Reinforced Concrete Slab Design
Example 1. One-way Slab
The specified design strength of the concrete, is 25 MPa () and the specified yield strength of the reinforcement, fy , is 400 MPa. The unit weight of reinforced concrete is assumed to be 24 kN/m3.
A one-way slab and beam system is shown below. The slab supports a live load of 4.8 kN/m2 and a superimposed dead load of 1.6 kN/m2. Assume an exterior exposure.
Because the ratio of the long to short clear spans of the slab panels is 2 or more, the behaviour of the slab panels is essentially one-way, spanning in the short direction. One design approach is to use CSA A23.3-04-Appendix B with m = 0.5. An alternative to this is to design the slab as a continuous one-way beam.
Step 1 - Slab Thickness (CSA Clause 18.104.22.168)
Deflection requirements (Table 9-2 of CSA A23.3)
Exterior span 1-1'
Interior span Try hs = 125 mm
Step 2 - Design Moments
For design strip 1 metre wide, wf = 12.95 kN/m
Limitations for use of Clause 9.3.3 are satisfied;
use coefficients in Clause 9.3.3.
Step 3 - Select Reinforcement
Assume 10M reinforcement
Try 10M at 400 c/c
Check crack control:
At least minimum reinforcement will be provided for bottom steel in both directions. The principal reinforcement in the North-South direction will be in the outside layer. The reinforcement in the non-spanning East-West direction is more critical for crack control.
For s = 400 mm
For an exterior exposure, z should be less than 25,000. This suggests that a spacing of 400 mm is too large. Revise spacing to 350 mm.
Resisting moment at minimum reinforcement:
This is adequate for all bottom steel requirements and for top steel on gridline 1. For top steel at interior supports try s = 300 mm.
This is sufficient at all interior supports.
Use: 10M at 350 c/c bottom each way.
10M at 350 c/c top North-South at perimeter beam.
10M at 300 c/c top North-South at interior beams.
Transverse reinforcement over girders spanning parallel to slab is governed by (10.5.3.2).
Use: 10M at 300 c/c top East-West over all girders.
Step 4 - Check Shear
dv is larger of:
Step 5 - Design Supporting Beams
Beams and girders designed for vertical reaction from slab, self-weight of beam stem, and any other loads applied directly to beams. Edge beams also designed for torsion corresponding to negative support moments assumed in slab design.