Slab participation in practical earthquake design of reinforced concrete frames

Recent amendments in the ACI318-99 that affect the estimation of nominal beam flexural capacity in seismic design of frame connections were the motivation for this paper. These changes concern the width of slabs considered effective in beam flexure and represent the culmination of a 15-year long concerted research effort in the U.S., Canada, New Zealand, and Japan aimed at understanding and quantifying slab participation in the lateral load resistance of frames through observed experimental findings. By accounting for slab participation, the Code recognizes the diaphragm action of slabs in the seismic response of frames, particularly when the slab is on the tension face of the beam. Note that until recently, it was an established design practice to neglect the presence of the slab in estimating beam stiffness and strength, except when the slab was located in the compression zone of the beam (known as T-beam design). Experimental evidence from tests on complete frames and slab-beam-column assemblies has illustrated that this practice resulted in the gross underestimation of beam flexural strength in the assumed plastic hinge regions (at the face of beam-column connections). This neglected source of beam flexural over-strength has significant consequences in the realization of the objectives of the established capacity design framework for reinforced concrete (RC) where beam shear design, joint dimensioning, and column flexure/shear detailing are controlled by the requirement of beam flexural yielding. This paper reviews the practical implications of the recent amendments in the sequence of seismic design and in assessment of the resulting response of RC frame structures. The effects (in terms of demand and supply) on strength and deformation capacity of the various mechanisms of response, and the hierarchy of failure modes in slab-beam-column connections are identified and discussed.