DRCS - Quick Reference (Unit 4)

Slabs:

A slab is a plate like structural element transferring the load on to the supporting beams some times directly on to Columns, basically by bending action.

For design purpose, a slab is considered to be a beam of unit width

Types of Slabs:

By Shape: 1) Rectangular 2) Non-Rectangular
By Support Condition: 1) Cantilever 2) Simply Supported 3) Continuous
By Span Ratio: 1) One-way Slab 2) Two-way Slab

One-way Slab: Ly/Lx >2
Two-way Slab: Ly/Lx <=2

Ly = Effective long span
Lx = Effective short span

Types of Problems:

1. Design of Simply Supported One-way Slab (Slab over load bearing walls)

Procedure:

1. Given Data
2. Assume Slab Thickness @ 40mm per metre of short span
3. Assume Clear Cover = 15mm
4. Assume dia. of main steel bar is 10mm
5. Find Effective Span = Clear Span + 2 x Half of Support Width
6. Find Load on Slab 1) Self weight of slab 2) Floor Finish Load at 1 KN/Sqm 3) Live Load
7. Find Max. BM = Wl²/8 for UDL and S.F. = Wl/2 for UDL
8. Check for Depth: MR = C x Z
9. Find Ast: MR = T x Z
10. Spacing of Main Steel Reinforcement, S = 1000 x (ast / Ast)
11. Check for Maximum spacing of main steel bars is not to exceed least of  1) 3d  2) 300mm
12. Provide Distribution Steel at 0.12% = (0.12/100) * bd  (where b = 1000mm)
13. Check for Maximum spacing of Distribution Steel bars is not to exceed least of 1) 5d 2) 450mm 
14. Check for Shear same as Singly Reinforced Beam
15. Check for Bond same as Singly Reinforced Beam
16. Draw Cross Section and Plan of Slab with Steel Reinforcement Details

2. Design of Continuous One-way Slab (Slab over load bearing walls)

Procedure: Same as Simply Supported Slab except the following:

• Assume thickness of slab around 1/30 of span subjected to a minimum of 100mm
• Support Moment, Mu -ve = W_dl²/12 + W_l²/9
• Mid Span Moment, Mu +ve = W_dl²/16 + W_l²/12

W_d = Dead Load

W_l = Live Load

3. Design of Cantilever Slab

Procedure: Same as Simply Supported Slab except the following:

• Assume thickness of slab around 1/7 of span subjected to a minimum of 100mm
• Support Moment, Mu -ve = Wl²/2 for UDL

Note:

1. Assumption of Live Load for Roof Slab (top floor) = 1.5 KN/Sqm
2. Assumption of Live Load for Floor Slab (Residential Building) = 3.0 KN/Sqm
3. Assumption of Live Load for Floor Slab (Commercial Building) = 4.0 KN/Sqm
4. Bent-up bars should be bent at an angle of 45 degree at a distance of L/7 from inner edge of support

4. Design of Simply Supported Two-way Slab (Slab over load bearing walls)

Methods:

Using Rankine Grashoff's Theory

M_x = α_x w l_x²

M_y = α_y w l_x²

^Where 

α_{x =}  r⁴ / 8 (1+r⁴)

α_{y =} r² / 8 (1+r⁴)

r = ly / lx

IS code Method:

Use IS 456:2000 (table 26) for difference ly/lx

Procedure:

1. Given Data
2. Assume Slab Thickness @ 40mm per metre of short span
3. Assume Clear Cover = 15mm
4. Assume dia. of main steel bar is 10mm
5. Find Effective Span = Clear Span + 2 x Half of Support Width
6. Find Load on Slab 1) Self weight of slab 2) Floor Finish Load at 1 KN/Sqm 3) Live Load
7. Find Max. BM in both directions: Mx and My (Use Any one of methods given above)
8. Check for Depth: MR = C x Z
9. Find Main (along short span )As:: Mx = T1 x Z1
10. Spacing of Main Steel Reinforcement, S = 1000 x (ast / Ast)
11. Check for Maximum spacing of main steel bars is not to exceed least of  1) 3d  2) 300mm
12. Find Distribution Steel (along long span) Ast: My = T2 x Z2
13. Spacing of Main Steel Reinforcement, S = 1000 x (ast / Ast)
14. Check for Maximum spacing of main steel bars is not to exceed least of  1) 3d  2) 300mm
15. Check for Shear same as Singly Reinforced Beam
16. Check for Bond same as Singly Reinforced Beam
17. Draw Cross Section and Plan of Slab with Steel Reinforcement Details

5. Design of Simply Supported Two-way Slab (with Provision for Torsion ) - Using IS code Method

Procedure:  Same as above except the following:

1. As per clause D-1.8 of Code, torsion reinforcement must be provided on all simply supported corners of the slab.
2. At each of the four corners this provided in 4 layers in a Square portion of lx/5
3. Ast for Torsion = (3/4 of Ast required for Short Span ) x (lx/5)