The slab is a two dimensional plane element with its depth “D” being much smaller than the other two dimensions namely the width and the span. Usually in the design of slab firstly it is checked whether the slab is one way slab and two way slab. Here we will be discussing all about the shear strength of RCC slabs its variation with depth.
We all know how a concrete beam behaves when it is under bending, it behaves as a flexural member. The same is the case with the RCC slab, reinforced concrete slabs are also flexural members subjected to both compression and tension loads.
But unfortunately bending failure is not the only kind of failure that might occur in reinforced Beam or slab. Another kind of failure that might occur is the shear failure.
Shear failure usually occurs near the supports where high loads are present. These high loads can make the RCC elements to just suddenly fail by shearing. These high loads can make the RCC elements to just suddenly fail by shearing.
But, as you know concrete has some shear resisting properties in it. Yes concrete can resist shear forces!! Then what is the problem? Read on…
Shear strength of RCC slabs its variation with depth :
Concrete can resist shear failure as it has got some amount of such properties. Usually for a normal one way or for two way slabs also (by normal I mean that a normal construction of a small home, not a big apartment with huge area and larger span slab!) but as the shear forces increases to very large magnitude (like in large span slabs) concrete will be unable to take that magnitude of shear force. Hence that demands shear reinforcements.
But, in many a case we know that some of us directly assume shear conditions for a slab will be satisfied if bending is satisfied. That is because:
- for a normal construction shearing forces are small
- since the depth is small compared to the length and breadth hence slab acts as a fairly slender member. So we can satisfy bending conditions and conclude.
However, at the end sections because of development of large forces huge shear to be as a outcome of this known as Punching shear.
Punching shear is be defined as a type of shear failure in a reinforced concrete slab which is subjected to a very high localized forces.
Shear and relation with depth :
According to IS 456-2012, which implies in its statements that with increase in depth of slabs shear strength of slabs decreases, and with decrease in depth of slabs shear strength of slabs increases.
Reason for increase in depth of slabs shear strength of slabs decreases and vice versa :
Assuming a situation with two different slabs/ beams subjected to the same type of shearing action, and all. Now, assume first case is with a larger depth or thickness than second case.
Here, in the latter case (with larger depth) larger width cracks are possible because of shear failure, these cracks would be smaller in thinner slabs.
Therefore, with increase in depth of slabs shear strength of slabs decreases, and vice-versa.