Estimation of effective slab strip width of RC floor systems for plane frame idealization

Abstract

There are two distinct methods (Direct Design Method and Equivalent Frame Method) of plane frame idealization for the analysis of RC frame buildings for vertical loads but there is a lack of well-guided code for plane frame idealization for lateral load analysis. In 2D plane frame analysis, an appropriate width of the floor panel needs to be selected, which is structurally representative of the actual 3D frame. The ratio of the width of this slab strip to the total panel width is known as effective slab strip ratio. In this work, an investigation is performed to study the effects of various parameters related to framed RC floor systems such as column dimensions, bay width, span length, slab thickness, number of floor, floor height etc, on the effective width of slab strip. To carry out the investigation, both 2D and 3D frames with floor systems were modeled using finite elements. In 3D model, the slab is modeled using shell elements, columns are modeled using solid elements and beams are modeled using frame elements. In 2D model, both columns and beams are modeled using frame elements. Applying a set of lateral load on the 3D frame, lateral deflection was computed. Then applying the same lateral load on the 2D frame having same column properties and at the same time adjusting the moment of inertia of the beams, the deflections are matched as closely as possible. When deflections are matched, the moment of inertia of the beam of 2D model corresponds to the inertia of the structurally active part of the floor slab system of 3D model. Since slab thickness is known, the width can be calculated from the inertia. The process is a simple trial and error method. The whole process is carried out under various parametric conditions. It has been found that some parameters like slab thickness, number of span, number of floors, floor height and column dimension along bay have no effect on the effective slab strip width for the flat plate structure. On the other hand, span length, bay width and column dimension along span have been found to have significant effects on effective slab strip ratio for the flat plate structures. Similarly, number of span, number of floor, floor height and column dimension along bay have no effect on the effective slab strip ratio for slab having column line beam. Whereas, slab thickness, span length, bay width, column dimension along span and beam depth have been found to have significant effects on effective slab strip width for slab having column line beam. Based on the study on both flat plate structure and slab with column line beams, two empirical equations have been developed to estimate the width of structurally effective part of the slab. The accuracy of the proposed equations have been demonstrated by determining the effective width for several examples with arbitrarily chosen parameters within the applicable range and comparing the result with those obtained from FE analysis. It has been shown that the suggested equations predict the effective width with acceptable accuracy.