untensioned length, they bond to the concrete and apply a compressive force. This “pre-compression” increases load-carrying capacity to the components and helps control cracking to specified limits allowed by building codes.

Precast components are used in various applications and projects of all types. Key components include:  Wall panels, which can include an inner layer of insulation and be load supporting if desired;

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Spandrels, which generally span between columns and are used with window systems in office buildings or in parking structures; Double tees, so named due to the two extending “stems” perpendicular to the flat horizontal deck. These tees are often used for parking structures and buildings where long open spans are desired; Hollow-core slabs, which are long panels in which voids run the length of the pieces, reducing weight while maintaining structural strength; Columns and beams, including columns and a variety of beam shapes; Bridge components for both substructure and superstructure designs, including girders in a variety of shapes, box beams, and deck panels; Piers, piles, caps and other supporting components for bridges.

1.7 Types of Prefabrication Elements The system of prefabricated construction depends on the extent of the use of prefabricated components, their material, sizes and the technique adopted for their manufacture and use in building. The various prefabrication systems are outlined below. 1) Small prefabrication 2) Medium prefabrication 3) Large prefabrication 4) Off-Site prefabrication system 5) Open prefabrication system 6) Large panel prefabrication system 7) Wall system 8) Floor system 9) Stair case system 10) Box type system 11) Frame system 1. Small Prefabrication: The first 3 types are mainly classified according to their degree of precast elements using in that construction. For example, brick is a small unit pre-casted and used in buildings. This is called as small prefabrication. That the degree of precast element is very low. 2. Medium Prefabrication: Suppose the roofing systems and horizontal member are provided with precast elements. These constructions are known as medium prefabricated construction. Here the degree of precast elements are moderate. 3. Large Prefabrication: In large prefabrication most of the members like wall panels, roofing/flooring systems, beams and columns are prefabricated. Here degree of precast elements are high. 4. Off-Site (Factory) Prefabrication

One of the main factors which affect the factory prefabrication is transport. The width of road walls mode of transport vehicles are the factors which factor the prefabrications which is to be done on site or factory. Suppose the factory situated at a long distance from the construction site and the vehicle have to cross a congested traffic with heavy weighed elements the cost in-situ prefabrication is preferred even though the same condition are the cast in site prefabrication is preferred only when number of houses are more for small elements the conveyance is easier with normal type of lorry and trailors. Therefore we can adopt factory (or) OFF site prefabrication for this type of construction. 5. Open Prefabrication System This system is based on the use of the basic structural elements to form whole or part of a building. The standard prefabricated concrete components which can be used are, a) Reinforced concrete channel units b) Hollow core slabs c) Hollow blocks and battens d) Precast plank and battens e) Precast joists and tiles f) Cellular concrete slabs g) Prestressed / reinforced concrete slabs h) Reinforced / prestressed concrete slabs i) Reinforced / prestressed concrete columns j) Precast lintels and sunshades k) Reinforced concrete waffle slabs / shells l) Room size reinforced / prestressed concrete panels m) Reinforced / prestressed concrete walling elements n) Reinforced / prestressed concrete trusses The elements may be cost at the site or off the site. Foundation for the columns could be of prefabricated type of the conventional cast in situ type depending upon the soil conditions and loads. The columns may have hinged or fixed base connections depending upon the type of components used and the method of design adopted. There are two categories of open prefabricated systems depending on the extent of prefabrication used in the construction as given below. i. Partial Prefabrication Open System The system basically emphasizes the use of precast roofing and flooring components and other minor elements like lintels, sunshades, kitchen sills in conventional building construction. The structural system could be in the form of in-situ frame work or load bearing walls. ii. Full Prefabrication Open System In this system, almost all the structural components are prefabricated. The filler walls may be of bricks or of any other local materials.

6. Large Panel Prefabrication System This is based on the use of large prefabricated components. The components used are precast concrete large panels for walls, floor roofs, balconies, stair cases etc. The casting of the components could be at the site or off the site. Depending upon the context of prefabrication, this system can also lend itself to partial prefabrication system and full prefabrication system. Hence construction is a time consuming labor-intensive process. Builders need to bring together all of the necessary materials and skilled workers to complete the project successfully within a given time frame one way to make the process easier is by using prefabricated components. Such as pre-built walls (or) larger wall panels.

The simple way of classification of precast wall panel is based on their size or the materials of which they are made. They can be classified According to size as small and large or as narrow vertical stirrups or as broad horizontal bands. The material that are used for precast wall panel are bricks, hollow clay blocks, normal density concrete light weight metal gypsum plastic & timber. Generally materials that are locally available or which can be easily obtained are used for the production of precast wall panels. Due consideration is also given to the structural and physical properties of the materials in their selection particularly in respective of their strength, thermal and sound insulation properties and relative cost. Another classification of precast concrete wall which is especially application to prefabricated construction is based on their function and location in the building. They can also be distinguished for their cross sectional characteristics. As regards their location the wall panels may be classified as exterior or interior location walls. Depended on their function they may be either structural (load bearing) or non-structural (non-load bearing) elements. They may be of solid ripped sandwich hollow core, or composite construction they can be either prestressed or conventionally reinforce. In large panel construction the load bearing wall may be laid out either perpendicular to the longitudinal axis of the building (cross wall system) or parallel to it ( spine wall system). A

mixed system consists of cross wall and spine wall system. In most Vertical load carrying elements transfer their loads directly to the foundation without an intermediate frame.

7. Wall System Structural scheme with precast large panel walls can be classified as 1) Cross wall system 2) Longitudinal wall system Cross Wall System In this system the cross walls are load bearing walls. The facade walls are non-load bearing. This system is suitable for high rise buildings. Longitudinal Wall System In this system, cross walls are non-bearing, longitudinal walls are load bearing. This system is suitable for low rise buildings. A combination of the above systems with all load bearing walls can also be adopted.

Precast concrete walls could be Homogeneous walls: The walls could be solid or ribbed. Non-homogeneous walls: Based on the structural functions of the walls, the walls could be classified as a. Load bearing walls b. Non-load bearing walls c. Shear walls a. Load bearing wall: Precast load bearing walls provide an economical solution when compared to the conventional column/ beam/ infill wall system. The primary advantages are speed of construction and elimination of wet trades. In adopting the wall thickness, structural adequacy is not the sole consideration. Other factors to be considered include: • Connection details for supported beams and slabs. • Sound transmission and fire rating. • Joint details at panel-to-panel connections. • Possible future embedded services, which could reduce the concrete area available.

Based on typical layouts and building configurations, a thickness of 180mm is recommended for the precast panels used for party walls.

b. Non-Loading Bearing Wall Curtain Wall Curtain wall is a non-load bearing concrete wall construction that protects covered and/or conditioned interior spaces from the outside environment. Often designers consider aluminum-framed walls of glass or thin in-fills of metal or other materials as curtain walls. Sandwich Walls Insulated sandwich wall panels can be strictly architectural, strictly structural, or a combination of both. The difference between typical panels and insulated sandwich wall panels is that the latter are cast with rigid insulation "sandwiched" between two layers of concrete. The insulation thickness can vary to create the desired thermal insulating property ("R" value) for the wall.

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The structural behavior is either: Composite in which the Wythes are connected using ties through the insulation that fully transfer loads. The structural performance is then based on the full thickness of the panel;

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Non-Composite in which the Wythes are connected using ties through the insulation, which limits performance to the individual capacities of each Wythe. Whether the panel is composite or non-composite depends on the configuration and material used for the ties. Insulated sandwich wall panels can be designed to be loadbearing and support floor and roof components. They make an ideal structural element for this purpose, typically by casting a thicker interior wythe to provide the necessary support. They can also be non-loadbearing to complete a façade. Finishes: As with typical wall panels, the panels are cast in a flat orientation, so the form side is typically the side that will be exposed to view in the final construction. This face can be made with virtually any type of finish. GFRC panels allow for great aesthetic details and extensions such as cornices, due to the manufacturing process. The back face is typically troweled smooth, but is not left exposed. The back-up systems are often used to attach drywall and/or other finish materials. Typical widths: 4 to 15 ft. Typical heights: 8 to 50 ft. Typical thicknesses: 1.5 to 3 in. Precast Non-Load Bearing Facade Wall Typically, the wall panels for the front and rear elevations are non-load bearing facade elements. Support of these panels is achieved by any of the following methods: • The facade panel is connected to main load bearing walls and is designed to carry its own weight between supports. • The facade panel is connected to the floor slab or beam, which is then designed to provide support to the wall. These panels will typically be designed for vertical loads due to self-weight and an allowance for floor loads, if applicable, in addition to horizontal loads due to external wind pressures. A typical panel thickness of 120mm is proposed on the basis of strength considerations and to accommodate window fixings and profiles around the window perimeter.

Facade panels will often require three-dimensional architectural features, such as hoods, sills and ledges. In cases where there is a reasonable degree of repetition, customized

moulds can be produced, enabling these features to be economically incorporated into the panels. As an alternative, when repetition is limited, it will be most economical to cast the façade panel flat and subsequently add the features, manufactured separately using materials such as precast concrete, GRC, Aluminum or steel. 8. Floor System Depending upon the composition of units, precast flooring units could be homogeneous or non-homogeneous. 1) Homogeneous floors could be solid slabs, cored slabs, ribbed or waffle slabs. 2) Non-homogeneous floors could be multilayered ones with combinations light weight concrete or reinforced / pre stressed concrete with filled blocks. Depending upon the way, the loads are transferred the precast floors could be classified as one way or two way systems. One Way System One way system transfers loads to the supporting members in one direction only. The precast elements of this category are channel slabs, hollow core slabs, hollow blocks and hollow plank system, channels and tiles system, light weight cellular concrete slab etc. Two Way Systems Transfer loads in both the direction imparting loads on the four edges. The precast element under this category are room sized panels two way ribbed or waffle slab system etc..

Typical Flooring / Roofing system. 9. Stair Case System Stair case system consists of single flights with inbuilt risers and treads in the element only. The flights are normally unidirectional transferring the loads to supporting landing slabs or load bearing walls. 10. Box Type System In this system, room size unit are prefabricated and erected at site. This system derives its stability and stuffiness from the box limits which are formed by four adjacent walls. Walls are joined to make rigid connections among

themselves. The box unit rest as plinth foundation which may be of conventional type of pre-cast type. 11. Frame System  Precast frames can be constructed using either linear elements or spatial beam column sub-assemblages.  The use of linear elements generally means placing the connecting faces at the beam-column junctions. The beams can be seated on corbels at the columns, for ease of construction and to aid the shear transfer from the beam to the column.  The beam-column joints accomplished in this way are hinged.  However, rigid beam-column connections are used in some cases, when the continuity of longitudinal reinforcement through the beam-column joint needs to be ensured.

Typical Precast Beams

Typical Precast Columns 1.8 Industrial Building Industrial type building (Workshops, warehouses, etc.) is governed by laws differing from those controlling housing building. Prefabrication in situ of the main load-bearing beams and other secondary members (trusses, floors, etc.) is by now of common use in any construction yard for the erection of a factory or an industrial building. Quite often the construction Company purchases the main beams and other load-bearing members directly from specialized firms expressly equipped for an industrial type production. This tendency is mentioned here because it is probably destined to assert itself even more in the presumable development of the building industry which will convert the construction companies into concerns for the assembly of industrially prefabricated structural elements. 1.8.1 Components of Industrial Building (Single-Storey)

The roofs of single storey shed type industrial buildings maybe constructed by purlins with covering of roofing slabs or corrugated asbestos cement sheet method. These are the most popular forms of roof covering used in central Europe. This is not surprising considering the simplicity of manufacture of purlins and the availability from stock of factory made lightweight roofing slab and panels. The structural system of the purlins maybe a) Freely supported beam b) Cantilever girder

c) Continuous girder The connections of purlins over the support are designed only to absorb a limited bending moment. Normal purlins spans between 5 and 10m.The purlins are spaced at intervals of 2m to 3m. Roofing members are classified as, Reinforced planks: Reinforced planks made of hollow tiles. The reinforced planks with longitudinal circular holes. Thickness of these tiles is 60mm, 80mm & 100mm & the width is 200mm & length is vary from 360mm to 400mm. On the upper side one longitudinal groove is provided. Reinforcement is placed into these grooves which are subsequently filled with cement mortar. In this way, roofs of length 2 to 3m & thickness of 60 to 100mm & width 200mm can be constructed. The end tiles resting on the support are provided with 3.11mm dia stirrups protruding from the tile. There are kept together over mortar of 40mm thickness & in further concreting of joint is completed. Light weight concrete roofing members: Light weight concrete roofing members play a role in addition to space bordering & load bearing in heat insulation. The thickness varies from 7.5 to 25cm for reinforcement of light weight concrete roofing members, welding nets is used. Steel reinforcement is given additional coating to prevent any corrosion care is taken to give good bonding of reinforcement with concrete. The unit weight of these members is 750kg/m3& width of 50cm.Its varies from 1.75mm to 6m.precast members can be made either in usual way using lightweight materials. Sand as aggregate & combination of high strength concrete. The top & bottom layer of about 2 to 3cm thickness is provided with high strength concrete. Its consists of prestressed 2.5mm dia embedded in these layers. The middle portion is made with light weight concrete. Small reinforced concrete roofing members: The Small reinforced concrete roofing members is essentially precast simply supported ribbed concrete slab width varying from 450 to 120cm & length varying from 2 to 4m. Purlins: Purlins are usually solid web members. For long span they maybe lattice girders or trussed beams. Freely supported purlins are designed as parallel flanged or fish- belly members. Purlins designed as cantilever girders (articulated girders) are usually parallel flanged members. The cross section of purlins is generally rectangular but it can also have trapezoidal, T, L and I shape. The c/s features depends on the spans of purlins and on the slope of the roof. The purlins for flat roofs are usually rectangular T- section or (prestressed concrete).

T-section members for steeply sloped roofs if the purlins are loaded also bi axial bending L-section and the approximate spans associated with them for a purlins spacing of 3m are indicated for the flat roofs. The dimensions relate to freely supported purlins. Precast purlins can be simply supported or cantilever beams & for the bearing of loads beyond these weight simply supported purlins can be transformed into continuous beams. It is very simple & easy to place. For cantilever purlins placing of hinges should be determined in a manner to develop positive & negative moments equal to each other. This can be arrived by placing the hinges @ 0.145 from the support where I is the spacing between the supports.

Purlins section with associated spans for a purlins spacing of about1.25m in the case of steeply sloped roofs with corrugated asbestos cement sheet are indicated. The L-section is popular with British Firms channels section purlins have been developed by among others professor VON HALASS. They are convenient to manufacture with the legs of the channel upwards whereby very thin webs can be produced. This type of purlins maybe conventionally reinforced or by prestressed, also they may be freely supported or be continuous over several spans. In case of L- section purlins usually only the flange of the section is supported The fish belly girder is very favorable with regard to material requirements and the pattern of forces in the girder, but it has the disadvantages of being rather unsatisfactory. From the point of view of architectural aesthetics when it is used it is generally designed as a reinforced concrete purlins. Large reinforced concrete roofing members: Large reinforced concrete rest on the main girders. These are generally used for large hall structures & these are most advanced type of precast structures. Members are manufactured corresponding to spacing of the frame length of about 6 to 10m & width of 1.3 to 1.8m. As they are most supported on main girder purlins are not required. Four kinds of members exist: 1. Normal members.

2. Intermediate members. 3. Members with cornice. 4. Members with gutter & eves border. Shell Roof: The shell structure can have ribs in the centre & provided with curved membrane like roof. There are many industrial structure are built by precast members with shells. The thickness of shell varies from 2 to 10cm.Some precast shell, are produced with dimensions which are very difficult to transport. To avoid such difficulty large size shells are precast near to the resting or construction place. The transportable or small size shell members can be precast in factories & these are transported to the site. Examples: Barrel shells, Saddle or hyperboloid shells. Cupola or parabolic shells. The advantage of shells is that it provides large column free area for the monolithic construction. The cost of shuttering & scaffolding is very high but if manufactured in a precast factory in large scale. The production cost can be considerably reduced. Type of Shell Constructions a).Single barrel Shell Structure The structure above is a single barrel with edge beams. The shell has been allowed to project beyond the edge of the stiffener in order to show the shape of the shell. Stiffeners are required at columns. They do not necessarily have to be complete diaphragms but may be arches with a horizontal tie. The thickness is based on design of a slab element, the thickness of the barrel shell is usually based on the minimum thickness required for covering the steel for fireproofing, plus the space required for three layers of bars, plus some space for tolerance. If these bars are all half inch rounds, a practical minimum would be 3 ¼ inches. Near the supports the thickness may be greater for containing the larger longitudinal bars. If more than one barrel is placed side by side, the structure is a multiple barrel structure & if more than one span, it is called as multiple span structure. b).Multiple barrel Shell Structure This structure shows a multiple barrel with vertical edge beams at the outside edges. The stiffeners have been place over a roof. The advantage of having the stiffeners on top is that there are no interruptions to the space inside the shell so both the inside appearance & the utility are better. The movable formwork may be used which will slide with little decentering lengthwise of the shell. The multiple span structure should have an occasional expansion joint to reduce shrinkage & thermal stresses. This can be accomplished by cantilevering half the span from each adjacentstiffener. A small upturned rib placed on each side of the joint & accordion type sheet metal flashing is arranged to prevent roof leakage.

The maximum spans for this type shell are again limited by the geometry off the cross section .Assuming the maximum width of barrel to be 50 feet & maximum end slope to be 45deg, the rise would be about 14 feet, the maximum span would be in the order of 150 feet. c).North light shells This type of shell structure is used to provide large areas of north light windows for factories requiring excellent natural lighting. The windows may be slanting or may be vertical. The member at the bottom forms a drainage trough with the curved shell & materially assists in stiffening the structure. The effective depth of the shell is not the vertical distance between the two ends but is merely represented b the depth if the shell is laid flat with the ends of the circle on the same horizontal line. The spans for the north light shell must be rather small in comparison to the vertical depth of construction. The edges of adjacent shall should be tied together by concrete struts serving as mullions between the window glazing. d).Long barrel shell Long barrel shell obtained hen the semicircle or a segment of same is translated along the longitudinal axis. Generally used for shed for industrially purpose & buildings for large column free areas. Generally the prefabricated barrels off sizes 3.5 to 5m & 10m long with edge beams having thickness of 60mm.The thickness of the shell should not be more than 40mm.The dimension of these members were finally limited by the load carrying capacity of the available hoisting machines using the girder system built of precast prestressed trusses with parallel chords, areas having a span of even more than 15m can be cover with barrel shell. 1.9 Materials Used Prefabricated building materials are used for buildings that are manufactured off site and shipped later to assemble at the final location some of the commonly used prefabricated building. The materials used in the prefabricated components are many. The modern trend is to use concrete steel, treated wood, aluminum cellular concrete, light weight concrete, ceramic products etc. While choosing the materials for prefabrication the following special characteristics are to be considered. foundations ermal insulation property

2.0 Characteristics of Materials

of foundations.

