Thumb Rules for Structural Design

Thumb rules for Structural Design | RCC Structures Design of RCC Structural Components In this article, we are going to discuss the minimum standards that are to be followed for the design of Structural components of a building such as Columns, beams, slab and foundation. We will also discuss the minimum safe standards for the reinforcing bars that are to be used for the design of the above mentioned Structural Components. Minimum cross-sectional dimension for a Column: is 9‖x9‖. But to avoid slenderness ratio problems in multistorey buildings, we prefer a rectangular column design of 9″x12″ which is safer. Also check out: Thumb rules for making a Column Layout

Construction on Site | Design of RCC Structures Minimum RCC beam size: is 9″x9″. But generally, to maintain uniformity and speed of construction, we design all beams of the same size 9″x12″. But 9″x9″ can also be used safely, according to design. Minimum thickness of RCC slab is 4.5″ because a slab may contain electrical pipes embedded into them which could be 0.5″ or even fatter for internal wiring, which effectively reduces slab depths at certain places, causing cracking, weakening and water leakage during rains. So, a minimum thickness of 4.5″ should be maintained. Minimum size of foundation for a single storey of G+1 building, where soil safe bearing capacity is 30 tonnes per square meter, and the oncoming load on the column does not exceed 30 tonnes, a size of 1m x 1m or 3′ x 3′ should be used. Even if the load is less (for example only 20 tonnes) then also the minimum is 3′x3′ and depth of footing should be atleast 4′ below ground level if not more…

Reinforcing bar details (minimum) 1. Columns: 4 bars of 12mm steel rods FE 415 2. Beams: 2 bars of 12 mm in the bottom and 2 bars of 10 mm on the top. 3. Slab a) One Way Slab: Main Steel 8 mm bars @ 6″ C/C and Distribution Steel of 6 mm bars @ 6″ C/C b) Two Way Slab: Main Steel 8 mm bars @ 6″ C/C and Distribution Steel of 8 mm bars @ 9″ C/C 4. Foundation: 6″ of PCC layer comes first. Over than, a tapered or rectangular footing of 12″ thickness is minimum. Steel mesh of 8 mm bars @ 6″ C/C should be laid. In a 3′ x 3′ footing, this would consist of 6 bars of 8 mm on both portions of the steel mesh. Those looking for more information

Thumb rules for designing a Column layout | Civil Engineering Guidelines to be followed for making a column layout | Building Construction Today, we will discuss something very general. Inspite of knowing these general thumb rules, Civil Engineers still end up making disastrous mistakes which would not only cost them but also cost the people living in the building designed by these engineers. Earlier, I wrote an article describing one of my projects where structural designing was executed on site (which was extremely pathetic) even before Architectural design was done. (Check out: Consequences of Wrong Structural Design | Live Project example) In this article, we will go through the essential thumb rules to be followed for giving a column layout. Ofcourse RCC columns have to be designed in accordance to the total load on the columns but apart from that it is essential for every Civil engineer and Architect to remember a few thumb rules so that they are prevented from making mistakes.

Three thumb rules to be followed are as follows: 1. Size of the Columns 2. Distance between Columns 3. Alignment of columns

Thumb rule no.1 Size of the columns The size of the columns depends on the total load on the columns. Minimum size of the column should not be less than 9‖x9‖. 9‖x9‖ columns are to be used for a single storey structure with M15 grade of concrete. In case, 9‖x9‖ column size is to be used for 1 and half storey structure, then it is advised to use M20 grade concrete.

A safe and structurally sound column size for a 1 and half storey structure should not be less than 12‖x9‖ using M15 grade concrete. This should be in your most preferred and practical options list. Thumb rule no.2

Distance between the columns Try to maintain equal distance between the centres of two columns. Always plan a column layout on a grid. The distance between two columns of size 9‖x9‖ should not be more than 4m centre to centre of column. If larger barrier free distances are required then going for larger column size is to be used.

