Structural responses considering the vertical component of earthquakes

The guidelines in the NEHRP Provisions and the Mexican Code regarding the e ffects of the vertical component of earthquakes on the response of frames a re re-evaluated. Using a time domain nonlinear finite element program devel oped by the authors, the seismic responses of frames are evaluated realisti cally by simultaneously applying the horizontal and vertical components of earthquake motion. Three steel frames and 13 recorded earthquake motions ar e considered. The same response parameters are then estimated using the two codes, and their error is evaluated. It is found that, if the frames remai n elastic, the NEHRP Provisions estimate the maximum horizontal deflection at the top of the frames and the bending moment in the columns very accurat ely; the Mexican Code overestimates them. If the frames develop plastic hin ges, the Mexican Code conservatively overestimates them, but the NEHRP Prov isions underestimate them in some cases. Both codes significantly underesti mate the axial loads in columns. The underestimation increases as the frame s develop plastic hinges. The underestimation is more for interior columns than for exterior columns. If the ratio R of the PGA of the vertical and ho rizontal components of an earthquake is higher than normal, the underestima tion increases as R increases. The underestimation is not correlated with f rame height. The vertical component may increase the axial load significant ly. Since they are designed as beam-columns, the increase in the axial load will have a very detrimental effect on the performance of the columns. In light of the results obtained in this study, the design requirements for th e vertical components need modification. At the very least, further study i s required. (C) 1999 Elsevier Science Ltd. All rights reserved.