MODULE-2 2.1 Modular Coordination  Modular coordination is a concept of coordination of dimension and space, in which building components are dimensioned and positioned in a term of a basic unit or module, which is also known as 1M and which is equivalent to 100 mm.  MC is internationally accepted by the International Organization for Standardization (ISO) and many other countries as well as Malaysia. A module is a unit of measurement and it means Standardized and easily fit components.  Generally the word Module is derived from Latin Word ‘MODULUS’ meaning a small dimension.  MC is the International system of dimensional standardization in building. The smallest Module is generally used to coordinate Position and Size of Components, Elements and their Installations.  Smaller Dimensions should be more clearly distributed than larger dimensions.  Modular coordination was first explored as an aid to design shortly after the introduction of prefabrication in the construction industry in the industrialization. It was conceived as a further step in the development of systematic design and construction of the building. This subject has been discussed and attempted in an actual building experiment in practically every developed country.  Modular coordination was first studied in Singapore in the early seventies. The housing and development board implemented the concept in 1973 in the new generation flats. Prefabrication and standard components were subsequently introduced. Modular blocks and bricks were introduced in 1983. There are merits to extend the use of modular coordination in other components as well. 2.2 Objectives  The principle objectives of modular system is to provide practical and coherent solutions for coordination of the position and dimensions of elements, components and space in building design.  This process can contribute to increase design freedom and improved balance between quality and cost in manufacture and construction. 2.3 Principle of Modular Coordination The main purpose of Modular Coordination is to achieve the Dimensional Compatibility between the Building Dimensions, Span or Spaces and the Size of Components and Equipment’s by using related Modular Dimensions. Modular Coordination generally provides the easy grasped layout of the positioning of the building components in relation to each other and to the building and facilitate collaboration between planners, manufactures, distributors and contractors.

2.4 Advantages of Modular Coordination  To facilitate collaboration between building designers, manufactures, distributors and contractors.  To permit the use of building components of standard size to construct the different types of building.  To optimize the member of standard sizes of building component.  MC increases the speed of construction.  Benefits through the increase use of computer aided design and drafting.  Reduction in manufacturing and installation cost.  MC minimize the wastage of materials, time and manpower in cutting and trimming on site  Facilitate prefabrication.  MC improved the balance between Quality and Cost. 2.5 Disadvantages of Modular Coordination  Uniformity.  Can lead to problems when modules are linked because link must thoroughly test.  It is difficult to manufacture to produce components based on mm tolerance. 2.6 Principle of Modular Coordination 1. Basic Module 2. Modular Dimension 3. Planning Module 4. Placing of Components 5. Modular Grid

1. Basic Module The fundamental module used in modular coordination the size of which is selected for general application to buildings and components.

2. Modular Dimension Traditionally designers have trained to use simple whole number ratios. Modular coordination provides a sound basis for an ordered selection of dimensions and accommodates a proportional flexibility that satisfies the needs of architectural aesthetics. 1) The planning grid in both directions of the horizontal plan shall be: a. 3M for residential and institutional buildings b. For industrial buildings, 15M for spans up to 12m 30M for spans between 12m and 18m and 60M for spans over 18m The center lines of load bearing walls shall coincide with the grid lines. 2) In case of external walls, the grid lines shall coincide with the center line of the wall 50mm from the internal force. 3) The planning module in the vertical direction shall be 1M up to end including a height of 2.8m, above the height of 2.8m it shall be 2M. 4) Preferred increments for sill heights, doors, windows etc. shall be 1M. 5) In case of internal columns, the grid lines coincide with the centre lines of columns. In case of external columns and columns near the lift and stair wells the grid lines shall coincide with centre lines of the column in the top most storey or a line in the column 50mm from the internal face of column in the top most storey. 3. Planning Module and Placing of Components There are different methods of locating components within dimensional frame mark of the building the distribution is made between load bearing walls, slab components vertically and horizontally. The placement of components either made on axial to the boundary planning. 4. Modular Grids To simplify the design process a mesh of lines, which have preferred space dimension, are plotted in three directions for all types of buildings. A rectangular coordinate reference system in which the distance between consecutive lines is the basic module or a multi-module. This multi-module may differ for each of the two dimensions of the grid. Basic Modular Grid The fundamental modular grid, is that in which the intervals between consecutive parallel lines is equal to the basic module, smallest planning grid. Multi - Modular Planning Grid In addition to the basic modular grid, multi-modular grids in which the intervals between consecutive lines are a multi-modular may be used.

Type of Modular Grid There are different types of grid patterns which are used to locate the positions and dimensions of building spaces components are A.Continuous grid Where all dimensions in either direction are based on one increment only. B. Superimposed grids When the modular grid of 100 mm increment is superimposed on a multi-modular grid. C. Displacement of grid or tartan grids Where there is a homogenous and repetitive relation between at least two basic increments. Eg:- 1M +2M (or) 3/2 M + 3M D. Interrupted grids (or) neutral zones Where there are non-modular interruptions of grids neutral zones are created to cope with the economics of building design. 2.7.Modular Coordination Design Rule: Basic Module 1M= 100mm Structural Grid 3M (1M as the second preference) Horizontal Multi-Module 3M (1M as the second preference) Vertical Multi-Module 1M (0.5M as the second preference) Doors Multiples of 1M (width and height) Windows Multiples of 1M (width and height) Sub-modular increment 0.5M and 0.25M Planning modules for main dimensions of framework especially the span (horizontal dimensioning) are shown in figure.

2.8 Notation and Symbols

2.9.Tolerance It is the sum of acceptable positive and negative discrepancies of actual dimensions from the theoretical one. The limits of tolerance are based on the manufacture and erection requirements. Types of Tolerance: 1. Manufacture Tolerance (T) • Deviation caused by shrinkage, creep and temperature changes. • Also due to Loadings. • Positive and negative manufacturing tolerances are assumed equal to T/2(mm) 2. Erection tolerance • These are the limits of deviation of the positioning in the assembly of the prefabricates.

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The position tolerance are normally defined by five components namely, deviation in positioning of the prefabricates in x,y,z directions ( x, y, z) and deviation in positioning with respect to another prefabricate and the deviation in the verticality of the elements. Tolerance as per IS: 15916-2011

2.10 Standardization Standardization is to the creation and use of guidelines for the production of uniform interchangeable components especially for use in mass production. It also refers to the establishment and adoption of guidelines for conduct to global marketing the term is used in describe the simplification of procurement & production to achieve economy. It is the extensive use of components, methods or processes in which there is regularity, repetition and a background of successful practice (e.g. standardisation of the dimensions of components such as doors and windows, uniform standards for certain common materials such as steel and concrete, etc.). The construction industry can improve its efficiency through wider use of components of standardised dimensions and standardised processes. To maximize these benefits, we need to factor in the use of standardised components at the design stage to ensure compatibility in design and to facilitate the manufacturing process. The industry will also need to act together in order to achieve the necessary economy of scale. Standardised processes and practices provide much greater predictability about what is performed, by whom, how and when and the possible outcomes. Wide adoption of standardised processes and practices across the industry would facilitate integration among industry participants, minimize development efforts, and promote learning sharing.

Typical Standard Precast Concrete Sections 2.10.1 Advantages of Standardization 1) Easier in design as it eliminates unnecessary choices 2) Easier in manufacture as there are limited number of variants. 3) Makes repeated use of specialized equipment in erection and completion 4) Easier and quicker.

2.10.2 Factors Influencing Standardization 1) To select the most rational type of member for each element from the point of production, assembly, serviceability and economy. 2) To limit the number of types of elements and to use them in large quantities. 3) To use the largest size of the extent possible, thus resulting in less number of joints. 4) To limit the size and number of prefabricate by the weight in overall dimension that can be handled by the handling and erection equipment and by the limitation of transportation. 5) To have all these prefabricates approximately of same weight very near to the lifting capacity of the equipment 2.10.3 Types of standardization 1. Generic standardization - where an element or process is by its nature standard and is usually recognized as such worldwide. E.g. steel, concrete, cement or plaster. International standards (ISO etc.) seek to rationalize standards internationally. 2. National standardization - where some items are standard for a country or group of countries, such as the European Union. The dimensions of a household brick would be an example of national standardization. National standards (BSI etc) seek to rationalize such items or processes into standards that are practices throughout the country. 3. Client standardization - where a particular client defines certain elements, processes or procedures in their business. 4. Supplier standardization - where a supplier, or in some cases a whole product or materials sector, stipulates that certain components, sub-assemblies, or even whole products are standard. 5. Project standardization - where a project team will decide to standardize certain procedures or building elements. For example, Quality Assurance procedures, column sizes, dimensional grids or module sizes. 2.11 Codal Provision (IS-15916: 2010) 2.11.1 Materials Use of materials for plain and reinforced concrete shall satisfy the requirements of IS 456. Connections and jointing materials shall be in accordance with 9.3. While selecting the materials for prefabrication, the following characteristics shall be considered: a) Easy availability; b) Light-weight for easy handling and transport; c) Thermal insulation property; d) Easy workability; e) Durability; f) Non-combustibility; g) Sound insulation;

h) Easy assembly and compatibility to form a complete unit; j) Economy; and k) Any other special requirement in a particular application. 2.11.2 Plans and Specifications The detailed plans and specifications shall cover the following: a) Such drawings shall describe the elements and the structure and assembly including all required data of physical properties of component materials. Material specification, age of concrete for demoulding, casting/erection tolerance and type of curing to be followed. b) Details of connecting joints of prefabricates shall be given to an enlarged scale. c) Site or shop location of services, such as installation of piping, wiring or other accessories integral with the total scheme shall be shown separately. d) Data sheet indicating the location of the inserts and acceptable tolerances for supporting the prefabricate during erection, location and position of doors/windows/ventilators, etc, if any. e) The drawings shall also clearly indicate location of handling arrangements for lifting and handling the prefabricated elements. Sequence of erection with critical check points and measures to avoid stability failure during construction stage of the building. 2.11.3 Components The dimensions of precast elements shall meet the design requirements. However, the actual dimensions shall be the preferred dimensions as follows: a) Flooring and Roofing Scheme — Precast slabs or other precast structural flooring units: 1) Length — Nominal length shall be in multiples of 1 M. 2) Width — Nominal width shall be in multiples of 0.5 M. 3) Overall thickness — Overall thickness shall be in multiples of 0.1 M. b) Beams 1) Length — Nominal length shall be in multiples of 1 M. 2) Width — Nominal width shall be in multiples of 0.1 M. 3) Overall depth — Overall depth of the floor zone shall be in multiples of 0.1 M. c) Columns 1) Height — Height of columns for industrial shall be 1 M and other building 1 M. 2) Lateral dimensions — overall lateral dimension or diameter of columns shall be in multiples of 0.1 M. d) Walls Thickness— The nominal thickness of walls shall be in multiples of 0.1 M. e) Staircase Width — Nominal width shall be in multiples of 1 M. f) Lintels 1) Length — Nominal length shall be in multiples of 1 M.

2) Width — Nominal width shall be in multiples of 0.1 M. 3) Depth — Nominal depth shall be in multiples of 0.1 M. g) Sunshades/Chajja Projections 1) Length — Nominal length shall be in multiples of 1 M. 2) Projection — Nominal length shall be in multiples of 0.5 M. 2.11.4 Design Considerations The precast structure should be analyzed as a monolithic one and the joints in them designed to take the forces of an equivalent discrete system. Resistance to horizontal loading shall be provided by having appropriate moment and shear resisting joints or placing shear walls (in diaphragm braced frame type of construction) in two directions at right angles or otherwise. No account is to be taken of rotational stiffness, if any, of the floor-wall joint in case of precast bearing wall buildings. The individual components shall be designed, taking into consideration the appropriate end conditions and loads at various stages of construction. The components of the structure shall be designed for loads in accordance with IS 875 (Parts 1 to 5) and IS 1893 (Part 1). In addition, members shall be designed for handling, erection, Ties and bearings. Handling stresses Precast units should not be inflicted with any permanent damage arising from their handling, storage, transportation and erection. Consideration should be given during design to:  Loads on erected elements at construction stage; Design considerations should also be given to:  Construction loads. A minimum load of 1.5 kN/m² should be used. However, due consideration should be given to any special requirements e.g. for plant loads or storage loads and the load increased accordingly;  Notional horizontal load. The lateral load should be taken as not less than 1.5% of the characteristic dead load;  Accidental loads such as earth movement, impact of construction vehicles.  Demoulding, storage, transportation and erection of precast units on site; Temporary stages/erection sequence The critical loading for precast elements is often not the permanent condition but can occur during the construction phase and, hence, the temporary condition may govern the design of elements. Consideration should be given to the loading imposed on precast elements during each phase of construction. Examples of such cases are as follows:  Precast sections of composite elements which are required to support self-weight plus construction load prior to casting of an in-situ topping;  Lower precast floor slabs or precast stair flights which support propping to upper levels during installation; and  Bearing or halving joints which support higher temporary construction loads because of back propping to upper levels.

The design should also take into consideration that the structural action and framing might be different during the temporary stages resulting in higher stresses in individual members. Design of Ties The types of ties to be provided for stability and interaction between precast units are as follows:

Types of tie in structural frame 1. Peripheral Ties At each floor and roof level an effectively continuous tie should be provided within 1.2 m of the edge of the building or within the perimeter wall. The tie should be capable to resisting a tensile force of Ft equal to 60 kN or (20 + 4N) kN whichever is less, where N is the number of storeys (including basement). 2.Internal Ties These are to be provided at each floor and roof level in two directions approximately at right angles. Ties should be effectively continuous throughout their length and be anchored to the peripheral tie at both ends, unless continuing as horizontal ties to columns or walls. The tensile strength, in kN per metre width shall be the greater of

where (gk + qk) is the sum of average characteristic dead and imposed floor loads in kN/m2 and lr is the greater of the distance between the centre of columns, frames or walls supporting any two adjacent floor spans in the direction of the tie under consideration. The bars providing these ties may be distributed evenly in the slabs or may be grouped at or in the beams, walls or other appropriate positions but at spacing’s generally not greater than 1.5 lr.

3. Horizontal ties to column and wall All external load-bearing members such as columns and walls should be anchored or tied horizontally into the structure at each floor and roof level. The design force for the tie is to be greater of, a) 2 Ft kN or ls × Ft × 2.5 kN, whichever is less for a column or for each meter length if there is a wall. ls is the floor to ceiling height, in meter. b) 3 percent of the total ultimate vertical load in the column or wall at that level. For corner columns, this tie force should be provided in each of two directions approximately at right angles. 4. Vertical ties (for buildings of five or more storeys) Each column and each wall carrying vertical load should be tied continuously from the foundation to the roof level. The reinforcement provided is required only to resist a tensile force equal to the maximum design ultimate load (dead and imposed) received from any one storey. Bearingfor Precast Units Precast units shall have a bearing at least of 100 mmon masonry supports and of 75 mm at least on steel or concrete. Steel angle shelf bearings shall have a 100 mm horizontal leg to allow for a 50 mm bearing exclusive of fixing clearance. When deciding to what extent, if any, the bearing width may be reduced in special circumstances, factors, such as loading, span, height of wall and provision of continuity, shall be taken into consideration. 2.11.5 Joints The design of joints shall be made in the light of their assessment with respect to the following considerations: a) Feasibility — The feasibility of a joint shall be determined by its load carrying capacity in the particular situation in which the joint is to function. b) Practicability — Practicability of joint shall be determined by the amount and type of material required in construction; cost of material, fabrication and erection and the time for fabrication and erection. c) Serviceability — Serviceability shall be determined by the joints/expected behavior to repeated or possible overloading andexposure to climatic or chemical conditions. d) Fire rating — The fire rating for joints of precast components shall be higher or at least equal to connecting members. e) Appearance — The appearance of precast components joint shall merge with architectural aesthetic appearance and shall not be physically prominent compared to other parts of structural components. 2.11.6 Design Requirements for Safety againstProgressive Collapse Prefabricated buildings shall be designed with proper structural integrity to avoid situations wheredamage to small areas of a structure or failure of singleelements may lead to collapse of major parts of the structure.

The following precaution may generally provide adequate structural integrity: a) All buildings should be capable of safely resisting the minimum horizontal load of 1.5 percent of characteristic dead load applied at each floor or roof level simultaneously. b) All buildings shall be provided with effective horizontal ties, 1) Around the periphery; 2) Internally (in both directions); and 3) Columns and walls. c) All buildings of five or more storeys shall be provided with vertical ties. In proportioning the ties, it may be assumed that no other forces are acting and the reinforcement is acting at its characteristic strength. Normal procedure may be to design the structure for the usual loads and then carry out a check for the tie forces. 2.12. Stages of Loadings There were two stages of loadings. In the first stage, the structure is loaded with the Ultimate Superimposed Design Load. In the second stage, going beyond the Ultimate Superimposed Design Load, the structure is loaded with a point load in the center, which was increased until the structure failed. In this case, the test is stopped before catastrophic failure because of space limitations of the test setup. 2.13Stages of Prefabricated Concrete Product

2.14 Materials Concrete: The most common grade of concrete for precast is M30 to M60. The type of concrete depending upon the structural requirements. The code specify SCC, Light weight aggregate concrete and Cellular Concrete. Steel: Generally High tensile hot rolled ribbed bars are used for precast reinforced construction. Diameter of steel varies from 6mm to 40mm.

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2.15 Methods of Prefabrication Site prefabrication: In this scheme, the components are manufactured at site near the site of work as possible. This system is normally adopted for a specific job order for a short period. The work is normally carried out in open space with locally a valuable labour force. The equipment machinery and moulds are of mobile nature. Therefore there is a definite economy with respect to cost of transportation. This system suffers from basic drawback of its non-suitability to any high degree of mechanization. It has no elaborate arrangements for quality control. Plant prefabrication: Factory prefabrication is restored in a centrally located plant for manufacture of standardized components on a long form basis. It is a capital intensive production where work is done throughout the year preferably under a covered shed to avoid the effects of seasonal variations high level of mechanization can always be introduced in this system where the work can be organized in a factory like manner with the help of constant team of workmen. The basic disadvantage in factory prefabricated, is the extra cost in occurred in transportation of elements from plant to site of work sometimes the shape and size of prefabrication. Semi-mechanized The work is normally carried out in open space with locally available labour force. The equipment machinery used may be minor in nature and mouldsare of mobile or stationary in nature. Fully-mechanized The work carried out under shed with skilled labour. The equipment’s used are similar to one of factory production. This type of precast yards will be set up for the production of precast components of high quality, high rate of production. 2.16 Process of Manufacture The various processes involved in the manufacture of precast elements are classified as follows: Main Process It involves the following steps. 1) Providing and assembling the moulds, placing reinforcement cage in position for reinforced concrete work, and 2) Fixing of inserts and tubes where necessary. 3) Depositing the concrete in to the moulds. 4) Vibrating the deposited concrete into the moulds. 5) Demoulding the forms. 6) Curing (steam curing if necessary) 34

7) Stacking the precast products. Secondary (Auxillary) Process This process is necessary for the successful completion of the process covered by the main process. 1) Mixing or manufacture of fresh concrete (done in a mixing station or by a matching plant). 2) Prefabrication of reinforcement cage (done in a steel yard of workshop) 3) Manufacture of inserts and other finishing items to be incorporated in the main precast products. 4) Finishing the precast products. 5) Testing the precast products. 2.17 Production Methods The term production of systems is describes a series of operation directly concerned In the process of making or more apply of moulding precast units on the face of it there are very many techniques since almost every type prefabricates requires a specific series of operation in its production. These techniques however may be grouped into three basic method of production. These are 1. Stand Method 2. Flow Method a) The ‘Stand Method’ where the mouldsremainstationary at places, when the various processes involved is carried out in a cyclic order at the same place. b) The ‘Flow Method’ where the precast unit under consideration is in movement according to the various processes involved in the work which are carried out in an assembly-line method. The various accepted precasting methods are listed in below Table (given in IS: 15916-2011) with details regarding the elements that can be manufactured by these methods.