The size of the columns increase because of two factors: 1. Increase in the distance between two columns (This increases the dimensions of the columns as well the depth of the beam.) 2. Height of the building (Increase in the number of floors is directly proportional to the dimensions of the columns. Thumb rule no.3

Alignment of Columns A rectangular grid is to be made for placing the columns. This helps in avoiding mistakes and placing in columns can be done in the right way.

The columns can preferably be arranged in two different fashions: 1. In a straight line with the help of a grid 2. In a circular fashion for circular buildings. Zigzag arrangement of columns is an absolutely wrong way of working out Structural design. It should be remembered that when columns are erected, beams are laid connecting the columns.

The Zigzag column placement causes three major issues: 1. Unbalanced load transfer 2. Problems in wall construction 3. Problems in laying beams If these three thumb rules are followed by Civil Engineering and Architecture students, implementation of wrong Structural design can be prevented.

Column Layout for a Residence | Civil Engineering Column Layout for a residence using the Thumb rules| Building Construction In my earlier article, we discussed three important thumb rules that are to be followed while making a column layout for any building. They are as follows: 1. Size of the Columns 2. Distance between the columns 3. Alignment of Columns

In this article, we will see an example of a residence of which column layout is done keeping the above three thumb rules in mind.

Column Layout for a residence The residential villa comprises of 1 and half floors. Initially, the column size 9″x12″ had been used with the use of M15 grade of concrete. The builder wanted to save on his budget by making the columns smaller in size. That is why, the columns in the Floor plans below are 9″x9″ in size but the Engineer made sure that M20 grade of concrete would be used for Columns.

Column Layout for a Ground Floor

Column Layout for First Floor

Thumb rule no1: Size of the Columns The size of the columns are 9″x9″ with the use of M20 grade of concrete.

Thumb rule no.2: Distance between the columns: The distance between the columns does not exceed 4.5m.

Thumb rule no.3: Alignment of Columns The Columns have been arranged on a iron grid pattern. So there is absolutely no question of zigzag walls and zigzag beams which reducing complications in the structure.

Guide to Doubly Reinforced RCC Beam Design RCC Beams RCC beams are cast in cement concrete reinforced with steel bars. Beams take up compressive and add rigidity to the structure. Beams

generally

carry vertical gravitational forces but

can

also

be

used

to

carry horizontal loads (i.e., loads due to an earthquake or wind). The loads carried by a beam are transferred to columns, walls, or girders, which then transfer the force to adjacent structural compression members. In Light frame construction the joists rest on the beam.

Doubly Reinforced Beam In this article, we are going to discuss types of beam construction and RCC design of Doubly reinforced beam…

RCC beam construction is of two types: 

Singly reinforced beam



Doubly reinforced beam

Singly reinforced beam

A singly reinforced beam is a beam provided with longitudinal reinforcement in the tension zone only.

Doubly reinforced beam 

Beams reinforced with steel in compression and tension zones are called doubly reinforced beams. This type of beam will be found necessary when due to head room consideration or architectural consideration the depth of the beam is restricted.



The beam with its limited depth, if reinforced on the tension side only, may not have enough moment of resistance, to resist the bending moment.



By increasing the quantity of steel in the tension zone, the moment of resistance cannot be increased indefinitely. Usually, the moment of resistance can be increased by

not more than 25% over the balanced moment of resistance, by making the beam overreinforced on the tension side. 

Hence, inorder to further increase the moment of resistance of a beam section of unlimited dimensions, a doubly reinforced beam is provided.

Besides, this doubly reinforced beam is also used in the following circumstances: 

The external live loads may alternate i.e. may occur on either face of the member.



A pile may be lifted in such a manner that the tension and compression zones may

For example:

alternate. 

The loading may be eccentric and the eccentricity of the load may change from one side of the axis to another side.



The member may be subjected to a shock or impact or accidental lateral thrust.

