Why slabs are designed for bending and shear?

Tejas Jain

Senior Member
Slabs are horizontal and are designed to withstand vertical loads such as gravity load, dead load, live load. Slabs are supported on beams or on walls. Under such conditions, slabs bend and bending is usually accompanied by shear.

Shear force at a section is equal to the rate of change of bending moment at that section Unless bending moment is constant, shear force is non-zero. That is why slabs are designed for bending and shear. Even when slabs are inclined, only thing that changes is that the gravity load is no longer normal to the plane of the slab. In such a case, predominant response is still bending and shear, but in addition, there is a small amount of in-plane axial force, which is usually neglected in design since it is small in comparison to the effects of bending and shear.

In buildings subjected to lateral loads (such as wind and earthquake loads), slabs are also subjected to in-plane forces. But it is common practice to consider slabs as rigid in their own plane (considering their large dimension in their own plane) and hence it is assumed that slabs do not deform in their own plane, no matter how large the in-plane forces. But if a slab has large openings, this assumption is no longer true. Building design codes usually do not specify how such slabs are to be designed.