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2.18 Quality Control At Factory Ordinance (“BO”) and the approved plans. to conditions stipulated by the BO and in the approved plans. e precast concrete works. “QAS”) provided by the manufacturer satisfies the purpose in that the manufacturer has made adequate provisions ensuring the production of the precast elements complies with the provisions of the BO and the approved plans. with the QAS prepared by the manufacturer in application for consent to commence work. ’s stream) to supervise the precast concrete production works at a frequency of not less than once a week. g book recording details of the supervisory personnel and details of the production, inspection, auditing and testing carried out for the production of the precast units. 36

book at the site office. elements at least once every month. Prepare audit reports for submission. s.

to carry out regular technical audits of the factory and the production of the precast units at a minimum frequency of once per month. to conditions stipulated by the BO and in the approved plans. tor (T3 TCP under the RC’s stream) to provide continuous supervision of the precast concrete production works. Provide supervisory personnel at the factory and an inspection log book recording details of the supervisory personnel and details of the production, inspection, auditing and testing carried out for the production of the precast units. Make sure the log book is available for inspection at all time by keeping the log book at the site office. d production of the precast units at least once every month by the Authorized Signatory of the RC. Prepare and submit the audit reports to the AP/RSE for endorsement and onward submission for record purposes. Duties of the manufacturer lements shall be manufactured by a factory possessing an ISO9000 quality assurance certification. subsequently to make application for consent to commence works. aintain the quality of the manufacturing of the precast elements. to carry out regular technical audits of the factory and the production of the precast elements at a minimum frequency of once per month. The QAS shall cover but not be limited to the following items: -bars, finishes and building services provisions. such as the frequency and standards adopted for the equipment used for the cube compressive strength test. employed and demoulding details. such as details of the curing procedure and associated controls. 37

by the independent parties employed by the manufacturer or the RC. quality assurance scheme and in accordance with the specification and the approved plans. Where Authorized Person (AP), Registered Structural Engineer (RSE) Registered Contractor (“RC”). 2.19 Quality Assurance System 1. Organization Chart 2. Casting yard set up of yard Number of moulds with estimated production rate Machinery employed 3. Production procedures (casting and transportation of the precast units within the yard). 4. Quality control procedures on materials and check points for Concrete re-bar Couplers finishes Building services installations 5. Quality control procedures on production and check points. Approved plans used. Shop drawings used. moulds assembling. re-bar fixing. Couplers fixing/welding work. Finishes and building services installation. concreting work. curing. 6. Calibration of testing equipment (responsible parties, frequency, and standards) 7. Testing of precast units such as dimensional check, cover meter test, pull-out test for tiled finishes, bonding test for building services installation, etc. 8. Concrete repair procedures. 9. Handling of non-compliant precast units with corrective/preventive action. 10. Inspection forms. 38

11. Identification system of the precast units. 12. Audit by independent parties. 2.20 Construction Methodology 1. Production Planning Generally, the production cycle is one day for a non-complicated precast element. In planning the production of precast elements, time of construction of each floor is a key factor in estimating the number of precastingmoulds. For example, in a project consisting of 15 precast façades per storey and a working cycle of 6 days per storey, the number of mould required is 3. A storage area in the precast yard should be sufficient to accommodate precastelements delivered to the construction site and extra precast elements in caseof emergency delivery. Example of a working schedule for production planning is shown in below figure.

Working schedule for precast unit. For the production of precast elements, the precast manufacturer requires about or at least 1.5 months for manufacturing the moulds and 0.5 month for production of the precast units. Therefore, the precast shop drawing (showing geometrical size) should be consolidated at least 2 months in advance of the scheduled date of delivered to site. All the above should be allowed for in addition to the time required for approval and consent by government or client. Embedded items including window frames, E/M pipe sleeves and openings in precast elements should be delivered to the precast yard before production.

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Unlike traditional in-situ construction, this requires coordination and approval of embedded items at an early stage. For projects with a large number of precast elements, time required for the above would be much longer and has to be taken into account. 2. Moulds 2.1 Materials Moulds can be made of any suitable material including steel, timber, glass reinforced concrete or a combination of these. The selection of the mould materials will depend on the several factors highlighted in the Code. Locally, the steel mould is the most common type owing to its robustness and precision. In general, the steel plate thickness adopted for mould design and fabrication varies from a minimum of 4.5mm to 6.0mm, which can be used over 100 times with proper care and maintenance. Material for moulds depends on the number of repetitions, required surface finish, quality and shape complexity of precast elements. • Steel moulds are preferred owing to its robustness and precision. • Minimum 8mm thick steel plate can be used for 500 repetitions. • Minimal number of demoulding parts of mould helps to ensure good maintenance of dimensional accuracy during production to facilitate easy assembly and dismantling. • Adjustable moulds for greater flexibility and variety in production of precast elements. 2.2 Tolerances To enhance cost competitiveness, adjustable moulds should be adopted where possible, for greater flexibility and variety in the production of precast concrete elements. 2.3 Recesses, sleeves and boxouts Moulds should be designed to allow for appropriate placing and compaction of the concrete. Adequate numbers of braces, ties and struts should be provided for proper casting and hardening of the concrete.

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Applying mould release Agent

Steel mould

Combined use of steel and timber mould

3. Cast-In Connection Three basic types of in-situ concrete connections commonly used in precast construction are A thin topping layer is cast to form a composite member, typically used with floor units such as hollow-core and double-Tee. It also acts as a leveling screed and may not be mechanically connected to the unit. Longitudinal shear due to bending is transferred by bonding and is also a function of the roughness of the interface. Composite construction is such that the in-situ concrete is a major component of the structural member. A typical example is a beam-shell where the precast unit forms the soffit and sides of the beam and contains the longitudinal reinforcement or prestressing wires and the shear steel. This type of construction allows continuous members to be easily formed by placing negative reinforcement in the in-situ concrete over supports. Simple spans are usually propped until the in-situ concrete attains sufficient strength to carry the dead weight on the composite section.

where beam or column continuity is required as in earthquake-resistant construction. Bond

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length of the bars being lapped dictates the length of the splice. It may be necessary to connect large main bars by welding.

Use of thin topping on Slabs and Beams

Cast-in connection on Beams and Columns 4 Lifting Inserts 4.1 General Three common types of lifting inserts used in precast concrete are: Reinforcement bar with omega “Ω” shape lifting insert. It is used in thin precast elements, such as a precast partition; and precast elements of shallow depth, such as a semi-precast slab. Lifting anchor with bulky head with U bars reinforcing the bottom head. Lifting anchor with eye for reinforcement bar to pass through. Lifting capacity of lifting inserts depends on the material strength of the insert and, more important, the strength of surrounding concrete. Clear instructions must be specified on concrete strength requirements for lifting, especially for the first lifting out of the mould. 42

4.2 Lifting position tolerance If the lifting anchor is offset substantially when compared with the drawings, the centre of gravity of the lifting point may be offset substantially from the centre of gravity of the precast element. This may cause the precast element to become out-of-balance and incline during lifting (i.e. not vertical in the lifting stage) making it difficult to handle, especially during installation. 5. Prefabricated Metal Frames Prefabricated frames such as windows should be protected to avoid damage by fresh concrete.

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Prefabricated frames. 6. Pre-Concreting Check Prior to concreting, the condition of the mould shall be inspected since it directly influences the quality of the precast concrete product: Themould form shall be level and the flatness of the base of the mouldand the squareness and stiffness of the mould form examined and the mould kept free from spillage. Manufacturers shall ensure that the dimensions of the mould are within the tolerances specified in accordance with Code. Themould shall be clean and free from debris (e.g. from the previous precasting operation). Form oil or release agent and retarder shall be applied to the surface of the mould to be in direct contact with concrete. They shall be applied in accordance with the manufacturer’s instructions. Over application may lead to puddling on the concrete surface. Reinforcing bars and cast in items such as lifting inserts, window frames and earthing lugs shall be fixed only after preparation of the mould: Rebar size, spacing, lap length and cover requirement shall be checked in accordance with the approved drawings and within the tolerance limits. Lifting inserts shall possess adequate length of embedment to prevent damage during lifting. A sufficient number of spacers, chairs and supports shall be properly placed and secured to achieve the required concrete cover during casting. Window frames shall be installed and fixed in place according to the approved shop drawings, and with full electrical continuity to the earthing lugs and façade. Reinforcing bars and all cast in items shall be clean and free from contamination by mould oil and cement grout before casting, as this may lead to poor bonding to the concrete.

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Checking of rebar, lap length, cover and lifting inserts before concreting. 7. Concrete Placing Density, uniformity and surface quality of precast concrete products depend on the workability of concrete, placing and the compacting procedures used during the production process. The workability of a concrete mix is presented in detail in the Concrete Code handbook. Attributes which relate to measuring the workability of concrete are as follows: Consistency depends on the degree of dryness and wetness of the concrete mix. Deformability is a measure of workability for low to medium workability concrete. Flowability is a measure of workability of high workability concrete. Passing ability measures the ability of a concrete mix to pass through narrow gaps. Segregation resistance or cohesiveness relates to the potential separation of some ingredients due to free falling and sliding along surfaces during the placing of concrete. Factors affecting workability include size and shape of aggregate, mix proportions, cement content, admixtures used and concrete temperature during placing and compaction. Procedures 45

and precautions for placing concrete are detailed in the Concrete Code 2013. Vibration and compaction of concrete is the principal method forconsolidating concrete. Fresh concrete must beproperly vibrated so that once hardened, its strength and durability are fully realized. Studies have shown that proper vibration enhances compressive, tensile and flexural strength and resistance to deterioration by increasing the density of concrete and eliminating voids, honeycombing and entrapped air due to poor placement of concrete. The use of vibration tables, external form vibrators, and surface vibrators are examples of external vibration/compaction techniques that are applicable in precast concrete production. Form vibrators shall be mounted on the form to induce vibration in the mould, which is then transmitted to the concrete. The number and locations of external vibrators used shall be strategically planned to best distribute their impact. Surface vibrators are installed on the concrete surface, exerting their effects at the top surface and consolidating from top down. They are used mainly in precast slab construction. Vibration tables are used to vibrate the frame that supports themould and are usually used for elements cast in small moulds. The table is isolated from the ground with springs or neoprene isolation pads to prevent undesirable vibrations affecting other production processes. Proper vibration and compaction shall be carried out, in particular in congested areas with a lot of steel reinforcement. When all air, entrapped in pockets and voids has been released, vibration is considered sufficient. This is demonstrated when air bubbles cease to emerge at the concrete surface.

(a) Using a vibration table

(b) Using an external vibrator

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Concreting and compaction of concrete. 8.Demoulding and Lifting A precast concrete product shall only be demoulded and lifted when the designed compressive strength of the concrete has been achieved. This can be assessed by compression tests on cubes cured under the same environment as the precast element itself. The minimum concrete strength at which a precast element can be lifted from the mould shall be based on the calculated concrete stresses at the lifting points, stresses caused by the transfer of prestressing forces or handling, the anchorage length of inserts and the type of precast element. To overcome additional suction and frictional forces during demoulding, the minimum concrete strength for lifting may be higher than the recommended value specified in the Code. It depends on the design and shape of the mouldand the precast element. Flat mould suction increases in the presence of water and can be relieved by first lifting one edge of an element gently. Frictional forces are induced by contact and bonding between concrete and the verticalsides of the mould. To reduce friction, the mould shall be designed with adequate draw or removable sides or vibrated gently while lifting one edge of the member. The Code also suggests the use of a high quality demoulding agent to reduced suction and frictional forces. Embedded hardware, threaded inserts, dowel connectors and removable sections of the moulds are usually attached to the mould with bolts, pins and clamps. Before demoulding, all bolts and pins connected to the mouldshall be loosened and all clamps removed. Side forms, window capping shall also be detached from the element. Failure to remove all bolts and pins is a common cause of failure of lifting insert and the formation of cracks on the concrete surface.

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Detaching the mould from a precast element.

Sequence of lifting of a precast façade (from top and left to right).

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9.Curing Curing has four major objectives: To maintain a suitable environment for new concrete to produce as much gel as possible so as to develop its full strength potential and reduce its permeability for better protection of the steel reinforcement from corrosion To avoid damage by plastic cracking and early age thermal cracking To avoid damage by shock vibrations due to nearby activities To avoid damage by premature loading caused by movement of adjacent parts of the structure Therefore, the scope of curing includes: Moisture control to prevent premature drying out of the concrete mix due to solar radiation and wind that may lead to plastic shrinkage cracking of the free surfaces not in contact with mould surfaces. Thermal control to prevent large temperature rises and drops, whichcould cause serious thermal cracking problems. Vibration control, which is particularly important if the precast plant is located on or near a construction site, or adjacent to any activities involving vibration. Movement and deformation control, required if the mould might move during the curing process. Curing of precast elements is usually achieved firstly by accelerated curing followed by a normal curing process (i.e. sprinkling water and keeping the elements moist with a curing membrane). Steam curing, described in the Code, is a subset of accelerated curing. The chemical cement hydration reaction takes place more rapidly with increased curing temperature and results in greater early strengths and efficiency in the production of precast products. In practice, elevated temperatures and addition of moisture during the steam curing process are both used in order to accelerate the rate of strength gain. The following explains the stages of a steam curing process: Stage 1 – Fresh concrete in the mould is allowed to achieve its initial set before putting the concrete in contact with steam or hot air. Steam is applied within a suitable enclosure that permits free circulation of the steam. Precautions shall be taken to prevent moisture loss from the concrete. Stage 2 – The precast element is heated to a maximum temperature of 700C at a heating rate usually within 100C per half hour. A curing temperature exceeding 700C may result in delayed ettringite formation which is detrimental to concrete strength. Gradual increase in temperature ensures a small thermal gradient between the surfaces and the interior of the concrete element. Stage 3 – Temperature and pressure of the environment are maintained for a sufficient duration, depending on the thickness and shape of the section. Stage 4 – The temperature is lowered at a rate not exceeding the rate of heating and the pressure is normalized. Low pressure steam curing refers to steam curing as mentioned in the Code and 49

above. High pressure steam curing, also known as autoclaving, is used for the production of concrete masonry units. This technique is designed especially for concrete of extremely low water cement ratio. Other methods of accelerated curing are also available. For example, the heatcuring technique increases concrete temperature by conduction and convection. The temperature of the mould is increased by a flowing fluid such as oil or water in contact with the mould. The direct contact between concrete and the mould results in conductive heat transfer and increases the curing temperature. Electrical resistance curing increases curing temperature by the dissipation of heat generated in the current carrying metal formwork, reinforcing steel bars or the concrete itself. 9.1 Normal curing Precast elements achieve design strength by curing at a certain temperature and for a certain length of time during production. In addition to Code, it is good practice to monitor the ambient temperature. If the environment is at 240C or above, normal curing is recommended. If the ambient temperature is hot (e.g. prolonged exposure to direct sunlight for several hours), it may be necessary to moisten the concrete surface by adding water to the surface. 9.2 Steam curing This method is usually used when the temperature of the environment is less than 240C. It can help to speed up the curing time in achieving adequate strength for demoulding. Table 3.1: Temperature versus curing time for different precast concrete products.

At the initial stage, concrete and moulds should be covered with a tarpaulin sheet before putting into the steam chamber. Steam is released in the first hour to increase the temperature to 700C. Water is sprayed inside the steam chamber for 3 to 5 minutes, once an hour, until the target curing time is reached. Afterwards, the steam chamber is allowed to cool down. The temperature inside the chamber then drops down to the surrounding environmental temperature within an hour. At the second stage, water must be poured on the precast elements until the 4thday of curing. Steam curing has the advantages of speeding up the production of precast elements and increasing the strength of precast elements in the early stages. Steam curing is appropriate when the ambient temperature is below 240C. In such a situation, the precast elements are placed 50

inside the steam room and sealed with a non-porous membrane. The temperature inside the steam room is sealed by a non-porous membrane and controlled by a centralized thermal system. The steam inlet injects the steam into the steam room. The temperature inside the steam room will increase to reach the target temperature. The time period required for curing depends on the temperatureat the time.

Application of water to precast unit for normal curing. 10.Handling The handling of precast concrete units encompasses demoulding, loading, transportation, unloading, storage and site erection. The optimum solution for economical handling is the ability to demould a unit and tilt it into a position similar to its storage position and its final configuration when installed in the buildings. Re-handling and turning of units between demoulding and final installation adds extra cost and increases the danger of accidental damage; hence handling must be reduced to a minimum. The following outlines some handling considerations for various construction stages: Delivery of precast elements, e.g. loading, transportation and unloading, shall be properly planned so that unnecessary site storage and handling is minimized to prevent damage. Transportation regulations shall be observed, as this may affect the size, weight and timing of shipments.

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The route shall be studied as conditions including bridge restrictions, nearby buildings or power lines, restricted turning radii and natural obstacles may determine the choice of transportation route. Once the unit is loaded onto the vehicle, it shall be attached firmly to the supporting members and fastened with a position locking device. Units shall be loaded with proper supports, frames, blocking, cushioning and tie downs to prevent or minimize in-transit damage. Dunnage shall be placed across the full width of each bearing location to separate stacked members. Prior to unloading the unit from the vehicle to the storage area, visual inspection shall be made before theremoval of bracing, chains, straps and protection for edges of the units. Storage sequences shall be properly planned to minimize the effects of handling during erection. Storage areas shall be relatively level, firm and well drained to avoid any differential ground settlement which may damage the stored elements and it shall be large enough to permit easy access for handling of the precast elements. Locations of supports, stacking configuration and sequences shall be in accordance with the approved shop drawings. Horizontal precast concrete elements such as precast slabs, beams and hollow core panels can be stacked and supported separately using strips of wood or battens across the full width of the designed bearing points. The support spaces shall be aligned to reduce induced stresses. Precast walls and façade panels are usually cast horizontally and rotated for storage in upright positions with racks and stabilizing walls to support their weight. For site erection, the rigging sequence and method shall be considered. Locations of lifting devices and lifting points shall be compatible with the method of shipment so that patching and repairing are minimal. It shall be aimed at ease of erection and connection of the precast unit to the structure. Lifting devices shall be standardized and installed in various precast elements so that frequent changes of lifting method can be avoided. Precast elements shall be rigged for balancing, remaining vertical and in line with their centres of gravity to prevent undesirable and excessive movement which may induce additional stresses on the elements and lifting devices due to dynamic loading.

Rotatable steel rack for easeof lifting. 52

Temporary supports for façade in the factory.

Proper storage to avoid excessive stresses and possible damage to precast units. Handling involves demoulding, loading and unloading operations. Proper handling of precast elements is important to ensure crack free concrete which will improve durability and ensure zero accident, thus safeguarding productivity at site. 11. Stacking of precast elements The storage yard provided at site should have the capacity to hold at least one week’s production. The area should be firm enough to hold layers of precast elements. The yard should have proper drainage system and should be easily accessible for trailers. 53

Stacking support system for horizontal and vertical elements should be based on the structural behaviour of the elements. Horizontal stacking for slabs and vertical stacking for wall elements should be done. Elements stored should be tagged for easy identification and should follow the erection sequence.

