Seismic Demand Prediction Models for the Inter‐Story Isolated Building Frame Subjected to Near‐Fault Pulse‐Type Ground Motions

ABSTRACT The present study investigates the effects of directivity‐induced near‐fault pulse characteristics on the inter‐story drift ratio (IDR) and peak floor acceleration (PFA) responses of inter‐story isolated (ISI) building frames. In the absence of a thorough understanding and general design methodology for ISI frames subjected to near‐fault pulse (NFP) type ground motions, this investigation addresses two key aspects: (i) evaluating the seismic behavior of ISI building frames under pulse characteristics using an equivalent mathematical pulse (EMP) model and (ii) proposing empirical seismic demand prediction equations based on responses from both recorded NFP‐type ground motions and the EMP model with pertinent modeling characteristics. These modeling characteristics are calibrated with the recorded NFP‐type ground motion to retain the same spectral characteristics. Non‐linear dynamic analysis is performed to assess IDR and PFA demands under both datasets, recorded NFP‐type ground motions and artificial time histories generated by the EMP model. The IDR response statistics obtained from the EMP model and recorded NFP‐type ground motions are compared, and the 84th percentile response shows a good agreement between the two observed datasets. However, the PFA response below the isolation layer is amplified under‐recorded NFP‐type ground motions compared to the EMP model. A detailed parametric investigation is carried out to understand the effects of amplitude, pulse period, oscillatory characteristics and phase angle of the EMP model. Moreover, significant deviations are observed in IDR and PFA responses for different isolation bearings when the pulse period approaches the fundamental period of the ISI building frame, while deviations diminish for higher modal periods. The ISI building system exhibits two distinct local peaks in the drift spectra corresponding to fundamental and second modal periods. Near the fundamental mode, the drift response is highly sensitive to the oscillatory characteristics of the EMP model. The maximum IDR ratios between the fundamental and second modal periods are observed as 3.24 and 1.75 for oscillatory characteristic values of 1 and 2.5, respectively. Finally, a set of new empirical seismic demand prediction equations is developed to obtain the expected IDR and PFA demands. The developed models demonstrate a coefficient of determination value greater than 0.80, which demonstrates the efficacy of the proposed model. In addition, a two‐step design procedure is recommended, considering the fault characteristics such as the moment magnitude and the closest distance to the rupture . Given the absence of general design guidelines for the ISI building frames, this investigation would be valuable to professional structural engineers, especially in the near‐fault region.