Permissible Deflection in Slab
Deflection limits depend on the codes being used for design of structures.
Mainly there are two types of deflections are describe as follow :
Immediate Elastic Deflection:
This occurs due to elastic behavior of structure under load. In other word this type of deflection occur during concreting or after some time of the concreting
Immediate elastic deflection Value can be found by following formula
Span/180 (ACI code)
Long Term Elasto-Plastic deflection:
This occurs due to plastic behavior of concrete and any additional load on the structure over and above the initial load causing immediate deflection.Creep and shrinkage mainly known as Long term Deflection
Long Term deflection Value can be found by following formula Span/360 to 480 (ACI Code)
Construction Measures to Reduce Deflection Slabs
- Cure the concrete to decrease creep and shrinkage
- Build camber into the floor slab
- Materials Selection to Reduce Deflection of Beams and Slabs
- Use concrete with a higher modulus of rupture
- Use fibers to the concrete mixture
- Cure the concrete to allow it achieve strength
Slab Design Guidelines
- 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 using cl.24.1,IS 456 provided short span is <3.5m and loading class is <3.5KN/m2
According Thumb rule, deflection due to all loads including effects of temperatures ,creep and shrinkage and measured from the as cast level of the support of floors , should not normally exceed L/250.
and the deflection due to effect of temperature , creep and shrinkage after erection of partitions and the applications of finishes should not normally exceed L/350 or 20 mm whichever is less.