12.Testing of finished components The component should be loaded for 1 h at its full span with a total load (including its own self weight) of 1.25 times the sum of the dead and imposed loads used in design. At the end of this time it should not show any sign of weakness, faulty construction or excessive deflection. Its recovery 1 h after the removal of the test load, should not be less than 75% of the maximum deflection recorded during the test. If prestressed, it should not show any visible cracks up to working load and should have a recovery of not less than 85% in 1 h. Maximum Deflection = 40*l2/D where l is the effective span, in m; and D, the overall depth of the section, in mm. 13. Post-Concreting Check 13.1 General The Code discusses items to be checked and the checking shall be performed in two stages; immediately after demoulding and when the precast units are ready for dispatch. A list of production tolerances, which defines the limits to variations of size and shape of individual precast concrete members, to ensure that the member will fit the structure without difficulty, is given in the Code. 13.2 Production Tolerances The tolerances given in the Code are for normal circumstances. More stringent dimensional tolerances may be required if the structure is sensitive to deviations in dimensions. In general, the tolerance limits are acceptance criteria. However, even if the tolerance limits are exceeded, they may be considered acceptable if: Exceeding the tolerances does not affect the structural integrity or architectural performance of the member. 54

The member can be brought within tolerance by structurally and architecturally satisfactory means. The total erected assembly can be modified economically to meet all structural and architectural requirements. When proposing a set of production tolerances, the following factors shall be taken into account: Effect of forms on dimensions: Rigid forms are used to fabricate architectural precast panels where appearance or function dictates the need for the closest tolerances. Their sides are permanently fixed to ensure the highest degree of dimensional accuracy. The downside of a fixed mould is the frictional forces induced by the sides of the formwork. Some moulds are designed as flexible forms with semi rigid joints so that the side forms can be removed during demoulding. Since the enddividers or removable side forms are not permanently and rigidlyattached to the form, they have the potential to move during placement and vibration of the concrete. As a result, they are less likely to achieve precision linear plan dimension, than is the case with rigid forms. emperature on a plane member can induce bowing and cambering. For long members, the temperature effects may lead to lengthening and shortening of the length of the product. 14. Lifting Equipment and Accessories Lifting equipment such as mobile cranes, gantry cranes, forklifts, etc., must be carefully selected to ensure that lifting of precast elements is carried out within the rated capacity of the lifting equipment. The supports for the lifting equipment must be checked to ensure that adequate supporting capacity is provided. Lifting accessories may comprise combinations of lifting beams or frames, slings or cables, hooks or shackles, etc. The selection of each of these components should be predetermined to take into account the forces exerted on them with respect to all aspects of the lifting operations. Lifting beams or frames are usually tailor made to suit the precast elements, especially for slabs or prefabricated volumetric units which may be easily damaged during handling. Lifting beams or frames shall be designed to withstand the required weight of the precast units as well as the axial compression in the beams or frames due to an inclined sling during lifting. An impact factor as prescribed in the relevant code of practice should be applied to the structural design. Personnel suitably qualified in accordance with the relevant regulations must regularly inspect all lifting equipment prior to and after use. Results of such inspections must be properly recorded and be available for subsequent inspection by the Engineer upon request. Some precast elements such as prestressed hollow-core floor slabs must be handled by means of lifting clamps, straps or slings as they may have no lifting inserts. Lifting equipment of this type can wear out easily and should be regularly inspected. Location of lifting points should be clearly indicated on the drawings. 55

Gantry cranes.

Lifting a precast unit

Precast staircase

Lifting a precast façade

Double-Tee panel slab

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Lifting pin for a 30 ton precast column.

Erection detail of a precast panel. 15. Factory and Site Storage Storage areas must be large enough for precast elements to be stored properly. They should provide adequate room for lifting equipment and for themmaneuvering of transporting vehicles. The storage ground area must be hard, level, clean and well drained to permit organized storage. Precast elements can be damaged by incorrect stacking and storage. Locationsof support points, e.g. dunnage, for precast elements are critical and should benoted on the shop drawings. Supports 57

must be arranged to avoid twisting or distorting of precast elementsand must be adequate to transfer the weight of the stacked units to the groundwithout excessive settlement. Stored and stacked units should be protected to prevent accidental damage. Support material should be nonstaining to prevent discoloration. Liftingpoints should be well protected and kept accessible while the units are in storage Precast elements must be stored safely with adequate supports so as not toendanger the workers.

Examples of supports. 16. Transportation Transport of prefabrication elements must be carried out and with extreme care to avoid the distress in elements and handled as far as possible to be placed in final portion. Requirements for Transportation and Erection i. Traffic regulations limit the maximum length, size and weight of an individual unit. ii. The capacity of cranes and hoists limits. iii. The load carrying capacity of trucks. Transported by road, typical dimension might be Height 4.0 to 4.5m; Length 22.0 m and width 2.55m. 16.1 Delivery Precast elements must be loaded carefully on to delivery vehicles to preventdamage. Weight and size of precast elements should be considered at thedesign stage to ensure that the precast elements can be transported by truck.

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Ensure no undesirable stresses to the precast elements due toflexing of truck.

Pre-plan sequence of liftingto avoid multiple handling of precast elements. 16.2 Loading and storage on transporters To protect the edges throughout their journey, proper devices should be usedto support, secure and wedge the precast units. The units should be adequately secured and supported to prevent them from overturning, shifting or being damaged during transportation. Chain blocks or tie wire, together with a steel frame, are usually used to hold precast elements in position during transportation.

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17 Erection on Site The type of erection must be taken into account: horizontal, i.e. elements positioned storey by storey with a tower crane, or vertical, i.e. bay by bay over the full height of the building with a mobile crane.

Typically, tower cranes can handle only relatively light loads, albeit at a large radius and through a full 360h. However, the largest tower crane used in Germany to date was able to handle a load of 30 t at a radius of 40 m. 17.1 Erection of Prefab Elements It is the process of assembling the Prefabrication element in the find portion as per the drawing. In the erection of prefab elements the following items of work are to be carried out. 1).Slinging of the prefab elements. 2).Tying up of erection slopes connecting to the erection hooks. 3).Cleaning the elements and the site of erection. 4).Cleaning the steel inserts before incorporation in the joints lifting and setting the elements to correct position. 60

5).Adjustments to get the stipulated level line and plumb. 6).Welding of cleats. 7).Changing of the erection tackles. 8).Putting up and removing the necessary scaffolding or supports. 9).Welding the inserts laying the reinforced in joints. The erection work in various construction jobs by using prefab elements differs with risk condition, hence skilled foremen, and workers to be employed on the job. 17.2 Equipment’s required for Erection Equipment’s required for the prefab elements in industry can be classified as. 1) Machinery required for quarrying of course and fine aggregates 2) Conveying equipment, such as but conveyor, chain conveyors etc. 3) Concrete mixers 4) Vibrators 5) Erection equipment such as cranes, derricks, chain pulley etc. 6) Transport machines 7) Work shop machinery for fabricating and repairing steel. 8) Bar straitening, bending and welding machines 9) Minor tools and takes, such as wheel barreriour, concrete buckets etc… 10) Steam generation a plant for accelerated curing 17.3 Installation of Vertical Elements 1. Setting out

2. Setting out Quality control point offset line

3. Hoisting, Rigging and Installation o designated location

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Check stability of prop before releasing hoisting cable. 4. Grouting works

-compacting to prevent cracking. oad bearing elements. 5. Connecting joints

d connections welding as required 17.4 Installation of Horizontal Elements 1. Setting out

during the erection process 2. Hoisting & Installation ort slab/beam

considerations

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3. Connections/Jointing -in-situ joints place the lap rebars as required

to form part of formwork joints

4. Installation using Big canopy er and efficient 5. Erection Purpose In Japan o Used to construct the 26 storey pre-cast concrete 30,763m2 o The system realized 60% reduction in labor requirement for the frame erection. In Singapore o DBS China square used the system to erect is efficient and faster 6. Installation constraints Management

7. Management panels lowered in roller platform

8. Mishandling of precast panels t was placed on the beam corbel

9. Common Defects in precast panels The common defects to hole in precast panels before installation panel before installation.

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duck choked 10. Precast failures Bridge Deck collapse Single T beam collapse

17.4 Erection safety In precast construction, erected precast elements are initially in a temporary condition before being connected to the parent in-situ structure. Since many are temporarily installed around the peripheral areas of buildings, the safety of workers during installation is of paramount importance. The absence of peripheral scaffolding in precast construction is a major difference from conventional construction. As a result, steel working platforms are provided for workers. The height, width, strength and connections of working platforms should be thoroughly considered to match the precast construction circumstances. In some cases, a temporary detachable handrail is necessary at the installation areas. 17.5Propping Erection is one of the critical steps to be attended to in precast construction. The Code specifies the requirements and considerations in respect of erectionpreparation, safety, sequences and tolerance as well as the temporary supporting system. A full method statement, which includes details of the handling of the precast elements, rigging arrangements, use of machinery as well as the erectionsequence, shall be prepared, checked and endorsed.

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2.21. Erection of precast columns

Elevation

Side View Stage-1-Column Erection 1-1 Erect P/C columns. 1-2 After the Grout Strength (For Column Base and Connection) Achieve 40MPa, Dismantle the Push and pull

Lifting of Columns 65

2.22.Construction sequence of precast columns, beams and slabs

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2.23 Disuniting of Structures The solution of problems connected with the transportation and placing of structures demands as a rule their disuniting in to smaller members. One bay frames not exceeding, 40 tonnes in weight may represent an exception because the problems of their hoisting and placing can be solved with the aid of modern available hoisting machines and equipment. In spite of this framed are frequently disunited at their corners or points of minimum moments into members to make the hoisting of these smaller members possible, using much simpler equipment. In general there is trend towards the use of larger members. This justified by more than one reason. One is that the bearing of a certain moment can be solved more economically by using one large girder instead of two or more smaller beams together having the same capacities.

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Benefits of Disuniting Hoisting of one large member as a rule, less expensive than that of 2 smaller members having the same combined weight. It is a direct consequence of the following circumstances: 1. The assembling of the lifting tackle, the transfer of the hoisting machine, the hoisting, placing and plumbing must be done for each member separately, independently of its weight. 2. The disuniting into larger members means lower costs of hoisting and placing as well as saving in joining costs. Types of System (a) Systems consisting of linear members disunited at joints. Advantage: Disuniting at joints gives linear member. This means that a great advantage and facilitates from the view point manufacture and assembly. Disadvantage: 1. Joints are at corners i.e. at points of maximum moment values, so forming the joint is difficult. 2. Joints must be over dimensioned to cope with in-situ concreting. And one alternate solution to replace moment resistant joints by hinged connection.

(b) System for the Prefabrication of Entire Rigid Frame In this system, to reduce the no of joints and to precast larger numbers I one piece leads to the prefabrication of entire frame. Production of the frames does not cause any particular trouble but the hoisting is more difficult and requires careful preparation. The stress distribution of straight members during hoisting is in general statistically determinate. Advantage: 1. It is ideal for site prefabrication. 2. Small number of joints so rapid prefabrication work is possible. 3. Suitable for long walls consisting of great number of uniform frames.

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(C) System Consisting of I, T, U of Straight Members Disunited At Points of Minimum Moment Another method of disuniting of structures is by division into different membranes at points where the moments are thin or smallest. This method is called as lambda method. Using this method hinge joints are made. Advantage: 1. Functions are made at points of minimum moments or at points of contra flexure. 2. Disuniting the main girder in this manner makes the application of different skylights possible.

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. Disadvantage: 1. Hosting and temperature bracing of L joined asymmetric frame members is particularly complicated. 2. Temperature resting of frame member on each other necessitates the use of cantilevers having half depth and proper forming of this cause difficulty. (D) Two hinged and three hinged arches Arched structures are normally two hinged and three hinged arches. Arched structures are normally used for bridging span more than 20-25m. Their production and placing is more difficult than straight members. Arch can be two hinged and three hinged but they can also be fixed at footings and can be constructed with or without tie. These members are generally precast and assembled in statistically determinant three hinged variance and middle hinge is only eliminated after placing is finished. The reinforcing bars protruding both sides are welded together and the joint between the members is filled in with insitu concrete. Arch structure can be precast in either vertical or horizontal positions. In the first case, shuttering made of timber or concrete is required having the same curvature s the arch itself. The prefabrication of larger arches in the horizontal position is found to be more economical. The construction of arch trusses can be properly carried out in the horizontal position only.

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2.24 Lateral Load-Resisting Systems Three common systems for resisting wind or earthquake lateral loads are given here. 1. Moment-resisting frames: This system are made up of interconnected beams and columns. Lateral loads are resisted by bending of the beams and columns. Such frames undergo relatively large lateral deflections. If all stories have beams and columns with sizes proportional to the shear in the story, the lateral deflection of each story would be similar. To simplify construction, however, the sizes of the beams and columns selected for the lower stories are commonly used throughout, or are changed only every third or fourth story. Hence, the beams and columns in a building tend to be oversized in the upper stories. Momentresisting frames are used for buildings up to 8 to 10 stories. May be impossible to make the beams stiff enough to prevent large deflections. In such cases, walls or other stiffening elements are used to control lateral deflections. 2. Bearing-wall systems: This system are used for apartment buildings or hotels. A bearing-wall building has a series of parallel transverse shear walls between rooms or apartments. The walls resist lateral loads by flexural action and deflect as vertical cantilevers. 3. Shear-wall–frame system: This system are used in buildings ranging from about 8 to about 30 stories. The lateral load is resisted in part by the wall and in part by the frame. Some of the most slender shear-wall–frame structures ever built are described by Grossman. He presents some of the wind modeling rationale and two case histories of buildings with heights up to 10 times the least width at ground level. 4.Very tall concrete buildings Structural systems: This system contain closely spaced columns in the upper stories transfer vertical loads much like a continuous closed tube would. At the top of the second floor the vertical loads are transferred to a continuous deep beam called a transfer beam. It, in turn, transfers the vertical loads to the 10large columns on the perimeter of the ground floor. In this region the more-or-less uniform compression in the tube is disrupted. 71

2.25 Shear Wall Shear walls are vertical elements of the horizontal force resisting system. Shear walls are constructed to counter the effects of lateral load acting on a structure. In residential construction shear walls are straight external walls that typically form a box which provides all of the lateral support for the building. Importance of shear wall When shear walls are designed and constructed property and they will have the strength and stiffness to resist the horizontal forces. In building construction a rigid vertical diaphragm capable of transferring lateral forces from exterior walls floors and roofs to the ground foundation in a direction parallel to their planes. Lateral forces caused by wind earthquake and uneven settlement loads. In addition to the weight of structure and occupants create powerful twisting (torsion) forces. These forces can literally shear a building apart. Reinforcing a frame by attaching or placing a rigid wall inside it maintains that shape of the frame and prevents rotation at the joints shear walls are especially important in high-rise building subjected to lateral wind and seismic forces. In the last two decades shear walls became an important part of mid high rise residential buildings. As part of an earthquake resistant building design these walls are placed in building plans reducing lateral displacements under earthquake loads. So shear wall frame structure are obtained. Shear wall buildings are usually regular in plan and in elevation. However, in some buildings lower floors are used for commercial purposes and the buildings are characterized with larger plan dimensions at those floors. Purpose of constructing shear walls 1. Shear walls are not only designed to resist gravity/vertical loads due to its self-weight & other living moving loads), but they are also designed for lateral loads of earthquakes/wind. The walls are structurally integrated with roofs/floors (diaphragms). 2. Shear wall structural systems are more stable because their supporting area (Total cross sectional area of all shear walls) with reference to total plans area of building is comparatively more, unlike in the case of RCC framed structures. 3. Walls have to resist the uplift forces caused by the pull of the wind walls have to resist the shear forces that try to push the walls over walls have to resist the lateral force of the wind that tries to push the walls in and pull them away from the building. 4. Walls floors and roofs to the ground foundation in a direction parallel to their planes.

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Comparisons of shear wall with construction of conventional load bearing walls Load bearing masonry is very brittle material. Due to different kinds of stresses such as shear, tensile...etc. Caused by the earthquakes the conventional unreinforced brick masonry collapses instantly during the unpredictable and sudden earthquakes. The RCC framed structures are slender. When compared to shear wall concept of box like threedimensional structures though it is possible to design the earthquake resistant RCC frame it requires extra-ordinary skills at design detailing and construction levels. Which cannot be anticipated in all types of construction projects. On the other hand even moderately designed shear wall structures not only more stable, but also comparatively quite ductile. In safety terms it means that during very severe earthquakes they will not suddenly collapse causing death of people. They give enough indicative warnings such as widening structural cracks yielding rods etc offering most precious moment for people to run out off structures before they totally collapse. For structural purposes we consider the exterior walls as the shear resisting walls. Forces from the ceiling and roof diaphragms make their way to the outside along assumed paths enter the walls and exit at the foundation. Location of Shear Wall Generally shear walls are either plane or flanged in section, while core walls consists of channel sections.

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In many cases, the wall is pierced by openings. These are called coupled shear walls because they behave as individual continuous wall sections coupled by the connecting beams or slabs.

Normally the walls are connected directly to the foundations. However, in a few cases where the lateral loads are relatively small and there no appreciable dynamic effects, then they can be supported on columns connected by a transfer beam to provide clear space. The shape and plan position of the shear wall influences the behavior of the structure considerably. Structurally, the best position for the shear walls is in the centre of each half of the building. This is rarely practical, however, since it dictates the utilization of the space, so they are positioned at the ends.

This shape and position of the walls give good flexural stiffness in the short direction, but relies on the stiffness of the frame in the other direction. This arrangement provides good flexural stiffness in both directions, but may cause problems from restraint or shrinkage. As does this arrangement with a single core, but which does not have the problem from restraint of shrinkage. However, this arrangement lacks the good torsional stiffness of the previous arrangements due to the eccentricity of the core.

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Forces on shear wall Shear wall resist two types of forces Shear Forces Shear forces are generated in stationary buildings by accelerations resulting from ground movement and by external forces like wind & waves. This action creates shear forces throughout the height of the wall between the top and bottom shear wall connections. Uplift Forces Uplift forces exist on shear walls because the horizontal forces are applied to the top of the wall. These uplift forces try to lift up one end of the wall and push the other end down. In some cases the uplift force is large enough to tip the wall over. Uplift forces are greater on tall short walls uplift shear walls need hold down devices at each end when the gravity loads cannot resist all of the uplift. The hold down device then provides the necessary uplift resistance. Classification of shear walls 1. Simple rectangular types & flanged walls 2. Coupled shear walls 3. Rigid frame shear walls 4. Framed walls with in filled frames 5. Column supported shear walls 6. Core type shear walls Types of shear walls RC shear wall It consists of reinforced concrete walls and reinforced concrete slabs wall thickness varies from 140 mm to 500 mm depending on the number of stories, building age and thermal insulation requirement. In general these walls are continuous throughout the building height however. Some walls are discontinued as the steel front or basement level to allow for commercial or parking spaces. Floors slabs are either cast in-situ flat slabs or less often. Precast hollow core slabs. Buildings are supported by concrete strip or mat foundations the latter type is common for buildings with basements. Ply wood shear wall Plywood is the traditional material used in the construction of shear walls the creation of prefabricated shear panels have made it possible to inject strong shear assemblies into small walls that fall at either side of a opening in a shear wall plywood shear wall consist of  Plywood to transfer shear forces.  Chords to resist tension/compression generated by the over turning moments.  Base connections to transfer shear to foundations. 75

Mid-ply shear wall The mid-ply shear wall is an improved timber shear wall that was developed by redesigning the joints between shearing and finishing members. So, that the failure modes observed in standard wall testing are virtually eliminated at lateral loads levels high enough to cause failures in standard walls RC Hollow concrete block masonry walls (RHCBM) These walls are constructed by reinforcing the hollow concrete block masonry by taking advantage of hollow spaces & shapes of the hollow blocks. It requires continuous steel rods (reinforcement) both in the vertical & horizontal directions at structurally critical locations of the wall panels packed with the fresh grout concrete in the hollow spaces of masonry bocks. RHCBC elements are designed both as load bearing walls for gravity loads and also as shear walls for lateral seismic loads to safety withstand earthquakes. Steel plate shear wall In general steel plate shear wall system consists of a steel plate wall boundary columns and horizontal floor beams. Together the steel plate wall and boundary columns act as a vertical plate girder. The column act as flanges of the vertical plate girder and the steel plate’s wall act as its web. The horizontal floor beams act more or less as transverse stiffeners in a plate girder steel plate shear wall systems have been used in recent years in highly seismic area to resist lateral loads. Thus shear walls are one of the most effective building elements in resisting lateral forces during earthquake. By constructing shear walls damages due to effect of lateral forces due to earthquake and high winds can be minimized shear walls construction will provide larger stiffness to the buildings there by reducing the damage to structure and its contents. Hence it is preferable to have all these prefabricate approximately of some weight very near to the lifting capacity of the equipment.

