Reinforcement

RCC slab Plane structural members which are used as floor and roof covering in a building. The thickness of the slabs looking is quite small as compared to other dimensions.

Classification of the slab:- 

On the basis of the ratio of longer span to shorter span. RCC slab classified into the following two types

1.  One- way slab
2.  Two-way slab

One way slab:- 

The slab in which the ratio of (Ly/Lx) longer span to short span is greater than or equal to 2 are called one-way slabs. These slabs are also called edge supported slabs

Slab supported on four edges also behaves as one-way slab if the ratio of longer span to shorter span is greater than or equal to 2. In these slabs bending take place only along the shortest span. Therefore, the main reinforcement is provided along the shorter span and distribution Steel is provided along with the largest span.

Reinforcement in one way slab:- 

Main reinforcement:-

Reinforcement is provided parallel to the shorter span and main reinforcement is provided to resist the bending moments due to transverse loads coming on the slab.

Distribution reinforcement:-

Distribution reinforcement is provided in a direction perpendicular to the direction of the main reinforcement. These bars are normally not provided with hooks. The distribution Steel bar should be tied above the main Steel bar, otherwise, the Lever arm shall be reduced resulting in the reduction of the moment of resistance.

Distribution Steel are provided

•  To resist concentrated load coming over the slab.
•  To register shrinkage and temperature stresses.
•  To keep the main reinforcement in position.

Two-way slab:-

 The slabs which are supported on all the four edges and having a ratio (Ly/Lx)of larger span to shorter span less than 2 are called two-way slabs. Two-way slabs are also called as slabs spanning in two directions because bending takes place in both the direction. Therefore the main reinforcement is provided along both the direction in the two-way slab.

In two way slabs, the total load is divided along both the spans instead of one span as in the case of the one-way slab. Therefore, the maximum bending moment and deflection for such a slab are much smaller than that of a one-way slab and hence thinner slab is required.

Reinforcement in slab 

(a) Minimum reinforcement:-  The area of reinforcement in either direction is slab should not be less than 

• For HYSD bar:- 0.12% of the total cross-sectional area
•  For Fe 250 Steel:- 0.15% of total cross-sectional area

(b) Maximum dia:- The maximum diameter of the reinforcing bar in the slab should not exceed 1/8 of the total thickness of the slab.

Spacing of reinforcement:- 

Main reinforcement:- The horizontal distance between parallel main reinforcement bars should be the minimum of the following :

• Three times the effective depth of the slab 
• 300 mm

Distribution reinforcement:-The horizontal distance between parallel distribution reinforcement bar Shall be mini. of the following :

• Five times the effective depth of the slab
• 300 mm