Design procedure for doubly reinforced beam Step 1

Determine the limiting moment of resistance for the given c/s(Mulim) using the equation for singly reinforced beam Mulim = 0.87.fy.Ast1.d [1 – 0.42Xumax] Or Balanced section Ast1 = (0.36.fck.b.Xumax)/(0.87fy) Step 2 If factored moment Mu > Mulim, then doubly reinforced beam is required to be designed for additional moment. Mu – Mulim = fsc.Asc (d – d‘)

[fsc value from page no. 70]

Step 3 Additional area of tension steel Ast2 Ast2 =Asc.fsc/0.87fy Step 4 Total tension steel Ast, Ast = Ast1 + Ast2

Guide to Design of RCC Columns RCC Column A column forms a very important component of a structure. Columns supportbeams which in turn support walls and slabs. It should be realized that the failure of a column results in the collapse of the structure. The design of a column should therefore receive importance. Supporting the slabs is the main function of the columns… Such slabs are called Simply Supported Slabs. Simply supported slabs could be either one way slab or a two-way slab. It depends on the dimensions of the slab.

Reinforced Cement Concrete Column Plan and Section

A column is defined as a compression member, the effective length of which exceeds three times the least lateral dimension. Compression members whose lengths do not exceed three times the least lateral dimension, may be made of plain concrete. In this article, we are going to discuss in detail the basis of classification of columns and different types of reinforcement required for a certain type of column.

A column may be classified based on different criteria such as: 1. Based on shape 

Rectangle



Square



Circular



Polygon



Short column, ? ? 12



Long column, ? > 12



Axially loaded column



A column subjected to axial load and unaxial bending



A column subjected to axial load and biaxial bending



Tied columns



Spiral columns

2. Based on slenderness ratio 3. Based on type of loading

4. Based on pattern of lateral reinforcement Minimum eccentricity Emin > l/500 + D/30 >20

Where, l = unsupported length of column in ‗mm‘

D = lateral dimensions of column

Types of Reinforcements for columns and their requirements 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 cross-section 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 

It maybe 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 exceed 

Least lateral dimension



16 x diameter of longitudinal bars (small)



300mm

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

Design of Staircase | RCC Structures RCC Structures RCC Structures are nothing but reinforced concrete structures. RCC structure is composed of building components such as Footings, Columns, Beams, Slabs, Staircase etc. These components are reinforced with steel that give stability to the structure. Staircase is one such important component in a RCC structure.

Dog Legged Stair In this article, we will discuss different types of staircases and study the RCC design of a dog-legged staircase…

Stairs Stairs consist of steps arranged in a series for purpose of giving access to different floors of a building. Since a stair is often the only means of communication between the various floors of a building, the location of the stair requires good and careful consideration. In a residential house, the staircase may be provided near the main entrance. In a public building, the stairs must be from the main entrance itself and located centrally, to provide quick accessibility to the principal apartments. All staircases should be adequately lighted and properly ventilated. Various types of Staircases 

Straight stairs



Dog-legged stairs



Open newel stair



Geometrical stair

RCC design of a Dog-legged staircase In this type of staircase, the succeeding flights rise in opposite directions. The two flights in plan are not separated by a well. A landing is provided corresponding to the level at which the direction of the flight changes. Design of Dog-legged Stairs Based on the direction along which a stair slab span, the stairs maybe classified into the following two types.

1. Stairs spanning horizontally 2. Stairs spanning vertically Stairs spanning horizontally These stairs are supported at each side by walls. Stringer beams or at one side by wall or at the other side by a beam. Loads 

Dead load of a step

= ½ x T x R x 25



Dead load of waist slab = b x t x 25



Live load

= LL (KN/m2)



Floor finish

= assume 0.5 KN/m

Stairs spanning Longitudinally In this, stairs spanning longitudinally, the beam is supported ay top and at the bottom of flights. Loads 

Self weight of a step

= 1 x R/2 x 25



Self weight of waist slab = 1 x t x 25



Self weight of plan



Live load

= LL (KN/m2)



Floor finish

= assume 0.5 KN/m

= 1 x t x 25[(R2 + T2)/T]

For the efficient design of an RCC stair, we have to first analyse the various loads that are going to be imposed on the stair. The load calculations will help us determine, how much strength is required to carry the load. The strength bearing capacity of a staircase is determined on the amount of steel and concrete used. The ratio of steel to concrete has to be as per standards. Steel in the staircase will take the tension imposed on it and the concrete takes up the compression. These are the essential steps that are to be followed for the RCC Stair Design.