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MODULE-3 3.1 Precast RC Floor Slabs-General Precast concrete flooring offers an economic and versatile solution to ground and suspended floors in any type of building construction. Worldwide, approximately half of the floors used in commercial and domestic buildings are of precast concrete. 3.2 Advantages of Precast concrete floors over conventional RC Floor Slabs 1. It offers both design and cost advantages over traditional methods such as cast in situ concrete, steel-concrete composite and timber floors. 2. There are awide range of flooring types available to give the most economical solution for all loading and spans. 3. The floors give maximum structural performance with minimum weight and may be used with or without structural toppings, non-structural finishes (such as tiles, granolithic screed), or with raised timber floors. 4. Offsite production of high-strength, highly durable units. 5. Fast erection of long span floors on site. 6. It require minimum amount of in-situ reinforcement and wet concrete. 3.3 Types of Floor Slabs Structural floors/roofs account for substantial cost of a building in normal situation. Therefore, any savings achieved in floor/roof considerably reduce the cost of building. Traditional Cast-insitu concrete roof involve the use of temporary Shuttering which adds to the cost of construction and time. Use of standardized and optimized roofing components where shuttering is avoided prove to be economical, fast and better in quality. Some of the prefabricated roofing/flooring components found suitable in many low-cost housing projects are: i. Precast RC Planks. ii. Prefabricated Brick Panels iii. Precast RB Curved Panels. iv. Precast RC Channel Roofing v. Precast Solid vi. Hollow Slabs vii. Single /Double Tee Beam Precast Slabs i. Precast RC plank roofing system This system consists of precast RC planks supported over partially precast joist. RC planks are made with thickness partly varying between 3 cm and 6 cm. There are haunches in the plank which are tapered. When the plank is put in between the joists, the space above 3 cm thickness is 77

filled with in-situ concrete to get tee-beam effect of the joists. A 3 cm wide tapered concrete filling is also provided for strengthening the haunch portion during handling and erection. The planks have 3 numbers 6 mm dia MS main reinforcement and 6 mm dia @ 20 cm centre to centre cross bars. The planks are made in module width of 30 cm with maximum length of 150 cm and the maximum weight of the dry panel is 50 kg (Figure 3.1). Precast joist is rectangular in shape, 15 cm wide and the precast portion is 15 cm deep (Figure 3.1). The above portion is casted while laying in-situ concrete over planks. The stirrups remain projected out of the precast joist. Thus, the total depth of the joist becomes 21 cm. The joist is designed as composite Teebeam with 6 cm thick flange comprising of 3 cm precast and 3 cm in-situ concrete (Figure 3.2). This section of the joist can be adopted up to a span of 400 cm. For longer spans, the depth of the joist should be more and lifting would require simple chain pulley block. The completely finished slab can be used as intermediate floor for living also In residential buildings, balcony projections can be provided along the partially precast joists, designed with an overhang carrying super imposed loads for balcony as specified in IS: 875-1964, in addition to the self-load and the load due to balcony railings. The main reinforcement of the overhang provided at the top in the in-situ concrete attains sufficient strength. The savings achieved in practical implementations compared with conventional RCC slab is about 25%.(P.K.Adlakha and H.C.Puri, 2002)

Fig. 3.1 Precast R.C. Planks

Fig. 3.2 R.C. Planks laid over partially precast joists 78

ii. Prefabricated brick panel roofing system The prefabricated brick panel roofing system consists of: (a) Prefab brick panel-Brick panel is made of first class bricks reinforced with two MS bars of 6 mm dia and joints filled with either 1:3 cement sand mortar or M-15 concrete. Panels can be made in any size but generally width is 53 cm and the length between 90 cm to120 cm, depending upon the requirement. The gap between the two panels is about 2 cms and can be increased to 5 cms depending upon the need. A panel of 90 cm length requires 16 bricks and a panel of 120 cm requires 19 bricks (Figure 3.3). (P.K.Adlakha and H.C.Puri, 2002)

Fig. 3.3 Brick Panel (b) Partially precast joist-It is a rectangular shaped joist 13 cm wide and 10 cm to 12.5 cm deep with stirrups projecting out so that the overall depth of joist with in-situ concrete becomes 21 cm to 23.5 cm, it is designed as composite Tee-beam with 3.5 cm thick flange.(Figures 3.4 and 3.5 ). (P.K.Adlakha and H.C.Puri, 2002)

Fig. 3.4 Details of Precast JoistFig.

Fig.3.5 Precast R C Plank and Joist System 79

iii. Precast curved brick arch panel roofing This roofing is same as RB panel roofing except that the panels do not have any reinforcement. A panel while casting is given a rise in the centre and thus an arching action is created. An overall economy of 30% has been achieved in single storeyed building and 20% in two or three storeyed buildings. (P.K.Adlakha and H.C.Puri, 2002) iv. Precast RC channel roofing Precast channels are trough shaped with the outer sides corrugated and grooved at the ends to provide shear key action and to transfer moments between adjacent units. Nominal width of units is 300 mm or 600 mm with overall depths of 130 mm to 200 mm (Figure 8). The lengths of the units are adjusted to suit the span. The flange thickness is 30 mm to 35 mm. Where balcony is provided, the units are projected out as cantilever by providing necessary reinforcement for cantilever moment. A saving of 14% has been achieved in actual implementation in various projects. (P.K.Adlakha and H.C.Puri, 2002) v. Solid Slabs Solid slabs are used as structural deck components similar to hollow-core slabs. They can be made in a long-line pre-tensioning facility and reinforced with pre-stressing strand or cast in individual forms with either pre-stressing strand or conventional reinforcing bars. They are typically cast in the same position as used in the structure.

Sizes can vary to satisfy the structural requirements. Typical widths: 1.2m to 3.6m Typical spans: 2.4m to 9.0m. Typical thicknesses: 100mm to 300mm.

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vi. Precast Hollow slabs roofing Precast hollow slabs are panels in which voids are created by earthen kulars. Without decreasing the stiffness or strength. These hollow slabs are lighter than solid slabs and thus save the cost of concrete, steel and the cost of walling and foundations too due to less weight. The width of a panel is 300 mm and depth may vary from 100 mm to 150 mm as per the span, the length of the panel being adjusted to suit the span. The outer sides are corrugated to provide transfer of shear between adjacent units. The kulars are placed inverted so as to create a hollow during precasting (Figure 9). Extra reinforcement is provided at top also to take care of handling stresses during lifting and placement. There is saving of about 30% in cost of concrete and an overall saving of about 23%.(P.K.Adlakha and H.C.Puri, 2002) Hollow-core slabs are used predominantly for floor and roof deck components for various structures such as residential, hotel, office buildings, schools, and prisons.

Typical widths: 0.6, 1.2, and 2.4m; some precasters offer 3.0 and 3.6m widths Typical depths: 150, 200, 250, 300, 375, and 400mm. Typical span-to-depth ratios: Floors: 30 to 40 / Roofs: 40 to 50 Hollow core slabs are the most widely used precast floor component in prefabricated buildings. Thesuccess is largely due to the highly efficient automated production method, good quality surface finish, saving of concrete, wide choice of structural depths, high strength capacity and rapid assembly on site. The hollow core slabs are manufactured using long line extrusion or slip-forming processes; the former process being the most popularly used. Cross section, concrete strength, and surface finish are standard to each system of manufacture. Other variations include increased fire resistance, provision of penetrations, opening of cores for on-site fixings, cut-outs for columns/walls, etc. The slabs are sawn after detensioning which normally takes place six to eight hours after casting and typically when the concrete strength reaches 35 N/mm2.

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12m x 1.2m Hollow Core Floor Slabs Holes in the floor may be created in the precast units during the manufacturing stage before the concreteas hardened. The maximum size of the opening which may be produced in the units depends on the size of the voids and the amount of reinforcement that can be removed without jeopardizing the strength of the unit. Holes should preferably be located within the void size which may vary in different sections.

viii. Single Tee Beam Combination beam and slab Spans up to 120’-0" Typical width = 8’-0" Typical depths of 36" and 48" Double Tees Beam Named for its shape, double-tees are used primarily as floor and roof deck components for any type of structure, including parking structures and all types of buildings. They are made either: Pre-topped using a flange thickness of 100mm, which creates the wearing surface in parking structures; orField- topped with a 25mm flange, on which a cast-in-place concrete composite

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topping of 25mm to 100mm is added in the field. For roof construction, there is typically no need to add topping on the 50mm flange.

Typical widths: 2.5m, 3m, 3.6m and 4.5m. Typical depths: 600mm, 650mm, 700mm, 750mm, 800mm, and 850mm. Typical span-to-depth ratios: Floors: 25 to 35 / Roofs: 35 to 40 Combination beam and slab Spans up to 30m. Typical width = 2.5m Depths of 380mm, 450mm, 600mm and 800mm.

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3.4 Analysis and Design of Precast Floor Slab General The design process for a hollow-core floor system should typically involve the following specifications: a. General structural arrangement including the assumed self-weight of the floor and any significant floor penetrations. b. All other permanent actions including partition loads and potentially removable loads. c. All design imposed actions on the floor including uniformly distributed live loads and point loads. d. Intended floor support arrangements including any special requirements for plastic hinge regions. e. Earthquake induced support displacements and rotations. f. Vertical seismic loads (accelerations). g. Floor diaphragm requirements. h. Extent of any expected beam elongations resulting from inelastic behavior in the support beams. i. Tolerance requirements. j. Any other requirements, such as minimum concrete strengths, propping restrictions fire rating, durability, specifically required details, etc. The design of hollow core slabs is governed by the American concrete Institute (318-95). Building Code Requirements for structural concrete. As with prestressed concrete members in general, hollow core slabs are checked for prestresstransfer 84

stresses, handling stresses, service load stresses, deflections and design (ultimate) strength in shear and bending. Design the Hollow core slab having the following details. 1. Clear span = 5m 2. No. of openings = 3 No.s /m of 100 mm dia 3. LL = 4 KN/Sq.m 4. Floor finish = 2 KN/Sq.m 5. M30 Grade of concrete and Fe-500 steel 3.5Reinforcement Details

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3.6 Production of Floor Slabs A. Principles of Product forming Systems Two basic manufacturing methods currently in use for the production of hollow core slabs are:Extrusion: In extrusion system a very low slump concrete is forced through the machine. The cores are formed with augers or tubes with the concrete being compacted around the cores. Extrusion is known for the high compaction capabilities and excellent structural capacity of the hollow core slab. One of the advantages in the modern extrusion is the silent and automatic operation. The key to an optimum compaction on dry cast is the frequency of vibration since the absence of water makes it more difficult for the concrete particles to flow. Some of the Extruders manufactured today do not use the high frequency vibrators, instead they operate based on pressing extrusion screws, which usually requires more water to achieve the same level of compaction than the Hollow Core Extruders based on high frequency of vibration. Slip form: This system uses a higher slump concrete. Sides are formed either with stationary, fixed forms or with forms attached to the machine with the sides being slip formed. This is a relatively simple principle where concrete is flowing on to the product in two to three phases (layers) each phase is compacted typically by vibrators. The Slip form system is used mainly for shallower hollow core cross sections and various other cross sections like inverted T-beams. However, there are slip formers, which can form 700mm hollow core sections. The production speed for each of the system varies depending on raw materials, depth of products and machine types being used. The speed ranges between 1 to 3 meters/min. Hollow coreslabs can be manufactured in different thicknesses such as150mm, 200m, 240mm, 265mm, 280mm, 300mm, 320mm, 350mm, 400mmetc as required. 86

1. Cleaning and oiling of the bed 2. Reinforcement 3. Lifting the extruder on the bed 4. Concrete mixing and transportation 5. Concrete dosing to extruder 6. Extruding 7. Making openings 8. Covering and curing of slab 9. Cutting of slab 10. Drilling of drainage holes 11. Transportation to storage 12. Handling of slabs in storage 13. Transportation to site.

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3.7Construction Sequence for Erection of Precast RC Slabs

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3.8 Lift-Slab System  The load-bearing structure consists of precast reinforced concrete columns and slabs.  Precast columns are usually two stories high.  All precast structural elements are assembled by means of special joints.  Precast concrete floor slabs are lifted from the ground up to the final height by lifting cranes.  The slab panels are lifted to the top of the column and then moved downwards to the final position.  Temporary supports are used to keep the slabs in the position until the connection with the columns has been achieved.

1. Pillars and the first package (e.g. 5 pieces) of slabs prepared at ground level. 2. Lifting boxes are mounted on the pillars + a single slab lifted to the first floor level. 3. Boxes are sequentially raised to higher positions to enable the slabs to be lifted to their required 4.Final position - slabs are held in a relative (temporary) positions by a pinning system

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3.9 Precast Stair Case Precast stair slabs are usually designed to span longitudinally into the landings at right angles to thestair flights or span between supporting beams. In monolithic construction, the stair slab can be designed with continuous end restraints over the supports. But in instances where staircases are precast, the construction is generally carried out after the main structure, with pockets or recesses left in the supporting slabs or beams to receive the stair flights. With no appreciable end restraints, a precast stair slab could therefore be designed as simple slab between supports. In design, the dead load is calculated along the sloping lengths of the stairs but the live and finishing loads are based on plan area. If the risers were to be covered with finishes, additional loads would have to be added in the design. The effective span is measured horizontally between the centers of the supports or the actual horizontal length of the precast stair slab where dry connections are used at the supports. The thickness of the waist is taken as the slab thickness. The basic span-effective depth ratio may be increased by 15% to 23 (=20 x 1.15) if the stair flight occupies at least 60% of the span. This will apply to precast stair slabs without landings. The supporting nibs of the precast stair slab may be constructed with either dry or wet connections (extended bearings). The design of reinforcement of the nibs can be based on: Simple bending. Strut and tie force model. Shear friction. 3.9.1 Standard Precast Staircase Dimensions In practice, the number of risers and the riser height of a staircase have always been dictated by the storey height of a building. This would result in different riser dimensions. Prefabricating stair flights with many different riser dimensions would not be economically viable. The limit the riser height to 165mm and 175mm, with a tread dimension to 250mm. These dimensions are suitable for fire escapes. For school development projects, 150mm riser with 300mm tread (instead of 250mm) are recommended dimensions required by the Ministry of Education, for safety reasons.

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In residential developments, a designer could use staircases with 175mm riser for a floor tofloor height of 3m, 150mm, for fire escape. In luxury residential developments, staircases with a 165mm riser would be appropriate for a floor to floor height of 3.3m. The recommended width of standard staircase is ideally set to allow for a 1000mm clearance between handrails and edging kerb. In addition, it allows designers to include or exclude an edging kerb (or buffer zone) of 75mm width to one side of the staircase. The provision is 92

intended for the fixing of balustrades, which could be welded to the base plate, cast in the welding pocket, or bolted to the concrete surface by cast in socket.

Isometric View of Precast Stair Case with Dry Joint

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Isometric View of Precast Stair Case with Wet Joint

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3.9.2. Method of Constructions

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3.10 Joints and Connections One of the most intricate and most difficult problems to be solved in both design of construction of structures assembled of prefabrication members is the joining. It is highly important that the construction of the joints should be easy that unavoidable smaller inaccuracies and deviations within dimensional tolerances should neither influence the designed stresses in a detrimental manner nor cause is admissible changes in the stress distribution of the structures. The forming and construction of joints requires owing to their intricacy, great increased control joints which cannot be inspected should be omitted. When solving the problem of joints the properties of reinforced concrete must be taken into consideration. This means in other words, that the design of the construction of the joint should harmonize with the materials to be used. The properties of steel of timber are quite different from those of concrete and reinforced concrete. Therefore joints similarly to those used in timber and steel construction are generally not appropriate for the purpose. Design of class based an efficiency of the material used: The plastic concrete can be used for the subsequent concrete of joints of the fluid cement mortar cast or pressed into the gaps less part of their water during the setting time of shrink, after setting the shrinkage of the insite concrete of mortar continue. With respect to two phase of shrinkage same codes on reinforced concrete construction permit only reduced stresses for a subsequent insite concrete of a mortar casting. There are generally determined as a function of width of the joint on the gap to be concrete as cast. Joints must be designed of executed so that compensation for the allowed dimensional tolerances is ensured. A relative displacement of the joined member should be impossible even as a result of a blow or of any other unfavorable force effect. The length of the section determined for the transmission of forces should be as short as possible but should excluded any excess of the permissible stress. The joints can be rigid hinge like or shed. Rigid joints are adequate in addition to the bearing of tensile, compressive of shear forces for resisting to bending moments too. Thesejoints make relative displacement and relative relation impossible. Hinge like joints can transmit forces passing through the hinge itself and also allow a certain motion and rotation. Rigid joints are generally used for the junction of column to footings, but they can also be applied for joining of individual groups with one another. The joints generally used in the construction with precast members are usually hinge like. Their execution is simpler and requires less working lime than rigid joints “shed joints” are only exceptionally used in industrial construction of are justified for a long span only. These joints are chiefly used in bridge construction for long span bridges depending on the necessity of insite concreting; two kinds of joints can be distinguished. 96

a) Dry joints = joint accomplished by simple placing of two members on each other of fasting. b) Wet joints = joint require not only casting with cement mortar but also subsequent concreting. Joints for different structural connections: eam on top of column.

(a) Joining Columns to Footing This joint is usually rigid but also can be hinge. A rigid joint can be made by placing the column into a calyx of the footing or by using a welded joint. The figure shows the three variations of this method.

For large footings

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Column to Foundation The depth of the calyx is dimensioned according to the long or side length of the column. The depth of the calyx should be equal to 12.5% of the length of the column. The opening of the calyx is 6-10 cm greater in all direction than the class of the column. This is enabling the vibrator to be operated while concreting at the bottom of the calyx of checked by levelling before concreting. A similar steel plate is also put on the lower end of the column when positioning the column. These two steel plates must be on each other. The dimensions ofthese steel plates are frame 100x100x10 to 150x150x10 mm a chord into the concrete after the column is put in placed properly plumbed two advantages of the calyx joint should be mentioned. 1. The placing plumbing and fixing of the column as well as the subsequent filling of the calyx with concrete is for simpler and requires less time than in the case of a welded joint. 2. The method is least sensitive to inaccuracies occurring during the construction. The disadvantages of the calyx joint are more suitable for small columns. In the case of large columns requiring a calyx depth of which is greater than 1 m. (b) Joining of Column to Beam at an Intermediate Junction One method of forming a hinge like joint consists either or placing to beam on to a small cantilever protracting frame the column or of putting it on the bottom of an adequately shaped opening left out of the column shaft. The beam rests temporarily on a tongue like extension on a steel plate placed in this opening on the supporting surface the tongue is also furnished with a steel plate anchored into the concrete. The other parts of the tongue are supported after the placing has been finished with concrete cast through an opening left for this purpose. Hinge like joining of girder to column: 1. Opening for casting. 98

2. Subsequent concreting. 3. Steel plate.

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Column to Beam (c) Lengthening of Columns Columns are usually lengthened at floor levels. An intermediate lengthening should be avoided it possible. The lengthening of columns can be executed similarly to the joining with footing, accordingly the upper columns rests on the lower ones by a tongue like extension. The steel bars of the main reinforcement are joined by overlapping looped steel bars a welding. There after the stirrups have to be placed of finally the joint must be concreted.

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Column to Column (d) Joining of Beams The functions of beams can be affected either by overlapping the protracting steel bars or by welding them together. Fig. shows the hinge like joint of purlins. In this method the whole shear must be both by both cantilevers (i.e.) by two separate structures therefore it is expedient to formthis joint at least for large girders. The method illustrated in the fig presents a dry joint of beams which is called a bolted front. The advantages of this joint are immediate bearing capacity.

