The role of ground motion duration and pulse effects in the collapse of ductile systems

The seismic collapse capacity of ductile single‐degree‐of‐freedom systems vulnerable to P‐Δ effects is investigated by examining the respective influence of ground motion duration and acceleration pulses. The main objective is to provide simple relationships for predicting the duration‐dependent collapse capacity of modern ductile systems. A novel procedure is proposed for modifying spectrally equivalent records, such that they are also equivalent in terms of pulses. The effect of duration is firstly assessed, without accounting for pulses, by assembling 101 pairs of long and short records with equivalent spectral response. The systems considered exhibit a trilinear backbone curve with an elastic, hardening and negative stiffness segment. The parameters investigated include the period, negative stiffness slope, ductility and strain hardening, for both bilinear and pinching hysteretic models. Incremental dynamic analysis is employed to determine collapse capacities and derive design collapse capacity spectra. It is shown that up to 60% reduction in collapse capacity can occur due to duration effects for flexible bilinear systems subjected to low levels of P‐Δ. A comparative evaluation of intensity measures that account for spectral shape, duration or pulses, is also presented. The influence of pulses, quantified through incremental velocity, is then explicitly considered to modify the long records, such that their pulse distribution matches that of their short spectrally equivalent counterparts. The results show the need to account for pulse effects in order to achieve unbiased estimation of the role of duration in flexible ductile systems, as it can influence the duration‐induced reduction in collapse capacity by more than 20%

Collapse risk and residual drift performance of steel buildings using post-tensioned MRFs and viscous dampers in near-fault regions

The potential of post-tensioned self-centering moment-resisting frames (SC-MRFs) and viscous dampers to reduce the collapse risk and improve the residual drift performance of steel buildings in near-fault regions is evaluated. For this purpose, a prototype steel building is designed using different seismic-resistant frames, i.e.: moment-resisting frames (MRFs); MRFs with viscous dampers; SC-MRFs; and SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as stiffness and strength deterioration. A database of 91 near-fault, pulse-like ground motions with varying pulse periods is used to conduct incremental dynamic analysis (IDA), in which each ground motion is scaled until collapse occurs. The probability of collapse and the probability of exceeding different residual story drift threshold values are calculated as a function of the ground motion intensity and the period of the velocity pulse. The results of IDA are then combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess and compare the collapse risk and the residual drift performance of the frames. The paper highlights the benefit of combining the post-tensioning and supplemental viscous damping technologies in the near-source. In particular, the SC-MRF with viscous dampers is found to achieve significant reductions in collapse risk and probability of exceedance of residual story drift threshold values compared to the MRF. © 2016 Springer Science+Business Media Dordrech

Updated ground motion spectral matching requirements in the 2015 NEHRP recommended seismic provisions

The Building Seismic Safety Council (BSSC) recently completed a multi-year effort to rewrite Chapter 16 of the ASCE/SEI 7-10 Standard (2010), which governs ground motion selection and modification for new building projects in the United States. This paper provides an overview of new recommended provisions regarding the use of spectral matching of ground motions. Example analyses of three buildings are shown to illustrate the reasoning behind the spectral matching language, and to demonstrate the relative differences in response estimates obtained from scaled versus spectrally matched motions. For these examples, the demands placed on the buildings by spectrally matched motions developed using the recommended provisions were comparable to the demands produced by amplitude scaled motions. The consistent demands produced by the procedures suggest that evaluations of a building design’s acceptability are comparable whether scaled or spectrally matched motions are used. The revised provisions should significantly advance the practice of ground motion selection and modification in the United States and elsewhere, by aligning practice with research outcomes

Updated ground motion spectral matching requirements in the 2015 NEHRP recommended seismic provisions

The Building Seismic Safety Council (BSSC) recently completed a multi-year effort to rewrite Chapter 16 of the ASCE/SEI 7-10 Standard (2010), which governs ground motion selection and modification for new building projects in the United States. This paper provides an overview of new recommended provisions regarding the use of spectral matching of ground motions. Example analyses of three buildings are shown to illustrate the reasoning behind the spectral matching language, and to demonstrate the relative differences in response estimates obtained from scaled versus spectrally matched motions. For these examples, the demands placed on the buildings by spectrally matched motions developed using the recommended provisions were comparable to the demands produced by amplitude scaled motions. The consistent demands produced by the procedures suggest that evaluations of a building design’s acceptability are comparable whether scaled or spectrally matched motions are used. The revised provisions should significantly advance the practice of ground motion selection and modification in the United States and elsewhere, by aligning practice with research outcomes