Question

For the column and beam layout, find the required slab thickness and spacing of #4 reinforcing bars, if the loading in addition to self-weight, is 2 point loads of 2 kips at the third points of each span (4 point loads total) f'c =4000psi fy = 60ksi d = hSLAB - 1in,
weight of reinforced concrete is 150pcf, width of beams - 12in, space between borders of beams - 7ft.

Answer 1

The below design of slab is based upon IS 456-2000,

a) Effective span of slab:

Effective span of slab shall be lesser of the two

1. L=Clear span+ d (Effective depth)

2. L=Center to center distance between the support

b) Depth of slab:

The depth of slab depends on bending moment and deflection criterion. The trail depth can be obtained using:

1. Effective depth d= Span/((L/d)_basic X modification factor)

2. For obtaining modification factor, the percentage of steel for slab can be assumed from 0.2 to 0.5%.

3. The effective depth d of two way slabs can also be assumed using cl.24.1, IS 456 provided short span is <3.5m and loading class is <3.5 kN/m².

Type of support for simply supported Fe-415 slab= L/28

Or, the following thumb rules can be used:

1. One way slab d=(L/22) to (L/28)

2. Two way simply supported slab d=L/20) to (L/30)

Two way restrained slab d=(L/30) to (L/32)

c) Loads on slab:

The load on slab comprises of dead load, floor finish and live load.The loads are calculated per unit area (load/m²)

Dead load D X 25 kN/m² (Where D is thickness of slab in m)

Floor finish (Assumed as)= 1 to 2 kN/m²

Live load (Assumed as)= 3 to 5 kN/m² (depending on the occupancy of the building)

Detailing requirements of reinforced concrete slab as per IS45:2000

a) Nominal cover:

For mild exposure- 20mm

For moderate exposure- 30mm

However, if the diameter of bar do not exceed 12mm, or cover may be reduced by 5mm. Thus for main reinforcement up to 12mm diameter bar and for mild exposure, the nominal cover is 15mm.

b) Minimum reinforcement:

The reinforcement in either direction in slab shall not be less than

1) 0.15% of the total cross sectional area for Fe-250 steel

2) 0.12% of the total cross-sectional area for Fe-415 & Fe-500 steel.

c) Spacing of bars:

The maximum spacing of bars shall not exceed

1) Main steel- 3d or 300 mm whichever is smaller

2) Distribution steel- 5d or 450 mm whichever is smaller. Where, d is the effective depth of slab.

Note: The minimum clear spacing of bars is not kept less than 75 mm (Preferably 100 mm ) though code do not recommend any value.

d) Maximum diameter of bar:

The maximum diameter of bar in slab, shall not exceed D/8, where D is the total thickness of slab.