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Beam to Beam (e) Joining of Precast To Monolithic Reinforced Concrete Structures It frequently occurs that a monolithic beam has to be joined to a precast column. In this case the function can be established in the same way as already been described in the previous paragraph an joining namely by placing the end of the beam either on to a cantilever protruding from the column or into an opening formed into the columns shaft. When making the joint, first of all a 2.5 cm deep cavity is chiselled out of the side of the precast column. The bottom of this cavity should be roughened so as to attain a belter band between the concrete of the monolithic beam and the precast column.

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Expansion Joint An Expansion joint is an assembly designed to safely absorb the heat –induced expansion and contraction of various construction material to absorb vibration or to allow movement due to ground settlement or earthquakes they are commonly found between sections of sidewalks, bridges, railway tracks, piping systems, and other structures. Expansion Joint Design A design specification shall be prepared for each expansion joint application. Prior to writing the expansion joint design specification it is imperative that the system designer complicity review the structural system layout and other items which may affect the performance of a expansion joint particular attention shall be given to the following items the system should be reviewed to determine the local and type of expansion joint which is most suitable for the application. Both the EJMA standards and most reliable expansion joint manufacturers. Catalogs provide numerals examples to assist the user in this effort. The availability of supporting structures of anchoring and guiding of the system and the direction and magnitude of thermal moments to be absorbed must be consider when selecting the type and location of the expansion joint. Conventional Rubber Expansion Joint Expansion joints are designed to provide stress relief in piping system that are loaded by thermal movements and mechanical vibration. To deal with the various forces on the joint they require fiber reinforcement which guarantees both flexibility and strength. conventional expansion joints are reinforced using prefabricated fiber pipes, the use of these fiber pipes makes it impossible to control the orientation of the fiber on complex shape such as the below of the expansion joint. In both case the inability to use the fiber in the optimal way leads to the following disadvantages. High material cost ded then necessary

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MODULE-4 4.1 Types of Wall Panels a. Cross wall system b. Longitudinal wall system Cross-wall system: The main walls that resist gravity and lateral loads are placed in the short direction of the building. Longitudinal-wall system: The walls resisting gravity and lateral loads are placed in the longitudinal direction.

Precast concrete walls could be 1) Homogeneous walls 2) Non-homogeneous walls Homogeneous walls: The walls could be solid or ribbed. Non-homogeneous walls: Based on the structural functions of the walls, the walls could be classified as a. Load bearing walls b. Non-load bearing walls c. Shear walls Based on their locations and functional requirements the walls are further classified as 104

(i) External walls which can be load or non-load bearing depending upon the layout. They are usually non-homogeneous walls of sandwiched type to impart better thermal comforts. (ii) Internal walls which provide resistance against vertical loads, horizontal loads, fire etc. and are normally homogeneous. These could be composite or sandwich panel based on the structural functions of the walls, the wall could be classified as i. Load bearing walls ii. Non load bearing walls iii. Shear walls Based on their locations and functional requirements the walls are also classified as 1. External walls 2. Internal walls 1. External Walls which can be load bearing depending up to the layout and are usually non homogeneous walls of sandwiched typed to impart better thermal comforts. 2. Internal walls providing resistance against vertical loads horizontal loads, fire etc. and are normally homogeneous walls. In addition, the walls are classified as: 1. Braced if the walls are supported laterally by floors or other cross-walls 2. Unbraced if the walls provide their own stability, such as cantilever walls. The walls are considered stocky if the slenderness ratio, does not exceed 15 for a braced wall and 10 for an unbraced wall. Otherwise, the walls are considered as slender. 4.2 Design of Precast Concrete Walls (Based on ACI-533R-93) General The functions of precast concrete walls can be identified by the type of buildings in which they are used: 1. Skeletal frame structures: precast concrete walls are used as non-load bearing in-fill walls and may be designed to provide stability to the building 2. Shear wall structures: the precast walls are reinforced, cantilevered walls designed to carry the vertical loads and horizontal, lateral and in-plane forces. They are used as stabilizing elements for the structure and may come in the form of single elements or forming boxes for staircases or lift shafts.

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In a mixed skeletal frame and shear wall building, both types of walls are provided.Precast concrete walls may also be used as non-load bearing partitions to replace brickworks so asto achieve better surface quality and minimize site plastering. The thickness of precast walls varies from 125 mm to 300 mm and is governed either by carnage constraints at the site or factory or by the ultimate shear and load carrying capacity in service. Walls are preferably designed as single elements. For wider walls, it may be necessary to assemble them in separate units with in-situ jointing. Openings for doors, windows and services may be accommodated provided their positions do not interrupt the structural integrity and continuity of the walls. This is particularly important when the wall design is based on the compressive diagonal strut model. Alternative load paths for the vertical and horizontal forces must be considered if the openings are large. The construction of box walls for staircases and lift shafts can be obtained from individual wall sections or from a complete or partial box in single or part storey high. There is a wide range of solutions in jointing the walls. These include: 1. in-situ concrete and steel tie 2. Welded connections made by fully anchored plates 3. Bolting 4. Shear keys with or without interlacing steel and 5. Simple mortar bedding The design forces at the vertical and horizontal wall joints primarily consist of compression, tension and shear forces. Panel Cases Non-load-bearing panel (cladding)-A precast wall panel that transfers negligible load from other elements of the structure; this type of panel is generally designed as a closure panel and must resist all applicable service and factored loads from wind forces, seismic forces, thermally induced forces, forces from time-dependent deformations, self-weight and those forces resulting from handling, storage, transportation and erection. Load-bearing panel-A precast wall panel that is designed to carry loads from one structural element toother structural elements; load-bearing panels must interact with other panels and the supporting structural frame to resist all applicable design loads in addition to thoselisted for nonload-bearing panels. Load-bearing panels also include panels designed to function as shear walls.

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Panel types Solid panel-A panel of constant thickness; anallowance for surface texture must be made in determining effective thickness. Hollow-core panel-A precast panel that has voids within the thickness in one direction for the full length of the panel. Sandwich panel-A precast panel consisting of two layers of concrete separated by a nonstructural insulating core. Ribbed panel-A precast panel consisting of a slab reinforced by a system of ribs in one or two directions.

Design Forces (As per ACI) Precast wall panels should be designed to resist all of the following forces wherever applicable:  Forces developed from differential support settlement, deformations from creep and shrinkage, structural restraint and the effects of environmental temperatures.  Forces due to construction, handling, storage, transportation, erection, impact, gravity dead and live loads, as well as lateral loads from soil, hydrostatic pressure, wind, and seismic action. Local stress concentrations in the vicinity of connections and applied loads must be considered.  Forces developed from thermal movement or bowing as well as volume change of the panel, with respect to the supporting structure, must be considered. 4.3 Stability analysis of wall 1. Effective Thickness: Walls thickness should not be less than 150mm and H/t of less than or equal to 25.

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2. Effective Width: The effective width is depending on concentrated load or bending moment. For concentrated load may not exceed the center-to-centerdistance between supports, nor the width of the loadedportion plus six times the wall panel effective thicknesson each side of the concentrated load. For bending moments may not exceed the effective thickness of the wall panel or the width of the corbelat the point of concentrated bending moment, whicheveris greater, plus three times the effective wall panelthickness each side of the concentrated bending moment. 3. Distance between supports - Spacing of lateral supports for a precast wall panel loaded in flexure only should not exceed 50 times the effective width of the compression flange or face. 4. The spacing between lateral supports of a precast panel carrying axial load and bending moment should not exceed 50 times the effective width of the compression face or flange. 5. Allowable direct compressive stress in concrete should not exceed =

6. Maximum Concrete Bearing Stress

. ∗𝛷∗

[ −(

∗

) ]

= . ∗𝛷∗ 7. Minimum Eccentricity not less than 25mm or e/t not less than 1/10, whichever greater. 8. The maximum slenderness (kl/r) of a precast wall panel should not exceed 65. 9. Stability under axial load – By Euler’s Method The critical load is the maximum load which a column can bear while staying straight. 𝜋 𝑃 = Pc = Euler's critical load (longitudinal compression load on column). E = modulus of elasticity of column material. I = minimum area moment of inertia of the cross section of the column. L = unsupported length of column. K = column effective length factor. 10. Reinforcement: Spacing not less than 3 times of wall thickness nor more than 450mm. Minimum vertical Ast= 0.0015 x gross cross section. Minimum Horizontal Ast=0.0025 x gross cross section. 11. Permissible Deflection criteria (Table from ACI-533R:93)

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Design example: 1. Design precast load bearing wall for data mentioned below. Number of storey-1. Dimension – 18m x 12m x 2.4m Material-M40; Fe-415 Loads- DL=2.5 KN/m2;LL=2.0 KN/m2 4.4 Wall Joints 4.4.1 General The successful performance of a building exterior is frequently defined by its ability to keep rain and the elements outside,away from the building’s occupants. Precast concrete panels are relatively impermeable to water. Moisture will not penetrate through precast concrete panels. The joints between precast concrete panels or between panels and other building materials must be considered to prevent water and air penetration through the building envelope. The design and execution of these joints is therefore of the utmost importance and must be accomplished in a rational, economical manner. Joint treatment also has an effect on the general appearance of the project. To ensure the joint and sealant give the desired performance, selecting the right product, appropriate joint design, and proper surface preparation and application technique is required. The penetration of moisture into a building envelope may enter directly (through an opening), by gravity, capillary action, and as a result of the mean (steady state) air pressure difference across the wall. 109

Joint sealants are fully exposed to the major agents of aging and deterioration—ultraviolet light and thermal cycling. High performance sealants with a low modulus and high movement capability must be used to ensure quality long-term performance. In new construction, labor to material costs are typically 4 to 1, while in renovation/rehabilitation the ratio may be 8 to 1 or more. Joints are required to accommodate changes in wall panel or structure dimensions caused by changes in temperature, moisture content, or deflection from applied design loads. The joints between panels are normally designed to accommodate local wall movements rather than cumulative movements. Sealants subjected to volume change movements, either horizontally or vertically at building corners, at adjacent non-precast concrete construction, or at windows not having similar movements must be given special consideration. Some wall designs handle water properly in two-dimensional blueprints, but fail in three-dimensional reality. Isometric drawings should be used to show the proper intersection of horizontal and vertical seals. These intersections are a prime source of sealant problems. 4.4.2 Types of Wall Joints Joints between precast concrete wall units may be divided into three basic types: one-stage, twostage, and expansion joints. One-Stage joint–As its name implies, the one-stage (face-sealed) joint has a single line of caulking for weatherproofing. This is normally in the form of a gun-applied sealant close to the exterior surface of the precast concrete panel. The principal advantages of face-sealed joints are their simplicity, ease of installation, and almost universal suitability for normal joints between precast concrete panels. No grooves or special shapes are necessary. Thus, one-stage joints are normally the most economical with regard to initial cost. However, the economics may change when maintenance costs are included in the evaluation.One-stage joints provide adequate air leakage and water penetration control in most climates. Their performance depends greatly on the quality of sealant materials, the condition of joint surfaces, quality of field installation, and the overall wall design.Because sealants are subject to deterioration from the elements and ultraviolet (UV) exposure, it is recommended that the sealant be set back into the joints by using recessed joints. This partially protects the sealant from rain, wind, and UV light. Two-Stage joint–Watertightness of sealant joints can be improved by installing a second line of sealant in each joint. The inner seal is placed inside the joint, generally from the exterior, and recessed a minimum of 50 to 63 mm from where the back of the front sealant and backing willbe located or to the back of insulation in a sandwich (insulated) wall panel. This layer provides redundancy in the system, as it is fully protected from weather and UV exposure by the outer layer of sealant, which is installed in the normal manner.This approach requires the installationof 10 mm weep openings in the exterior seal to allow water contained by theinner seal to exit the

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cavity between joint seals. Near the junction of the horizontal and vertical joints, the inner seal must turn out to the plane of the exterior seal at regular intervals to force water out of the joint. 111

This termination requires care in detailing and construction. Failure to provide these weep openings results in trapped water within the joint and ponding against both seals; this accelerates deterioration of the sealant material and its bond to the substrate.These joints are based on the open rainscreen principle. They are sometimes known as ventilated or pressure equalization joints and are favored for exterior wall construction in Canada. The rain screen principle is based on the control of the forces that can move water through small openings in a face-sealed wall system, rather than the elimination of the openings themselves. These joints have two lines of defense for weatherproofing. The typical joint consists of a rain barrier near the exterior face and an air retarder close to the interior face of the panel. The rain barrier is designed to shed most of the water from the joint, and the wind-barrier or air retarder is the demarcation line between outside and inside air pressure. Openings in the rain barrier allow air to rapidly enter until the pressure inside the chamber is equal to the wind pressure acting against the outer wall, which prevents water from entering the chamber. The pressure difference across the exterior layer is essentially zero, and wind pressure is transferred to the inner, airtight layer. Rain does not penetrate to the air chamber and, subsequently, to the interior of the building because there is no wind pressure forcing it through the exterior layer. Any moisture entering the joint will cling to the joint walls and then be drained out by the transverse seal. The airtightness of the air retarder is critical in governing the speed at which pressure equalization occurs. Pressure equalization must take place almost instantaneously for a rain screen wall to be effective. The size of vent opening must reflect the size of the joint to be pressure equalized. Typical details of two-stage vertical joints are shown in below Figs.

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This system is especially applicable to high-rise buildings subject to severe climatic exposure (greater than 5000 degree days). The warm, moist air moving from the building interior to the exterior usually carries moisture, which could cause condensation. Air must be prevented from contacting cold surfaces in the wall. In northern climates, thermal bridges can occur and allow condensation to form a buildup of frost in or on the walls, which may be thought to be a failure of the joint sealant. This frost later can melt and run back inside the building, giving the impression that the building is leaking. 113

Water either from penetration or condensation in the joint should be drained from the joint by proper sealant installations. The second line of sealant should be brought to the front face at regularly spaced intervals along the height of vertical joints, usually near the junction of the horizontal and vertical joints at each floor level. Therefore, if any moisture does come out of the system, it will run down the face of the joint sealant and not over the face of the panels. A spacing of two or three stories may be sufficient for low-rise buildings and in areas of moderate wind velocities. Factors to consider when using two-stage joints are: 1. Higher initial cost due to labor and materials required for their successful application. 2. Sealants are not easily placed at the back of the two-stage joint unless 25 to 35 mm joints are used. Therefore, conscientious workers or intensive supervision throughout the installation procedure is necessary, because inspection of the completed installation is difficult. Panel configurations and joint widths should permit a careful applicator to successfully install both lines of sealant from the exterior. The special tools required may include an extension for the nozzle of the caulking gun and a longer tool for tooling the interior sealant. The architect, precaster, erector, and sealant applicator must all understand the function of the two-stage joints if optimum results are to be achieved. The dimensions of the joints must be maintained at all times. The most common mistake in the installation of two-stage joints is leaving gaps in the air seal. 4.4.3 Expansion Joints Cumulative movements, as well as differential expansion movement of adjacent wall materials, are generally taken by specially designed expansion joints. Because an expansion joint may have to accommodate considerable movement, it should be designed as simply as possible. Although this might result in an appearance somewhat different from a normal joint, the architect is urged to either treat it as an architectural feature or simply leave it as a different, but honest, expansion joint. Seismic seals are a special case of expansion joints. Such joints are generally quite large and are used between new and existing buildings to protect the joint from moisture and allow the structures to move from thermal expansion, wind drift, and seismic motions without damage. Seismic joints are designed to accommodate both vertical and horizontal movement. They are available in sizes from 0 to 305 mm. Wider openings can be accommodated by joining seal sizes together. Materials for expansion joints must be chosen for their ability to absorb appreciable movement while performing their primary function of controlling the movement of moisture and air. Figure shows bellows-type expansion seals of neoprene that accommodate thermal movement and seismic movement. Joints must be designed first for weather protection longevity, then for movement, and finally for appearance.In most cases, this requires that special gasket materials be

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used, rather than sealants. Otherwise, the requirements for expansionjoints are similar to those listed previously for other joints. 4.4.4 Number of Joints The number of joints in the architectural design should be minimized. This will result in a lower overall-cost for the joints, potentially lower maintenance costs, and will increase economy by working with larger panels. Limiting panel sizes to minimize movements in the joints is not recommended. It is generally more economical to select larger panels and design the joints and sealants to allow for anticipated movements. Optimum panel size should be determined by erection conditions, available handling equipment, and local transportation limitations as to panel weight and sizes. If the desired appearance demands additional joints, false joints may be used to achieve a more balanced architectural appearance. In order to match appearance of the two joints, the finish of the false joints should simulate the gaskets or sealants used in the real joints. Caulking false joints adds unnecessary expense. 4.4.5 Location of Joints Joints are simpler to design and execute if they are located at the maximum panel thickness. If there are any ribbed projections at the edges of the panels, joints should be placed at this location. Ribs at the edges improve the structural behavior of the individual unit. Also, panel variations—possible warping or bowing—are less noticeable when the joints are placed between ribs than when the joints are located in flat areas. However, complete peripheral ribs are not recommended because they are likely to cause localized water runoff resulting in unsightly staining. Instead, ribs should be placed at vertical panel edges. If the ribs are too narrow to accommodate joints, the full rib may be located in one panel only. Vertical joints should be located on grid lines. Horizontal joints should be near, but above, floor lines. The designer should allow the precaster to optimize panel sizes for economy with false joints, if necessary. The location of joints between precast concrete panels should be considered as an integral part of the evaluation of economical fastening of the units. Locating and detailing joints (real or false) is an important factor in creating weathering patterns for a building. Joints should be made wide and recessed to limit unexpected weathering effects. Recessed joints screen the joint from rain by providing a dead-air space that reduces air pressure at the face of the sealant. Also, the joint profile channels the rain runoff, helping to keep the building façade clean from unsightly runoff patterns. The designer should determine where the water will finally emerge. Set-backs should be provided at window perimeters and other vulnerable joints in the wall system to reduce the magnitude and frequency of water exposure. Figure shows an elevation where some of the false vertical joints, into which water is channeled, discharge this water over a verticalconcrete surface with fewer joints than at higher levels. This 115

causes a marked washing effect at termination of the joint; the water should be directed until it reaches the ground or a drainage system.Joints in forward-sloping surfaces are difficult to weatherproof, especially if they collect snow or ice. This type of joint should be avoided, whenever possible. When forward sloping joints are used, the architect should take special precautions against water penetration. All joints should be aligned, rather than staggered, throughout their length. Non-aligned joints subject sealants to shear forces in addition to the expected compression or elongation forces. The additional stress may cause sealants to fail. In addition, non-aligned joints force panels to move laterally relative to each other, inducing high tensile forces. 4.4.6 Width and Depth of Joints Joint width must not only accommodate variations in the panel dimensions and the erection tolerances for the panel, but must also provide a good visual line and sufficient width to allow for effective sealing. The performance characteristics of the joint sealant should be taken into account when selecting a joint size. Joints between precast concrete units must be wide enough to accommodate anticipated thermal expansion, as well as other building movements and proper sealant installation. Joint tolerances must be carefully evaluated and controlled if the joint sealant system is to perform within its design capabilities. When joints are too narrow, bond or tensile failure of the joint sealant may occur and/or adjacent units may come in contact and be subjected to unanticipated loading, distortion, cracking, and local crushing (spalling). Joint widths should not be chosen for reason of appearance alone, but must relate to panel size, building tolerances, joint sealant materials, and adjacent surfaces. The required width of the joint is determined by the temperature extremes anticipated at the project location, the movement capability of the sealant to be used, the temperature at which the sealant is initially applied, panel size, fabrication tolerances of the precast concrete units and panel installation methods. The following factors take precedence over appearance requirements: 1. Temperature extremes and gradients. The temperature range used when selecting a sealant must reflect the differential between seasonal extremes of temperature and temperature at the time of sealant application. Concrete temperatures can and normally will vary considerably from ambient air temperatures because of thermal lag. Although affected by ambient air temperatures, anticipated joint movement must be determined from anticipated concrete panel temperature extremes rather than ambient air temperature extremes. 2. Sealant movement capability. A sealant’s performance within joints is rated as the allowable movement expressed as a percentage of the effective joint width. The minimum design width of a panel joint must take into account the total anticipated expansion and contraction movement of the joint and the movement capability of the sealant. This evaluation should include volume changes from creep,shrinkage, and temperature variations. 116

