FLAT SLAB

Flat Slab Floor System. A flat slab is a one-way or two-way system with thickenings in the slab at the columns and load bearing walls called ‘drop panels’ Figure 9. Drop panels act as T-beams over the supports. They increase the shear capacity and the stiffness of the floor system under vertical loads, thus increasing the economical span range.

This form of construction has become less popular in recent years because of the limit on economical spans of about 9.5 m for reinforced slabs and about 12 m for prestressed slabs. Reinforced flat slabs may need to be sensibly pre-cambered (not overdone) to control deflection. The plan dimensions of the drop panels are a minimum of 1/3 of the span in the direction under consideration, usually rounded to the nearest 100 mm. The overall depth of the drop panel is typically taken as 1.75 to 2 times the depth of the slab, again rounded to suit timber sizes or the nearest 25 mm.

The principal features of a flat slab floor are a flat soffit, simple formwork and easy construction. The economical span ‘L’ of a reinforced concrete flat slab is approximately D x 28 for simply supported, D x 32 for an end span and D x 36 for an interior span. Prestressing the slab increases the economical span to D x 35, D x 40 and D x 45 respectively, where D is the depth of the slab excluding the drop panel.

Various Flat Slab Systems

Advantages and Disadvantages of Flat Slabs Advantages:

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Simple formwork

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No beams—simplifying under-floor services outside the drops

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Minimum structural depth

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Usually does not require shear reinforcement at the columns. Disadvantages:

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Medium spans

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Generally not suitable for supporting brittle (masonry) partitions

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Drop panels may interfere with larger mechanical ducting

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Vertical penetrations need to avoid area around columns

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For reinforced flat slabs, deflection at the middle strip may be critical.

Flat slab: An important development in RCC floor is that of a flat slab RCC floor. In this method slab is built monolithically, with supporting columns only without provision for beams. The flat slab may either be with drop or without drop. The term drop is used to indicate that part of slab around the column having greater thickness than the rest of the slab. See the sketch above to give various nomenclature of flat slab. The reinforcement in the flat slab can be arranged either in two – way system or four – way system. For ordinary loading conditions, the two-way system of reinforcement is generally adopted. The flat slab construction grants the following advantages. i). as the slab is constructed without any beam, a flat ceiling is obtained which gives attractive appearance and greater height without obstructions for services. ii). It is considered less vulnerable in case of fire as compared to the ordinary slab-beam construction. iii). The concrete is more logically used in this type of construction and hence it proves to be economical for large spans with heavy live loads. iv). The formwork is simple and economical. v). The plain ceiling grants better diffusion of light.

Hollow block and rib floors: In this type of floors, the hollow blocks of clay or concrete are used to reduce the total weight of the floor. In one form, the blocks are placed 100 mm apart and in this space, the mild steel bars are placed as shown in sketch. A minimum cover of 80 mm is kept at the top . suitable flooring at top and ceiling finish are provided. The blocks are provided with rough or grooved surface so that they can develop enough bond with concrete. This type of floor is economical, fire-proof, sound-proof and light in weight. If properly designed, this type of floor can even be used to carry heavy loads. The plumbing and electrical installations can be conveniently carried through the hollow blocks without affecting the appearance. As hollow blocks replace a portion of solid concrete, the dead weight is reduced and thereby the economy is achieved. These floors are widely used for buildings like hospitals, hotels, schools, offices, etc. the structural hollow clay floor tiles should be free from cracks and they should be of uniform color and texture. It is desirable to keep projections on one of the extreme face of such tiles so as to facilitate the application of plaster. The weight per meter length of structural hollow clay floor tiles does not exceed 140 N. The length, width and height of structural tiles are from 450 mm or 900 mm, 190 mm or 240mm and 70 mm or 90 mm respectively. Pre-cast concrete floors: With the development of pre-cast concrete construction technique, it is possible to prepare the pre-cast units for the floor. These pre-cast units are available in suitable sizes as can be conveniently handled, transported and fixed. They may be supported either on walls or on rolled steel joists. The sides of each unit contain grooves which are used to connect the adjacent units.

Advantages of pre-cast concrete floors: Following are the advantages of the precast concrete floors: i). The construction work can be carried out speedily as there is absence of operations like watering, curing, etc. ii). The members are light in weight and hence the foundation cost proves to be economical. iii). They are fire – proof and sound – proof. iv). They do not require formwork during construction. v). They have good thermal insulation. Disadvantages of pre-cast concrete floors: Following are the disadvantages of the pre-cast concrete floors: i). There are chances for breakages to occur during their transport from factory to the site of work. ii). They prove to be economical and cheap only when manufactured on a large scale. iii). They require careful supervision and skilled labor during their manufacturing process. iv). They require fairly uniform spacing of beams and columns throughout the structure which becomes practically difficult. v). They should be properly designed to resist the stresses developed during transport and handling operations.

A flat slab is a two-way reinforced concrete slab that usually does not have beams and girders, and the loads are transferred directly to the supporting concrete columns. For more detailed definition: Flat Slab Floor System : Definition & Description

Advantages of Flat Slab Flat Slabs are used by engineers in many building due to its advantages over other reinforced concrete floor system in different cases. The most important advantages of flat slabs are given below:

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Flexibility in room layout. 

Partition walls can be placed anywhere.

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Offers a variety of room layout to the owner.

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False ceiling can be omitted.

Reinforcement placement is easier. As reinforcement detailing of flat slab is simple, it is easier to place

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Ease of Framework installation. Big table framework can be used in flat slab

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Building height can be reduced. 

As no beam is used, floor height can be reduced and consequently the building height will be reduced.

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Approximately 10% in the vertical member could be saved

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Foundation load will also reduce.

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Less construction time. Use of big table framework helps to reduce construction time

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Prefabricated welded mesh. 

Standard sizes

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Less installation time

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Better quality control.

Auto sprinkler is easier.

Disdvantages of Flat Slab Flat slabs have some disadvantages also. The major disadvantages are given below.

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Span length is medium. In flat plate system, it is not possible to have large span.

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Not suitable for supporting brittle (masonry) partitions

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Use of drop panels may interfere with larger mechanical ducting

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Critical middle strip deflection In flat slabs, the middle strip deflection may be critical.

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Higher slab thickness Compared to typical reinforced concrete two way slab system, the thickness of flat plat slabs are higher.

Most engineers are familiar with the limitations on the size of drop panels in two-way non-prestressed slabs. ACI 318-14, Section 8.2.4 describes two qualifications: (A) Depth of the drop panel must be at least ¼ of the slab thickness away from the drop panel; and (B) The drop panel must extend 1/6 of the span length from the center of support in each direction It is important to note that these limitations do not apply for prestressed slabs, which allows the designer tremendous flexibility in sizing drop panels to increase negative moment capacity and reduce non-prestressed reinforcement over the columns. For example, you do not need to design large drop panels that are 1/6 of the span length in each direction in order to take advantage of the increased section for flexure.

We recommend that the depth of the drop below the slab soffit be at least 4 in., which is the nominal depth of a 2 x 4 plus ¾ in. plywood sheathing (plyform). The depth of the drop should be increased if required to increased punching shear capacity. I further recommend using a minimum drop panel size of 4 x 4 ft in plan. The size of the drop should be increased if necessary to increase flexural or punching shear capacity and fit shear reinforcement within the dimensions of the drop.