Revisiting flexural overstrength in RC beam-and-slab floor systems for seismic design and evaluation

Abstract

Beam flexural overstrength is an important parameter in seismic design and evaluation of reinforced concrete (RC) moment resisting frames. It affects the column-to-beam flexural strength ratio (CBSR) which is a key parameter to avoid unfavorable column-driven mechanisms. In RC beam-and-slab floors, interaction between beam growth and slab panels is of a very complex nature which significantly alters the beam flexural overstrength. Enhancement of beam flexural capacity in slab floor systems depends on several parameters that should be considered on a case-by-case basis. To include some of the effects that were not considered in previous studies due to physical limitations of experimental works and lack of appropriate computational tools, a generic two-by-two bay archetype story model with a beam-and-slab floor system were considered for further investigations by means of numerical modelling. In order to consider the spatial effects of the floor system, several computational models were assembled by changing the span and cross section of beams in terms of depth, width and reinforcement ratio as well as the thickness of slabs to perform a parametric study on beam flexural overstrength ratio. Multilayer nonlinear shell elements were used for finite element modelling of slab panels to account for the kinematic effects of beam elongation along with the restraining effects of the floor slab. According to the results of analyses, variation of positive and negative flexural overstrength ratios of beams at different performance levels were presented for different parameters. Results demonstrate that the flexural overstrength ratio is greatly affected by the level of chord rotation (beam lateral drift or performance level) reinforcement ratio and the type of connection. However, beam-to-slab flexural stiffness ratio as well as beam length are not overly important parameters for flexural overstrength evaluation. This study reveals that, despite code prescriptions, CBSR should not be considered identical for all cases; hence, it should be revisited based on the desired performance level, type of connection and the beam reinforcement ratio.