PCI Design Handbook supplies figures for estimating volume changes directly related to the size of the panel. Most drying shrinkage occurs in the first weeks following casting, and creep normally levels out after a period of months. For these reasons, movements caused by ambient air temperature variations are more important than those caused by shrinkage. For loadbearing panels, the effect of creep may be cumulative, thus may be more important. Many factors may be involved in actual building joint movement. These include, but are not limited to, mass of material, color, insulation, building load, building settlement, method of fastening and location of fasteners, differential heating due to variable shading, thermal conductivity, differential thermal stress (bowing), building sway, and seismic effects. Material and construction tolerances that produce smaller joints than anticipated are of particular concern. Tolerances in overall building width or length are normally accommodated in panel joints, making the overall building size tolerance an important joint consideration. Where a joint must match an architectural feature (such as a false joint), a large variation from the theoretical joint width may not be acceptable and tolerances for building lengths may need to be accommodated at the corner units. One-stage joint is a simple butt joint with sealant applied against a backer rod at the external face of the wall. One-stage joint offers only a single line of defense against water seepage. Pressure drop may occur across the one-stage joint and water may seep through micro cracks or hairline cracks. Two- stage joint, on the other hand, provides two defense lines against water ingress. Experience has shown that two stage joints give better watertightness performance than onestage joints. The required sealant depth is dependent on the sealant width at the time of application. The optimum sealant width/depth relationshipsare best determined by the sealant manufacturer, however, generally accepted guidelines are: 1. For joints designed for 19 to 25 mm width: The sealant depth should be equal to one half the width. The sealant should have a concave shape providing greater thickness at the panel faces. The sealant should have a minimum 6 mm contact with all bonding surfaces to ensure adequate surface adhesion. 2. For joints greater than 1 in. (25 mm) wide: Sealant depth should be limited to 1/2 in. (13 mm) maximum, preferably 3/8 in. (10 mm). For sealant widths exceeding 2 in. (50 mm), the depth should be determined by consultation with the sealant manufacturer. The depth of the sealant should be controlled by using a suitable sealant backing material. To obtain the full benefit of a well-designed shape factor, the backing material must also function as a bond breaker. When it comes to sealant depth, more is not better. If too much sealant is applied, the stresses on the sealant bead are magnified and the chance of premature deboning at the precast concrete interface is increased. If the bead is too shallow, there may be insufficient material to accommodate the joint movement and the sealant will split.

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4.4.7 Sealant Materials and Installation The most common joint materials are sealants meeting ASTM C920. These sealants are used in both one-stage and two-stage joints. If used as an air seal, they may be applied from the front provided joint width and depth permit, or from the interior if access to the joint is not blocked by edge beams or columns. Designers should consult with the various sealant suppliers to ensure they are specifying an appropriate sealant for the specific needs of the project, as well as the sealant’s proper installation. For a comprehensive discussion of joint sealants used between wall panels, refer to ASTM C1193, Standard Guide for Use of Building Sealants. Table 4.1 provides a list of common sealants and their qualities. Non-staining joint sealants should be selected to prevent the possibility of bleeding and heavy dirt accumulation, which are common problems with sealants having high plasticizer contents. Also, care should be taken to avoid sealants that collect dirt as a result of very slow cure or long tack-free time. Dirt accumulation is more a function of specific product formulation rather than generic sealant type.When specifying a sealant, a current sample warranty should be obtained from the manufacturer and the contents studied to avoid uncalculated risks. The warranty period for a polyurethane material can be up to 10 years, and up to 20 years for a silicone. This doesn’t imply that the sealant will deteriorate during that time. Some polyurethane-based products maintain their appearance and integrity for more than 15 years. Warranties can be written to cover either the material or the material and the labor needed to replace them. The specifier should be familiar with the available sealants and associated warranties prior to selecting a sealant for the building. The following characteristics should be considered when making the final selection of sealants from those with suitable physical (durability) and mechanical (movement capability) properties: 1. Adhesion to different surfaces—concrete, glass, or aluminum. 2. Surface preparation necessary to ensure satisfactory performance—priming, cleaning, and drying. 3. Serviceable temperature range. 4. Drying characteristics—dirt accumulation, susceptibility to damage due to movement of joint while sealant is curing. 5. Puncture, tear, and abrasion resistance. 6. Color and color retention. 7. Effect of weathering—water and ultraviolet (UV) light—on properties such as adhesion, cohesion, elasticity. 8. Staining of adjacent surfaces caused by sealant or primer. 9. Ease of application. 10. Environment in which the sealant is applied. 11. Compatibility with other sealants to be used on the job. 12. Long term durability. 118

13. Life expectancy.

The sealants used for specific purposes are often installed by different subcontractors. For example, the window subcontractor normally installs sealants around windows, whereas a 119

different subcontractor typically installs sealants between panels. The designer must select and coordinate all of the sealants used on a project for chemical compatibility and adhesion to each other. In general, contact between different sealant types should be avoided by having one sealant contractor do both panel and window sealant application with compatible materials. The recommendations of the sealant manufacturer should always be followed regarding mixing, surface preparation, priming, application life, and application procedure. Good workmanship by qualified sealant applicators is the most important factor required for satisfactory performance. Sealant installation should be specified to meet at least the requirements of ASTM C1193. Prior to sealant application, the edges of the precast concrete units and the adjacent materials must be sound, smooth, clean, and dry. They must also be free of frost, dust, laitance, or other contaminants that may affect adhesion, such as form release agents, retarders, or sealers. It may be more economical and effective to prepare joint surfaces prior to erection if a large number of units require surface preparation. It may also be desirable to conduct pre-project adhesion tests in accordance with ASTM C794, “Test Method for Adhesion-in-Peel of Elastomeric Joint Sealants,” and field adhesion tests using ASTM C1521, “Standard Practice for Evaluating Adhesion of Installed Weatherproofing Sealant Joints,” to determine the adhesion of the sealant with each contact surface. Adhesion (ASTM C794 or C1521) and stain testing (ASTM C510 or C1248) of the substrates and sealants in the early project planning stage of a building are recommended by most sealant manufacturers. This early testing will prevent most problems before they start and will give the construction team the assurance of a problem-free job. Even when performed on a limited basis, inspecting sealants during installation significantly improves the probability they will be installed in accordance with the contract documents. Performing this evaluation early in the project provides a method for obtaining feedback on installation workmanship. This way, modifications or corrections can be implemented before any problem becomes widespread. ASTM C1521 provides guidance for two tests. The first is non-destructive, and consists of applying pressure to the surface of the sealant at the center of the joint and the bond line with a probing tool. The second procedure involves removing sealant to evaluate adhesion and cohesion. The latter test offers tail and/or flap procedures, depending on whether similar or different substrates are present on adjacent surfaces of the sealant joint. The sealant pulled from the test area should be repaired by applying new sealant to the test area. Assuming good adhesion was obtained, use the same application procedure to repair the areas as was used to originally seal them. Care should be taken to ensure that the new sealant is in contact with the original sealant so that a good bond between the new and old sealants will be obtained. ASTM C1521 can be used to evaluate installed sealant during mockups, at the start of work to confirm application methods, and throughoutthe work to confirm installation consistency. ASTM C1521 provides guidelines for the frequency of destructive testing when evaluation is part of a

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quality control program for a new installation. All results should be recorded, logged, and sent to the sealant applicator and manufacturer for warranty issuance. In the construction of a mockup for water penetration testing, the actual field construction techniques must be used. If a leak develops, which usually occurs at the window to precast concrete interface, the details need to be examined and modified. Putting more sealant on to make the system pass the test is not realistic, as this will generally not occur during construction. Sealants that chemically cure should not be applied to wet or icy surfaces, as they may cure or set before they can bond to the concrete surface. Some methyl methacrylate resin sealers inadvertently sprayed in the joints may peel away from the concrete surface, leaving a void between sealant and concrete. Silicone water repellents in the joints may prevent adhesion of sealants to the concrete surface. Therefore sealant/sealer compatibility should be verified. Abrasion cleaning using a stiff wire brush, light grinding, or sandblasting followed by air blowing may be necessary to remove surface contaminants. The sealant should be cured 14 days before applying water repellents. Care should be taken to caulk first, as sealer may prevent proper adhesion of sealant. Also, before caulking, the joint may require solvent cleaning with a lint-free cloth dampened with an acceptable cleaning-grade solvent followed by wiping with a dry cloth. Isopropyl alcohol (IPA) is soluble in water and may be appropriate for winter cleaning, as it helps in removing condensation and frost by picking up surface moisture as it evaporates. Xylene and toluene are not soluble in water and may be better suited for warm weather cleaning. Follow the solvent manufacturer’s safe handling recommendations and local, state, and federalregulations regarding solvent usage. Sometimes, smooth concretes that are very shiny exhibit a “skin” on the surface. The skin may peel off, leaving a gap between it and the concrete after the joint sealant has been applied to the concrete. It may be necessary to remove the skin by using a stiff wire brush followed by a highpressure water rinse. The joint must be dry before applying the sealant. Wet concrete should be allowed to dry for at least 24 hours, under good drying conditions, before applying sealant or primer. The caulking gun should have a nozzle of proper size and should provide sufficient pressure to completely fill the joints. An extension for the nozzle of the caulking gun and a longer tool for tooling the inner seal of a two-stage joint are necessary. Joint filling should be done carefully and completely, by thoroughly working the sealant into the joint. Under-filling of joints normally leads to adhesion loss. After joints have been completely filled, they should be neatly tooled to eliminate air pockets or voids, and to provide a smooth, neat-appearing finish.Tooling also provides a slightly concave joint surface that improves the sealant configuration and achieves a visually satisfactory finish. Jointtooling should be performed within the allowable time limit for the particular sealant. The surface of the sealant should be a full, smoothbead, free of ridges, wrinkles, sags, air pockets, and embedded impurities. 121

Large daily temperature swings during curing (warm days, cold nights) may cause adhesive failure. A practical range of installation temperatures, considering moisture condensation or frost formation on joint edges at low temperatures and reduced working life at high temperatures, is from 40 to 80 °F (5 to 27 °C). This temperature range should be assumed in determining the anticipated amount of joint movement in the design of joints. A warning note should be included on the plans that, if sealing must take place for any reason at temperatures above or below the specified range, a wider-than-specified joint may have to be formed. Alternately, changes in the type of sealant to one of greater movement capability or modifications to the depth-to-width ratio may be required to secure greater extensibility. The applicator should know the joint size limitation of the sealant selected. When it is necessary to apply sealant below 40 °F (5 °C), steps must be taken to ensure clean, dry, frost-free surfaces. The area to be sealed should be wiped with a quick-drying solvent that is slightly water soluble, such as IPA, just before sealing. The area may be heated, if possible, or at least the sealant should be slightly warm (60 to 80 °F [15 to 27 °C]) when applied. It is recommended that tools be used dry. Tooling solutions such as water, soaps, oil, or alcohols should not be used unless specifically approved by the sealant manufacturer as they may interfere with sealant cure and adhesion and create aesthetic issues. It is imperative that uncured silicone or polyurethane sealants are not allowed to contact non abradable surfaces such as polished stone, metal, or glass. These surfaces must be masked or extreme care taken to prevent any contact with the sealant during the application process. Excess sealant cannot be completely removed with organic or chlorinated solvents. Once an uncured sealant comes in contact with an exposed surface it will leave a film that may change the aesthetic or hydrophobic surface characteristics of the substrate. Surfaces soiled with sealant materials should be cleaned as work progresses; removal is likely to be difficult after the sealant has cured. A solvent or cleaning agent recommended by the sealant manufacturer should be used. Sealant Backing. For sealants to perform to their optimum movement parameters, they must adhere only to the joint sides and never to the base. Closed-cell expanded polyethylene, or nongassing polyolefin sealant backing are the recommended backing materials for horizontal and vertical joints. For two-stage joints, open-cell polyurethane backing should be used on the interior seal unless the interior seal is allowed to cure for seven days before installing the exterior seal. Proper selection and use of backing material is essential for the satisfactory performance of watertight joints. When selecting a backing material and/or bond breaker, the recommendations of the sealant manufacturer should be followed to ensure compatibility with the sealant. The principal functions of sealant backing materials are: 1. Controlling the depth and shape of the sealant in the joint (proper width to depth ratio). Also, profiles the rear surface to an efficient cross-section for resisting tensile forces. 122

2. Serving as a bond breaker to prevent the sealant from adhering to the back of the joint. The sealant must adhere only to the two surfaces to which it bridges. If it also adheres to the back of the joint (three-sided adhesion), the stresses on the sealant bead are greatly increased and this increases the likelihood of premature sealant failure. 3. Assisting in tooling of the joint by providing back pressure when tooling. The combination of tooling and back pressure ensures full-sealant contact with the sides of the joint, which is vital if proper adhesion is to take place. 4. Protecting the back side of the sealant from attack by moisture vapors trying to escape from the building. Use of two-stage joints and backing is recommended where high vapor pressure occurs at the immediate back surface of the sealant. The backing should not stain the sealant, as this may bleed through and cause discoloration of the joint. Sealant backing materials should be of suitable size and shape so that, after installation, they are compressed 25 to 50%. Compression differs with open- and closed-cell rods; refer to manufacturer’s recommendations. Adequate compression is necessary so that the shape will stay in the opening and not be dislodged or moved by sealant installation. Primers. Some sealants require primers on all substrates; others require primer for specific substrates or none at all. Absence of a required primer will cause premature sealant adhesion failure. A primer often helps sealant adhesion in cold weather. Primers are recommended by the sealant manufacturer for the following reasons: 1. To enhance adhesion of sealants to porous surfaces, such as concrete, or to reinforce the surface. 2. To promote adhesion of sealants to surfaces such as porcelain enamel, unusual types of glass, certain metals and finishes, and wood. 3. To promote adhesion of sealants to an existing surface treatment which is difficult to remove. Special care should be exercised to avoid staining the visible face of the precast concrete unit because some primers leave an amber-colored stain if brushed along the surface. This stain will have to be mechanically removed, which will be expensive. The primer should be allowed to cure before application of the sealant. Sealant must be applied the same day the surfaces are primed. The sealant and primer should always be supplied by the same manufacturer.

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Module-5 5.1 Components of Industrial Building (Single-Storey)

5.2 Precast Reinforced Concrete Truss 5.2.1 General  Reinforced concrete truss having all the members in reinforced concrete only.  Solid Purlin - Purlin having its tensile and compression zones connected by concrete along its length such as purlin with T, L, trapezoidal, or rectangular cross-section.  Thickness of Member - The dimension in the plane ‘of the truss perpendicular to the axis of the truss.  Trussed Purlin - Purlin having members in triangulated or virendeel shapes.  Width- of the Truss - The dimension in the plane perpendicular to the plane of the truss.  Concrete for truss members shall be controlled concrete conforming to IS: 456-2000 and of grade not weaker than M 20 in case of reinforced concrete trusses.  For reinforced concrete members, the steel reinforcement shall be the following: a) Mild steel and medium tensile steel bars conforming to IS: 432 ( Part 1 )-1982. b) High strength deformed bars conforming to IS: 1786-1985.

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5.2.2 Analysis of Truss  In case of triangulated trusses where statical indeterminacy is due only to rigidity of ‘n’ joints, the truss may be treated as pin-jointed for the purpose of finding forces in various members for preliminary design. All loads may be assumed as acting at the nodes. In large span trusses (span exceeding 30 m), the secondary moments due to relative deflection of the joints in the end members near the support should be considered even in preliminary design.  The secondary moment and the secondary forces in various members may be arrived at by using the preliminary sections.  The secondary moments may be computed by finding the relative displacement of each member with the help of a Williot Mohr diagram or by any other method, including computer.  The fixed end moment produced at each end of the member due to deflection is given as:

The next moment for preliminary design may be taken as 0.7 times the fixed end moment, computed as above for preliminary design. Flatter diagonals are preferable to steep diagonals to steep diagonals to reduce the net moments in the truss members.  The moment due to self-weight of each member or any other superimposed load shall also be taken into account while calculating the secondary moments due to relative deflection and the rigidity of the joints.  The equilibrium of joints is obtained by further distribution of unbalanced moment at each joint by any of the method, such as the moment distribution method, or the Kani’s interation method of frame analysis.  The analysis may also be made by treating the whole truss as rigid jointed without making any simplifying assumptions.  However, for the usual triangulated trusses up to a span of 30 m, the secondary stresses due to loads and those due to prestress, resulting from rigidity of joints, compensate each other at most of the joints and so it is accurate enough to find the forces .in the members assuming the truss to be pin-jointed. 5.2.3 Design of Truss Loading - For the purpose of design of precast reinforced concrete trusses, the following loads shall be considered: a) Dead load, b) Live load, c) Wind load, d) Seismic load, and e) Handling and erection loads.