Calculation of loads for Foundation Design | Structural Design How to calculate the total load on the footing? | Building Construction This article has been written on the request from my readers. Engineering students generally get confused when it comes to calculating loads for footings. They ask weird and inappropriate questions regarding the load calculations. This is because they haven‘t understood what loads are to be calculated when footing/foundation for a building is designed. Calculation of loads is extremely simple. I hope after reading this article, the queries of many of my readers would get a satisfactory answer.

Four loads are to be considered in order to measure total load on the footing: 1. Self load of the column x Number of floors 2. Self load of beams x Number of floors 3. Load of walls coming onto the column 4. Total Load on slab (Dead load + Live load) If you get well versed with load calculations, then calculating the size of the footing and following the procedure for foundation design wouldn‘t be a problem.

Guide to Foundation Design | Column Footings Foundation Design Foundation is the base of any structure. Without a firm foundation, the structure cannot stand. That is the reason why we have to be very cautious with the design of foundations because our entire structure rests on the foundation.

Laying of Column Footing Reinforcement The strength of the foundation determines the life of the structure. As we discussed in the earlier article, design of foundation depends on the type of soil, type of structure and its load. On that basis, the foundations are basically divided into Shallow Foundations and Deep Foundations. In this article, we are going discuss the step by step guide to Column Footing Design….

Reinforced Concrete Footings Footing comprises of the lower end of a column, pillar or wall which i enlarged with projecting courses so as to distribute load.

Footings shall be designed to sustain the applied loads, moments and forces and the induced reactions and to ensure that any settlement which may occur shall be as uniform as possible and the safe bearing capacity of soil is not exceeded. In sloped or stepped footings, the effective cross-section in compression shall be limited by the area above the neutral plane, and the angle of slope or depth and location of steps should be such that the design requirements are satisfied at every section.

Design Procedure of Column Footings Here is a step-by-step guide to Column Footing Design:

Column Footing Plan and Section

Step 1 Area required for footing Square = B = (w+w1)/P0 Where, Po = safe bearing capacity of soil w1 = self weight of footing w = self weight of footing For Rectangle = b/d = B/D

A=bxd Net upward pressure on the footing q/p = W/A

Step 2 Bending Moment Critical section for maximum bending moment is taken at the face of the column For a square footing, Mxx = q x B/8 (L – a)2 Mxx = q x L/8 (B – b)2 Myy = q x B/8 (L – a)2

Step 3

To fix the depth of the footing shall be greater of the following: Depth from bending moment consideration d =square root(M/Qb) where, Q = moment of required factor Depth from shear consideration Check for one way shear Check for two way shear or punching shear Critical shear for one way shear is considered at a distance ‗d‘ from face of the column. Shear force, V = qB [ ½(B – b) d] Nominal shear stress, Tv = k . Tc Tc

= 0.16square rootfck

Step 4

Check for two way shear Critical section for two way shear is considered at a distance at a distance d/2 from all the faces of the column. SF, V = q [ B2 – (b + d)2] SF, V = q [L x B – (a + d)(b + d)] Nominal shear stress, Tv = V/2((a+d)(b+d)d) ——- {for a rectangle Tv

= V/4((b+d)d)

Tv

= k . Tc

k = 0.5 + Beta > 1

——- {for a square ; [Beta = ratio of sides of the column

Tc

= 0.16square rootfck

Area of steel, Ast = M/((sigma)stjd)

