Structural responses considering the vertical component of earthquakes

The guidelines in the NEHRP Provisions and the Mexican Code regarding the effects of the vertical component of earthquakes on the response of frames are re-evaluated. Using a time domain nonlinear finite element program developed by the authors, the seismic responses of frames are evaluated realistically by simultaneously applying the horizontal and vertical components of earthquake motion. Three steel frames and 13 recorded earthquake motions are considered. The same response parameters are then estimated using the two codes, and their error is evaluated. It is found that, if the frames remain elastic, the NEHRP Provisions estimate the maximum horizontal deflection at the top of the frames and the bending moment in the columns very accurately; the Mexican Code overestimates them. If the frames develop plastic hinges, the Mexican Code conservatively overestimates them, but the NEHRP Provisions underestimate them in some cases. Both codes significantly underestimate the axial loads in columns. The underestimation increases as the frames 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 horizontal components of an earthquake is higher than normal, the underestimation increases as R increases. The underestimation is not correlated with frame height. The vertical component may increase the axial load significantly. 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 the vertical components need modification. At the very least, further study is required.