Prediction of Seismic Demands in Building Structures

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Prediction of Seismic Demands in Building Structures

Author: Earl Kalkan | Size: 6.8 MB | Format: PDF | Quality: Unspecified | Publisher: Earl Kalkan | Year: 2006 | pages: 256

The precise characterization of ground motions incorporating site, source, distance and other effects and the accurate prediction of seismic demands at the component and system level are essential requisites for advancing performance-based design and evaluation methodologies. This research effort focuses on issues related to ground motion characteristics with particular emphasis on near-fault records and its interrelationship with seismic demand and ultimately in developing enhanced procedures for estimating deformation demands in structures for performance-based evaluation. Recent earthquakes have revealed an enhanced level of hazard imposed by ground motions recorded in the vicinity of causative faults associated with directivity effects. Both forward-rupture directivity and fling effects produce ground motions characterized by a strong pulse or series of pulses of long period motions. To highlight their potential damaging effects on building structures, the energy content of near-fault records were investigated by devoting special attention to forward-rupture directivity and fling effects and the influence of apparent acceleration pulses. A new demand measure called the effective cyclic energy (ECE) is developed and defined as the peak-to-peak energy demand imparted to structural systems over an effective duration that is equivalent to the time required for reversal of the system effective velocity. This energy term led to the evolution of a non-dimensional response index ( y cff ) as a new descriptor to quantify the destructive power of near-fault records. Based on validation studies conducted on numerous instrumented buildings, the ECE spectrum is proposed to estimate the input energy demand of multi-degree-of-freedom (MDOF) systems without performing nonlinear-time-history (NTH) analysis. In the final phase of the study, a new pushover analysis methodology derived from adaptive modal combinations (AMC) is developed to predict seismic demands in buildings. This procedure integrates concepts built into the capacity spectrum method recommended in ATC-40 (1996), the adaptive method originally proposed by Gupta and Kunnath (2000) and the modal pushover analysis advocated by Chopra and Gael (2002). A novel feature of the procedure is that the target displacement is estimated and updated dynamically during the analysis by incorporating energy based modal capacity curves in conjunction with constant-ductility demand spectra. Hence it eliminates the need to approximate the target displacement prior to commencing the pushover analysis. The methodology was applied to several vertically regular instrumented steel and reinforced concrete (RC) moment-frame buildings, and also validated for code-compliant vertically irregular steel and RC moment frame buildings. The comprehensive evaluation study including individual and statistical comparisons with benchmark responses obtained from NTH analyses demonstrate that the AMC procedure can reasonably estimate key demand parameters such as roof displacement, interstory drift, plastic rotations for both far-fault and near-fault records, and consequently provides a direct reliable tool for performance assessment of building structures.

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