TWO WAY REINFORCED CONCRETE SLABS AND THE BENDING MOMENTS CALCULATION

When the slabs are supported on the all four edges the load is transferred to all the four supports and, therefore, the bending moments and deflections in such slabs are considerably less than those in slabs supported on two opposite edges only. In case the slab is simply supported on all the four supports the corners get lifted up when such a slab is loaded .In case the corners are held down ,the deflections in the central portion are further reduced and thus the bending moments are reduced in such slabs .Where the corners are held down due to monolithic construction of the slab with the walls or with edge beams , there will be torsion at the corners and therefore the torsion reinforcement is to be provided. In this type of construction of slabs main beams and secondary beams are provided, because of the monolithic construction of slab with beams the corners of slab are held down ,as slab is spanning in the both directions steel in the each direction is considered as main steel .The secondary beams are in turn supported on main beams and are designed as continues beams .The main beams are supported on Columns or walls .The load transferred to main beams from secondary beams is the reaction coming from continuous beams .The load carried to each support and the bending moments in slabs mentioned above are not easy to calculate if all the conditions are to be satisfied.Therefore ,empirical formulae and the approximate theories have been formulated which gives bending moments in the slabs supported on all four edges .The method applicable in particular case depends on the support conditions and type of the load . Bending moments can be obtained by one of the following methods The theory of Grashoff and Rankine , Method of Dr Marcus ,ISI code method and Mr pigeaud's theory of load distribution of concentrated loads in two directions , It can also be used for UDL on slabs .