Guide to Design of Simply Supported Slabs | Design of RCC Structures What are Simply Supported Slabs? Before we discuss the technical design rules of Simply Supported slabs, lets just go through its definition and learn why they are named so… As the name suggests, simply supported slabs are supported on columns or stanchions…

Simply Supported Slab Simply supported slabs don‘t give adequate provision to resist torsion at corner to prevent corner from lifting. The maximum bending moment will be given if the slabs are restrained. But atleast 50% of the tension reinforcement provided at the mid span should extend to the support. The remaining 50% should extend to within 0.1Lx or Ly at the support as appropriate. RCC Slab Design depends on the on the dimensions of the slab after which the slab is termed as a one-way slab or a two-way slab… In the design of RCC structures, Column Design and Beam Design are to be done before we start with RCC Slab Design…

Basic Rules followed in the design of simply supported Slab Thickness of slab l/d ratio should be less than the following: 

Simply supported slab



Continuous slab, l/d = 26



Cantilever slab, l/d = 7

In any case of the above, the thickness should not be less than 100mm

Effective span 

Distance between centre to centre of support



Clear span plus effective depth

Minimum main reinforcement 

0.15% gross c/s of slab – for MS bars



0.12% gross c/s of slab – for HYSD bars

Spacing of main bars

The spacing or c/c distance of main bars shall not exceed following: 

Calculated value



3d



300mm

Distribution or Temperature reinforcement This reinforcement runs perpendicular to the main reinforcement in order to distribute the load and to resist the temperature and shrinkage stresses. It should be atleast equal to; 

0.15% gross c/s of slab – for MS bars



0.12% gross c/s of slab – for HYSD bars

Spacing of distribution bars

The spacing or c/c distance of distribution bars shall not exceed the following 

Calculated area



5d



450mm

Diameter of bars The diameter of the bars varies from 8mm to 14mm and should not exceed 1/8th of the overall depth of the slab. For distribution steel, the diameter varies from 6mm to 8mm.

Cover The bottom cover for reinforcement shall not be less than 15mm or less than the diameter of such bar.

Various types of RCC Slabs | Design of RCC Structures Reinforced Cement Concrete Slab 

A Reinforced Concrete Slab is the one of the most important component in a building. It

is

a

structural

element

of

modern

buildings.

Slabs

are

supported

on Columns and Beams. 

RCC Slabs whose thickness ranges from 10 to 50 centimetres are most often used for the construction of floors and ceilings.



Thin concrete slabs are also used for exterior paving purpose.

RCC Slab Construction 

In many domestic and industrial buildings a thick concrete slab, supported on foundations or directly on the sub soil, is used to construct the ground floor of a building.



In high rises buildings and skyscrapers, thinner, pre-cast concrete slabs are slung between the steel frames to form the floors and ceilings on each level.



While making structural drawings of the reinforced concrete slab, the slabs are abbreviated to ―r.c.slab‖ or simply ―r.c.‖.

Design of various types of slabs and their reinforcement For a suspended slab, there are a number of designs to improve the strength-to-weight ratio. In all cases the top surface remains flat, and the underside is modulated: 

Corrugated, usually where the concrete is poured into a corrugated steel tray. This improves strength and prevents the slab bending under its own weight. The corrugations run across the short dimension, from side to side.



A ribbed slab, giving considerable extra strength on one direction.



A waffle slab, giving added strength in both directions.

Reinforcement design 

A one way slab has structural strength in shortest direction.



A two way slab has structural strength in two directions.

These slabs could be cantilevered or Simply Supported Slabs.

Construction 

A concrete slab can be cast in two ways: It could either be prefabricated or cast in situ.



Prefabricated concrete slabs are cast in a factory and then transported to the site ready to be lowered into place between steel or concrete beams.



