Why Pedestal is provided over footing & how does it economises footing design
It is very essential to understand the concept behind deciding in which kind of footing pedestal is required.
Actually, for raft foundation, if you provide pedestal, then it will reduce the overall thickness of raft as pedestal will help against punching shear(Two Way Shear) which normally require higher section.Ultimately cost of raft foundation is reduced by providing pedestal. But for basement areas, we should keep in mind that by providing a bigger pedestal can cause hinderance against smooth movement of vehicles.So, in basement areas, we should carefully provide pedestal. Generally columns near drive should be free from pedestal.
On the otherhand, in ioslated foundation, one way shear is more predominatnt than punchin shear for lesser loads. So, if we provide a pedestal having lesser thickness, it will reduce stresses caused by one way shear and it will ultimately helps us to reduce the over all thickness of footing and thus foundation design become economical. On contrary, if load is very high then isolated footing sizes becomes large. In that case too, a pedestal provided will reduce one way shear stress and will reduce overall thickness of footing.A provided pedestal over a footing subjected with large load will also withsatnd high punch stress. Hence, in isolated footing, a pedestal works against punching stresses sas well against one way shear. And this ultimately leads us to an safer and economical design of foundation.