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The effect of shrinkage and temperature variation shall also be taken into account in the design of the truss seatings. Dead Load –  For trusses supporting asbestos cement/galvanized iron/aluminum or such light roofing material transmitted through precast concrete purlins, the following intensity of loading from self-weight of truss for preliminary design may generally be assumed in absence of more specific data:

 For trusses supporting ribbed concrete roof panels, the average dead load per square meter of the plan area will be approximately 1000 N/m2 more than the above values. .  For the dead weight of the roofing sheets and ceiling, if any, actual ascertained weights shall be used; but if these are not available, unit weights given in IS : 875 ( Part 1 )-1987 may be used.  For dead load initially assumed shall be checked after the design is completed and the design shall be revised if the actual calculated dead load exceeds the assumed dead load. Live Load - Live load on the pitched roofing with precast reinforced concrete trusses shall be in accordance with the relevant provisions of IS : 875 (Part 2)- 1987. The other superimposed loads from the services on bottom chord or other members of the truss should be considered. Wind Load - Wind load shall be considered in accordance with the relevant provisions of IS: 875 (Part 3)-19881. Seismic Load - The seismic load shall be considered in accordance with the relevant provisions of IS: 1893-1984. Handling and Erection Load - Trusses are normally cast in simple moulds on the floor in flat position and have to be tilted to vertical position at the time of demoulding. The demoulding is normally done in 2 to 3 days after casting. The concrete stress induced during this stage should be limited to one half the concrete strength attained in the period before tilting the truss.  Trusses are normally erected only after 75 percent of the 26-day strength is obtained. The handling of the trusses should be done carefully with slings around node points and should be accomplished in such a way that no adverse stresses are induced during this phase. The stresses induced should be limited to 50 percent of the strength of concrete during the handling phase. 126

Crack width analysis should be made in accordance with the accepted methods in case of reinforced concrete member’s subjected to predominantly direct tension or direct tension and bending. 5.2.4 Design Aspects as per IS: 3201-1988  Span - The span of precast reinforced trusses shall preferably be in increments of 3 m ranging between 9 to 30 m and 6 m for spans in the range of 30 to 60 m.  Rise of the Truss - The central rise of the truss may preferably be not less than one-sixth of the span * for straight chord trusses and between one-eighth and one-tenth of the span for curved chord trusses.  Spacing of Trusses - The spacing of trusses shall be decided on economical grounds. The spacing of trusses should usually be 6 m. Other preferred dimensions are 4.8, 7.5, 9.0 and 12.0 m.  Spacing of Purlins - The spacing of purlins shall be governed by the standard widths of roofing sheets or pretensioned planks or other roofing materials available which shall conform to the requirements of relevant Indian Standards.  Shape of the Truss - Shapes commonly used in steel trusses are possible for concrete trusses as well as portal frames in trussed form. For trusses carrying concrete roofing panels, bow trusses with curved chords are economical and induce constant tension in the bottom chord and reduced secondary moments in the top chord due to load transfer being uniformly distributed along the top chord.  Design of Members Thickness  The thickness of all the members, that is, truss thicknesses shall preferably be uniform except at the bearing surface of the truss where the section may be widened to provide adequate seating.  To reduce the effect of secondary stresses, the thickness of the members shall preferably be less than the width of the members. Reinforced Concrete Compression Members  The reinforced concrete compression members of the trusses shall be designed in accordance with the requirements for compression members specified in IS: 4562000. For the purpose of taking the effective length of the members, the ends may be assumed as pin-jointed.  The stresses is members due to combination of direct load and bending moment shall not exceed the permissible stresses for bending multiplied by the appropriate reduction coefficient treating the member as a long column in the appropriate direction taking due effect of ties provided by purlins or any other lateral stiffeners. 127

Reinforced Concrete Tension Members Members subjected to tension shall be designed conforming to the following requirements.  There should be sufficient reinforcement to resist all the tension at the permissible tensile stress of steel,  The calculated tensile stress on the effective section shall not be greater than the value specified in 4.4.1.1 of IS : 456-2000, and  Local bending should be checked in case of suspension oi transfer of loads away from node points of loads eccentric to the axis of the truss. Minimum Reinforcement –Minimum reinforcement of four 6 mm diameter corner bars for thick members and two 6 mm diameter bars for members less than 75 mm thick shall be provided irrespective of the nature and magnitude of the forces in the member. Design of Purlins  Purlins are usually subjected to bending in two planes resulting from the wind load acting normal to the sheeting and the dead acting vertically downwards.  Where solid reinforced concrete purlins are proposed the section shall be adequate to resist moments in both the planes and the resulting stresses shall be .within permissible limits as specified in IS : 456-2000.  Alternatively where trussed purlins are proposed, they shall primarily be designed to carry the component of the load in its own plane and the load component normal to the plane of the truss shall be resisted entirely by the top member of the trussed purlins. Transverse Reinforcement  The diameter of the transverse reinforcement shall be not less than 4 mm.  Transverse reinforcement either in the form of helical or stirrups shall be provided in all members irrespective of whether they are in tension or compression or carrying no load.  Reinforcement shall have a concrete cover and the thickness of such cover (exclusive of plaster or other decorative finish) shall be in accordance with the provisions of IS: 4562000.

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5.3 Design of RC Columns 5.3.1 Introduction  Compression members are structural elements primarily subjected to axial compressive forces and hence, their design is guided by considerations of strength and buckling. Examples: Pedestal, column, wall and strut.  These compression members may be made of Bricks or Reinforced concrete.  Pedestal: Whose effective length does not exceed 3 times of b (least lateral dimension). The other horizontal dimension D shall not exceed four times of b.  Column: Whose unsupported length L shall not exceed 16 times of b (least lateral dimension), if restrained at the two ends.  Wall: Whose effective height /thickness (least lateral dimension) shall not exceed 30 (cl. 32.2.3 of IS 456). The larger horizontal dimension i.e., the length of the wall L is more than 4t. 5.3.2 Classification of Columns Based on Type of Reinforcement

(Courtesy: Design of RCC Structures by Pillai &Deadas menon)

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Based on Type of Loading

  

 

(Courtesy: Design of RCC Structures by Pillai &Deadas menon) The occurrence of axial loading (concentric) is relatively rare. Columns in RC framed buildings usually subjected to biaxial loadings. Due to rigid frame action, lateral loadings and practical aspects of construction, there will be bending moments (Mx and My) and horizontal shear in all the internal columns. Similarly, side columns and corner columns will have the column shear along with the axial force and bending moments in one or both directions, respectively. This results in biaxial eccentricities ex= Mx /P and ey = My /P. However, the code ensures that the design of such columns is sufficiently conservative to enable them to be capable of resisting nominal eccentricities in loading. The effects of shear are usually neglected as the magnitude is very small. Moreover, the presence of longitudinal and transverse reinforcement is sufficient to resist the effect of column shear of comparatively low magnitude. The effect of some minimum bending moment, however, should be taken into account in the design even if the column is axially loaded. Accordingly, Cls.39.2 and 25.4 of IS 456 prescribes the minimum eccentricity for the design of all columns. In case the actual eccentricity is more than the minimum that should be considered in the design.

Based on Slenderness Ratios 1. Short columns; 2. Slender (or long) columns.  ‘Slenderness’ is a geometrical property of a compression member which is related to the ratio of its ‘effective length’ to its lateral dimension. This ratio, called slenderness ratio.  Columns with low slenderness ratios, i.e., relatively short and stocky columns, invariably fail under ultimate loads with the material (concrete, steel) reaching its ultimate strength, and not by buckling.

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 On the other hand, columns with very high slenderness ratios are in danger of buckling (accompanied with large lateral deflection) under relatively low compressive loads, and thereby failing suddenly. Design codes attempt to preclude such failure by specifying ‘slenderness limits’ to columns.  According to the IS Code (Cl. 25.1.2), a compression member may be classified as a ‘short column’ if its slenderness ratios with respect to the ‘major principal axis’ (lx/Dy) as well as the ‘minor principal axis’ (ly/Dy) are both less than 12; otherwise, it should be treated as ‘slender column’.  However, above definition is not suitable for non-rectangular and non-circular sections, where the slenderness ratio is better expressed in terms of the radius of gyration ‘r’ (as in steel columns), rather than the lateral dimension D. 5.3.3 Effective Length of Column  Distance between the points of inflection in the buckled configuration of the column in that plane. The effective length depends on the unsupported length l (i.e., distance between lateral connections, or actual length in case of a cantilever) and the boundary conditions at the column ends introduced by connecting beams and other framing members. An expression for Leff may be obtained as Leff = K * L Where ‘K’ is effective length ratio (i.e., the ratio of effective length to the unsupported length — also known as effective length factor) whose value depends on the degrees of rotational and translation restraints at the column ends. ‘L’ Unsupported length (Clear distance between the floor and the shallower beam framing into the columns in each direction at the next higher floor level). 5.3.4 Code Recommendations for Idealized Boundary Conditions 1. Columns braced against sideway: When relative transverse displacement between the upper and lower ends of a column is prevented, the frame is said to be braced (against sideway). a) both ends ‘fixed’ rotationally: 0.65 l. b) one end ‘fixed’ and the other ‘pinned’: 0.80 l. c) both ends ‘free’ rotationally (‘pinned’): 1.00 l.

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2. Columns Unbraced against sideway: When relative transverse displacement between the upper and lower ends of a column is not prevented, the frame is said to be unbraced (against sideway). a) both ends ‘fixed’ rotationally: 1.20 b) one end ‘fixed’ and the other ‘partially fixed’: 1.50 c) one end ‘fixed’ and the other free: 2.00

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5.3.5 Code Requirements Slenderness Limits for Columns:  The unsupported length between end restraints shall not exceed 60 times the least lateral dimension of a column.  If, in any given plane, one end of a column is unrestrained, its unsupported length, 1, shall not exceed-100 b2 / D. Minimum Eccentricity

Reinforcement and Detailing Longitudinal Reinforcement  Minimum area of cross-section of longitudinal bars must be atleast 0.8% of gross section area of the column.  Maximum area of cross-section of longitudinal bars must not exceed 6% of the gross crosssection area of the column.  The bars should not be less than 12mm in diameter.  Minimum number of longitudinal bars must be four in rectangular column and 6 in circular column.  Spacing of longitudinal bars measures along the periphery of a column should not exceed 300mm.

  

Transverse reinforcement (Ties) It may be in the form of lateral ties or spirals. The diameter of the lateral ties should not be less than 1/4th of the diameter of the largest longitudinal bar and in no case less than 6mm. The pitch of lateral ties should not exceedLeast lateral dimension16 x diameter of longitudinal bars (small)300mm

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Helical Reinforcement The diameter of helical bars should not be less than 1/4th the diameter of largest longitudinal and not less than 6mm. The pitch should not exceed (if helical reinforcement is allowed);  75mm  1/6th of the core diameter of the column Pitch should not be less than,  25mm  3 x diameter of helical bar Pitch should not exceed (if helical reinforcement is not allowed) Least lateral dimension  16 x diameter of longitudinal bar (smaller).  300mm. 5.3.6 Columns Subjected to Axial Loads: All compression members are to be designed for minimum eccentricity of load in two principal directions. Clause 24.4 of the Code specifics the following minimum eccentricity,

However, as a simplification, when the value of the minimum eccentricity calculated as above is less than or equal to 0.05D, 38.3 of the Code permits the design of short axially loaded compression members by the following equation:

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Example.1 Design the reinforcement in a column of size 450 mm × 600 mm, subject to an axial load of 2000 kN under service dead and live loads. The column has an unsupported length of 3.0m and is braced against sideway in both directions. Use M 20 concrete and Fe 415 steel. SOLUTION: Step: 1. Check for Short Column or Slender Column Given: lx = ly = 3000 mm, Dy = 450 mm, Dx = 600 mm

As the column is braced against sideway in both directions, effective length ratios kx and ky are both less than unity, and hence the two slenderness ratios are both less than 12. Hence, the column may be designed as a short column. Step: 2. Check for Minimum Eccentricities. ex,min=+3000/500+600/30 = 26.0 mm (> 20.0 mm) ey,min=+3000/500+450/30 = 21.0 mm (> 20.0 mm) As 0.05Dx = 0.05 × 600 = 30.0 mm >ex,min = 26.0 mm and 0.05Dy = 0.05 × 450 = 22.5 mm >ey,min = 21.0 mm, Code formula for axially loaded short columns can be used. Step: 3.Factored Load Pu = service load × partial load factor = 2000 × 1.5 = 3000 kN. Design of Longitudinal Reinforcement 3000 × 103 = 0.4 × 20 × (450 × 600) + (0.67 × 415–0.4 × 20) Asc Asc = (3000–2160) × 103/270.05 = 3111 mm2 Provide 4–25 φ at corners: 4 × 491 = 1964 mm2 And 4–20 φ additional: 4 × 314 = 1256 mm2 Asc = 3220 mm2> 3111 mm2 p = (100×3220) / (450×600) = 1.192 > 0.8 (minimum reinf.) — OK.

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Example 2.Design the reinforcement in a spiral column of 400 mm diameter subjected to a factored load of 1500 kN. The column has an unsupported length of 3.4 m and is braced against sideway. Use M 25 concrete and Fe 415 steel. Step: 1.Short Column or Slender Column Given: l = 3400 mm, D = 400 mm Slenderness ratio = le/D ≤ 3400/400 = 8.5 (as column is braced). As le/D < 12, the column may be designed as a short column. Step: 2 Minimum eccentricities • emin=(3400/500)+(400/30)=20.1 mm (> 20.0 mm) As 0.05D = 20.0 mm ≈ emin, the Code formula for axially compressed short columns may be used. Step: 3.Factored Load • Pu = 1500 kN (given) = 1.05 [0.4fck Ag + (0.67 fy – 0.4 fck) Asc] for spiral columns (appropriately reinforced)

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Provide 6 φ spiral @ 28 mm c/c pitch.

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5.3.7 Compression Members Subject To Biaxial Bending: Exact design of members subject to axial load and biaxial bending is extremely laborious. Therefore, the Code permits the design of such members by the following equation:

Where Mux, Muy, are the moments about x and y axes respectively due to design loads, Mux1, Muy1 are the maximum uniaxial moment capacities with an axial load Pu, bending about x and y axes respectively, and αn is an exponent whose value depends on Pu/Puz (see table below) where Puz = 0.45 fck A, + 0*75fy As

For intermediate values, linear interpolation may be done. Chart 63 can be used for evaluating Puz. For different values of Pu/Puz, the appropriate value of αn has been taken and curves for the equation

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Typical Design of Critical Column (Bi-Axial): Size of column=200 x 600 mm. Grade of concrete=25N/mm2 Grade of steel=415N/mm2 𝑃 = 682 KN = . − = − − Unsupported length = 3.0m Effective cover = 40 mm Column are held in position & restrained against rotation. = 0.65 L = 0.65 x 3 = 1.95 m 𝐷

= 1.95/ 0.6 = 3.25 < 12 = 1.95/0.20 = 9.75 < 12

Hence column is design for short column. Check for eccentricity: = L/500 + D/30 = 1950/500+600/30=23.9mm >20mm = L/500 + D/30 =1950/500+200/30= 10.56 mm < 20 mm 𝑖 = 20 mm = 682 x0 .0239 = 16.29 KN-m = 682 x 0.02 = 13.64 KN-m Assume Pt = 3% 𝑃 / 𝑘 = 3/25 = 0.12

Uniaxial moment capacity of the sec about X-X axis Assume 25mm; ′ = = .

From Chart 44 SP- 16

𝑃 𝑘∗ ∗

=

.

= .

/

Mux1 = 0.18 x 25 x 200 x 547.52 = 269.78 KN-m. Uniaxial moment capacity of the sec about Y-Y axis ′ = =

.

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𝑃 𝑘∗ ∗

From chart 45 SP -16

=

.

= .

/

Muy1 = 0.19*25*200*547.52 = 284.76 KN-m. 𝐴 = 𝑅 𝑖 From chart 63 𝑃 = N/mm 𝐴 𝑃

𝑃 𝑃

= =

∗

/

/

For value of

∗

= .

𝑟

∗ %=

∗

=

∗𝐴 = 𝐴

∗ ∗

. = %

=

N/mm

= 16.29/269.78=0.06N/mm2 = 13.64/284.76=0.047 N/mm2 = .

From chart 64 SP- 16 Muy /Muy1 = 0.98 > 0.047 N/mm2 safe. Main reinforcement: Use 25 mm  bars = 3600/491 = 7.33 = 8 nos. Dia of Lateral ties: 1 Dia = x25  6.25  8mm 2 4 Adopt 8 mm. Spacing of lateral ties: 141

Least lateral dimension = 375mmc/c 16 x 25= 400 mm c/c  Provide 8 # lateral tie @ 200 mm c/c

5.4 Design Accepts of Corbels 5.4.1 Introduction Corbel or bracket is a reinforced concrete member is a short-haunched cantilever used to support the reinforced concrete beam element. Corbel is structural element to support the pre-cast structuralsystem such as pre-cast beam and pre-stressed beam. The corbel is cast monolithic with the column element or wall element.

5.4.2 General Consideration as per IS-456:2000 A corbel is a short cantilever projection which supports a load bearing member and where: a) Shear span/depth ratio (av/d) is less than 1.0. b) The Depth of Df is should not less than one-half of the depth of Ds.

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5.3.3 Initial Dimensioning of Corbels (as per BS 8110) 1. The ultimate bearing pressure on concrete should not exceeded the allowable pressure.

5.4.4 Modes of Failures

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5.4.5 Analysis of Forces in Corbel 1. Struct and Tie Method

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Alternatively, the value of z/d can be obtained from chart for given values of a/d and once z is known, we can calculate the values of x, ft and εs.

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5.4.6 Step by Step Procedure

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147

Design Example: (Ref: Limit state method by P.C.Varghese)

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149

150

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2. Shear Friction Method General Since the corbel is cast at different time with the column element then the cracks occurs in the interface of the corbel and the column. To avoid the cracks we must provide the shear friction reinforcement perpendicular with the cracks direction. ACI code uses the shear friction theory to design the interface area. Shear Friction Theory In shear friction theory we use coefficient of friction μ to transform the horizontal resisting force into vertical resisting force. The basic design equation for shear reinforcement design is : φVn ≥ Vu Where: Vn = nominal shear strength of shear friction reinforcement Vu = ultimate shear force φ = strength reduction factor (φ = 0.85)

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Step by Step Procedure: 1. Step 1 : Check a/d < 1 (else, it is probably a shallow or deep beam) 2. Step 2: Find factored shear Vu and tensile force Nuc If Nuc is not specified, use a minimum value of Nuc = 0.2 Vu (ACI 11.9.3.4) Compute nominal values of shear and tensile force Vn = Vu / 0.85 ; Nnc = Nuc / 0.85 If Vn > 0.2 fc’ b d OR Vn > 800 b d (lb. units) then section size is inadequate (ACI 11.9.3.2) 3. Step 3: Compute shear-friction reinforcement (ACI 11.7.4.1) V Avf  n  fy Where  = 1.4 for concrete placed monolithically,  = 1.0 for concrete placed against hardened concrete (see ACI 11.7.4.3) 4. Step 4: Calculate required flexural reinforcement (11.9.3.3) V a  N nc (h  d ) Af  n f y ( jd )

(assume jd = 0.85)

5. Step 5: Reinforcement to carry tensile force (ACI 11.9.3.4) An 

N nc fy

  2 Avf  An  3 larger of  A f  An  f 0.04 c bd f y 

6. Step 6: Required main flexural steel (As) is given by (ACI 11.9.3.5 and 11.9.5) 7. Step 7: Provide closed horizontal stirrups (ACI 11.9.4): Ah = 0.5 (As – An) 8. Step 8: Ensure adequate detailing (ACI 11.9.6 & 11.9.7)

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References 1. Kim.S.Elliott and Tovey, Precast Concrete Frames Building, Design Guide, British Cement Association Publication,47.024. 2. Code of Practice for Precast Concrete Construction 2003, Hong Kong. 3. Precast Concrete Construction Handbook-An explanatory handbook to Code of Practice for Precast Concrete Construction 2003. 4. Hubert Bachmann and Alfred Steinle, Precast Concrete Structures, ISBN 978-3433-02960-2, Wilhelm Ernst & Sohn Publication, Germany,2011. 5. Kim.S.Elliott, Precast Concrete Structures, Butterworth-Heinemann Publication, 2002. 6. The Prefabricated Reinforcement Handbook, developed jointly by CIDB, TEG Engineering Consultants, Ho & Chang Consultants, B.R.C. Weldmesh (S.E.A) Pte Ltd and Eastern Wire Pte Ltd. 7. Standard Prefabricated Building Components, Reference Guide published by the Buildability Development Section, Innovation Development Department, Technology Development Division of the Building and Construction Authority, © Building and Construction Authority, August 2000. 8. The Structural Precast Concrete Handbook, 2nd Edition is published by the Technology Development, Division of the Building and Construction Authority, © Building and Construction Authority, May 2001. 9. Indian Standard, IS 15916: 2010, Building Design and Erection using Prefabricated Concrete- Code of Practice 10. Indian Standard, IS: 3201–1988, Criteria for Design Construction of Precast and Concrete Trusses and Purlins. 11. Indian Standard IS: 10297–1982, Code Of Practice For Design And Construction Of Floors And Roofs Using Precast Reinforced Prestressed Concrete Ribbed Or Cored Slab Units. 12. Indian Standard IS: 11447–1985, Code Of Practice For Construction With Large Panel Prefabricates. 13. Indian Standard IS 13990: 1994 Precast Reinforced Concrete planks And Joists for Roofing and Flooring –Specification. 14. ACI 533R-93, Guide for Precast Concrete Wall Panels. 15. Gopala krishnan, Prefabricated Structures, Report. Sasurie College Of Engineering, Vijayamangalam. 16. www.google.com

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