The design of a roof slab consists of thickness of slab, size of bars, spacing of bars, hooks, cranks and laps of bars, minimum cover of concrete etc. The mix of concrete for the slab is usually by volume and is of proportion 1:2:4 ( cement, sand and aggregate)
Due to the loading conditions the slab is subjected to possible or sagging bonding movement on the bottom surface at the centre of the span and negative or hogging bending moment at top near supports. To resist these moments main reinforcement is provided at bottom surface along the direction of span to resist positive bending moment.
Depending upon the span, an RCC slab may be designed as either one way reinforced slab or two way reinforced slab. The slab which is supported on two opposite supports is called as one way slab. This slab is considered to be supported on two longer opposite sides. When the ratio between the length and breadth of the room over which the slab is provided is more than 2. One way slab consists of main reinforcement spanning between the supports with distribution of reinforcement at right angles to it. Two way slab is said to be spanning in two directions, when the length to breadth ratio falls below 2. In such case, main reinforcement is provided in both directions in the form of a mesh.
The overall thickness of slab is usually kept 10 cm though the minimum thickness can be 7.5 cm. The minimum area of reinforcement per unit width of a slab shall not be less than 0.15%. of the gross cross sectional area of concrete. But due to other limitations, it becomes necessary to provide higher percentage of steel in the slab.
Spacing of bars of main reinforcement shall not be more than 3 times the effective depth of slab and that for the distribution bars not more than five times the effective depth. Spacing of not more than 20 cm diameter for main bars and not more than 40 cm for distribution bars is usually provided.
Bars smaller than 6 mm diameter or greater than 18 mm are not used for slabs. 10 mm or 12 mm plain M.S. bars or 8 mm or 10 mm tor-steel bars are generally used. They are provided at the bottom of the slab allowing minimum cover of 25 mm of concrete.
In addition to bars along the main direction, provided at the bottom, bars along the longer span are also placed on top of the main bars and at right angles to them. These are called ‘ distribution’ bars or ‘temperature reinforcement’ or ‘ transverse bars’. The distribution bars function to resist cracks due to variation of temperature and shrinkage stresses. They also assist in distribution of loads. The area of such reinforcement is usually 20 % of that of main reinforcement. Plain M.S. bars of 6 mm diameter are usually used for this purpose.
In all buildings, slabs covers the full width of the wall over which walls are raised. This creates partially fixed conditions for the slab. In such cases, every alternate bar is bent up. This arrangement is economical as compared to providing additional bars at the top. The main bottom bar is cranked at one-fifth of the span at either end and taken up.
Main and distribution bars are tied together with black mild steel wire of 16 guage by providing a cross and twisting the ends. If smaller diameter of wire is used, double strands of wire shall be provided in place of single strand. About 5 kg. of binding wire is required for tying 1 tonne of reinforcing bars.
Considering a 2.5 M or 8’0” wide verandah by way of example, the thickness of slab is 10 cm. Main bars are 10 mm plain M.S. or 8 mm tor-steel and are placed at 20 cm centre at bottom of the slab along the short span allowing a cover of 25 mm. Every alternate bar is cranked up to 0.5 M from the edge of wall and at both ends 6 mm plain M.S. bars are used as distribution bars and are placed at 30 cm centres at the top of the bottom bars. For the cranked bars near the supports at the top of the slab distribution bars of same size and same spacing are provided below the top bars.
Two-Way Slab
In providing steel for a two-way slab, the bars in both directions shall be of the same diameter and of the same kind i.e. either plian or tor-steel. The bottommost layer will be along the shorter span. Same size bars along the longer span shall be placed on top of these bars and at right angles to them. No extra distribution bars are necessary. Cranking of alternate bars shall be done at 1/5th span from support on all the four sides and cranked bars taken to the top which come at the top of slabs over the supports allowing usual cover.
If the slab of one room continues over the adjacent room the portion of the slab over the common intermediate support needs top reinforcement. The bars in the middle portions in short span direction will be at the bottom whereas those over the intermediate supports will be at the top. The spacing of a given size of bar in all these positions depends upon the lengths of spans and upon the loading conditions and is determined by design considerations.
It is desirable for sake of convenience of workmen and the supervisor to adopt the same spacing for the bottom bars at the midsection of the slab as that of the top bars over intermediate supports. Every alternate bottom bar when cranked up at 1/5th of the span serves as the top bar over supports which should be extended upto a distance equivalent to 1/4th of the adjacent span. When the cranking of bottom bars in two adjacent spans is done in a staggered manner, the spacing of top bars at supports becomes the same as that of the bottom bars at the mid-span.
Shear stress in concrete is generally small and bond stress within allowable limits. Stirupps are not used in slabs whether it is reinforced one way or both ways.