They may be pre-stressed (in the factory), post-stressed (on site), or unstressed. Care should be taken to see that the supporting structure is built to the correct dimensions to avoid trouble with the fitting of slabs over the supporting structure.



In situ concrete slabs are built on the building site using formwork. Formwork is a boxlike setup in which concrete is poured for the construction of slabs.



For reinforced concrete slabs, reinforcing steel bars are placed within the formwork and then the concrete is poured.



Plastic tipped metal, or plastic bar chairs are used to hold the reinforcing steel bars away from the bottom and sides of the form-work, so that when the concrete sets it completely envelops the reinforcement.



Formwork differs with the kind of slab. For a ground slab, the form-work may consist only of sidewalls pushed into the ground whereas for a suspended slab, the form-work is shaped like a tray, often supported by a temporary scaffold until the concrete sets.

Materials used for the formwork 

The formwork is commonly built from wooden planks and boards, plastic, or steel. On commercial building sites today, plastic and steel are more common as they save labour.



On low-budget sites, for instance when laying a concrete garden path, wooden planks are very common. After the concrete has set the wood may be removed, or left there permanently.



In some cases formwork is not necessary – for instance, a ground slab surrounded by brick or block foundation walls, where the walls act as the sides of the tray and hardcore acts as the base.

Span – Effective Depth ratios 

Excessive deflections of slabs will cause damage to the ceiling, floor finishes and other architectural details. To avoid this, limits are set on the span-depth ratios.



These limits are exactly the same as those for beams. As a slab is usually a slender member the restriction on the span-depth ratio becomes more important and this can often control the depth of slab required in terms of the span – effective depth ratio is given by,

Minimum effective depth = span/(basic ratio x modification factor) The modification factor is based on the area of tension steel in the shorter span when a slab is singly reinforced at midspan, the modification factors for the areas of tensions and compression steel are as given in the figure 2 and 4 of the code.

Solid Slab spanning in two directions 

When a slab is supported on all four of its sides, it effectively spans in both directions, and it is sometimes more economical to design the slab on this basis. The moment of bending in each direction will depend on the ratio of the two spans and the conditions of restraint at each support.



If the slab is square and the restraint is similar along the four sides, then the load will span equally in both directions. If the slab is rectangular, then more than one-half of the load will be carried in the shorter direction and lesser load will be imposed on the longer direction.



If one span is much longer than the other, a large portion of the load will be carried in the shorter direction and the slab may as well be designed as spanning in only one direction.



Moments in each direction of span are generally calculated using co-efficients which are tabulated in the code.



The slab is reinforced with the bars in both directions parallel to the spans with the steel for the shorter span placed farthest from the natural acis to five the greater effective depth.



The span-efective depths are based on the shorter span and the percentage of the reinforcement in that direction.

Guide to the Construction of Sunken Slabs | RCC Construction Construction of Sunken Slabs | Building Construction Sunken slabs are used in the toilets, bathrooms and washing place where we have our washing machines. The purpose of having a sunken slab is to conceal all the pipes below the floor. Since the pipes that carry water are concealed below the floor, care has to be taken to avoid leakage problems. It is seen that people are not much aware of the idea of waterproofing the Sunken slab before the floor finish is done. In this article, we will discuss the method of the construction of Sunken slab and waterproofing technique.

Method of construction of a Sunken Slab 

The concrete of the R.C.C. (floor and sunken slab) should mixed with a waterproofing material to get a denser, watertight concrete.



Then cement and waterproofing material should be diluted in water and splashed onto the RCC sunken slab. Over that a layer of plaster should be provided using a mortar plasticizer with the cement mortar.



Brick laying of walls and plastering (prior to tiling) of the walls and floor should be done with cement mortar mixed with a mortar plasticizer.



Tile fixing for the floor and walls tiles should be done with non-shrink, waterproof tile adhesives to make the tiled area waterproof.



Sanitary pipe joints should be sealed with sealants specially manufactured for Sealing Sanitary joints firmly so that no water can leak through.