Column Design with Spreadsheets

RCC Slab Design Guidelines

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:
• Effective depth d= Span /((L/d)Basic x modification factor)
• For obtaining modification factor, the percentage of steel for slab can be assumed from 0.2 to 0.5%.
• The effective depth d of two way slabs can also be assumed use cl.24.1,IS 456
Type of support Fe-250 Fe-415
Simply supported L/35 L/28
Continuous support L/40 L/32
Or, the following thumb rules can be used:
• One way slab d=(L/22) to (L/28).
• Two way simply supported slab d=(L/20) to (L/30)
• Two way restrained slab d=(L/30) to (L/32)

c) Load 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 RCC Slab as per IS456: 2000

a) Nominal Cover:
For Mild exposure – 20 mm
For Moderate exposure – 30 mm
However, if the diameter of bar do not exceed 12 mm, or cover may be reduced by 5 mm. Thus for main reinforcement up to 12 mm diameter bar and for mild exposure, the nominal cover is 15 mm.

b) Minimum reinforcement: The reinforcement in either direction in slab shall not be less than
• 0.15% of the total cross sectional area for Fe-250 steel
• 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
• Main Steel – 3d or 300 mm whichever is smaller
• 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.

Important unit weights and Unit conversions.

UNIT WEIGHTwater-1000 kg/m3

cement-1400 "

steel - 7850 "

wood - 1200 "

sand - 1650 "

P.C.C - 2400

"R.C.C - 2500 "

UNIT CONVERSION;0.3048 M = 1 ft

1 m2 = 10.76 sq.ft

2.83m3 =100 C.F.T

1 C.F.T = 28.7 litres

1 cent = 435.6 sq.ft

1ground =2400 sq.ftGRADE OF CONCRETE

;M10 = lean mix

M15 = 1:2:4

M20 = 1:1.5:3

M25 = 1:1:2

Weight. OF STEEL;

6 mm = 0.22 kg/m

8 mm = 0.39 "

10 mm = 0.62 "

12 mm = 0.89 "

16 mm = 1 kg/m