STUDIES IN STRUCTURAL EFFICIENCY OF WAFFLE FLOOR SYSTEMS

Abstract

Waffle-slab has had its genesis in a rather thick solid-slab floor from which the bottom layer concrete in tension is partially replaced by their ribs along orthogonal directions. The concrete replacement and rib formation is done through appropriate size waffle formwork. The ribs are reinforced with steel to resist flexural tensile stresses. At this stage, the dimensions and spacing of ribs become a matter of choice to be decided in a manner so as to achieve better load distribution (structural efficiency). The spacing of the ribs are so kept as to obviate the need for shear reinforcement as also to make the system structurally behave as a slab rather than a system of interconnected parallel beams (Grid floor). Waffle-slab may be rested on a system of vertical supports (walls) in which case the floor moment and deflection get concentrated near the mid-span. The floor, on the other hand, can be framed into the vertical structural system such that there is dispersal of floor moment and deflection. There is lateral shifting of moment and deflection from the mid-span zone towards the supports. Thus, floor framing with the help of wide Band beams rigidly connected to a system of columns becomes an obvious choice. In the present study, Waffle-slab floor system has been studied with a view to achieving better load dispersion through suitable configurations of ribs. For a given floor plan, rib dimensions and the number of ribs have been incremented in an attempt to achieve better moment dispersion at the lowest dead load percentage. The waffle-slab has been taken as monolithically connected to Band beams. Attempts have been made to achieve structural efficiency by ascertaining the above mentioned parameters which would minimize the dead load and hence its moment for a given floor plan. Analysis has been carried out by Stiffness Matrix Method. Feasibility of structural design of members has been ensured under the provision of IS 456:2000. Results have been presented in suitable tabular fashion so as to arrive at appropriate conclusions.