STRUCTURAL EFFICIENCY OF FLAT SLAB FLOOR SYSTEMS BY EQUIVALENT FRAME METHOD

Abstract

The structural function of floor system is to collect the gravity load in all forms including its own weight and transmit them to vertical structural elements through the combined capacity in flexure and shear. In the case of flat slab floor systems, the slab is directly supported on the columns without beams. As a result all loads supported by slab converge to the columns. Moments and shear values are usually the largest over the columns. However when spans are relatively small and imposed load is low, the thickness of slab can be increased to reduce the stresses at slab-column joint which would result in providing greater effective depth for negative moment occurring near the column. With increasing span and live load intensity this does not make an efficient arrangement due to the requirement of very thick slab. This results in catering primarily to dead load stresses. An ingenious approach is to flare the column as it meets the slab (column capital) which reduces the shear stress substantially alongwith the dead load of the floor. The column head stiffens the slab and thereby helps in controlling the floor deflection. Thus, floor spans can be increased to some extent. Beyond a certain range of span, further stiffening of the slab is required. This is achieved by increasing the slab thickness over a portion of the slab panel around the column capital. This portion of the slab is referred to as drop panel. In the present study, attempts have been made to ascertain the range of spans for square and rectangular slab panel which would be structurally most efficient for a given intensity of live load. Structural parameters have been varied in a systematic manner as to obtain a structural floor of minimum dead load. The minimum dead load of the floor would also result in saving of reinforcing steel. Results of the study have been presented in appropriate tabular form as to arrive at suitable conclusions.