No abstract

No abstract

This study examines the influence of ground motion duration on the collapse capacities of a modern, five-story steel moment frame and a reinforced concrete bridge pier. The effect of duration is isolated from the effects of ground motion amplitude and response spectral shape by assembling sets of "spectrally equivalent", long and short duration records, and employing them in comparative non-linear dynamic analyses. For the modern steel moment frame, the estimated median collapse capacity is 29% lower when using the long duration set, as compared to the short duration set. For the concrete bridge pier, the collapse capacity is 17% lower. A comparison of commonly used duration metrics indicates that significant duration is the most suitable metric to characterize ground motion duration for structural analysis. Sensitivity analyses to structural model parameters indicate that structures with high deformation capacities and rapid rates of cyclic deterioration are the most sensitive to duration.

This study applies nonlinear dynamic analyses to assess the risk of collapse of reinforced concrete (RC) special moment frame (SMF) buildings, with the goal of quantifying the seismic safety implied by modern building codes. Thirty archetypical RC SMF buildings, ranging in height from 1 to 20 stories, are designed according to ASCE 7-02 and ACI 318-05 for a high seismic region. The results of performance-based seismic assessments find that, on average, these buildings have an 11% probability of collapse under ground motion intensities with a probability of exceedance of 2% in 50 years. The average mean annual rate of collapse of 3.1x10-4 collapses/year corresponds to an average 1.5% probability of collapse in 50 years. The study further examines the influence of specific design provisions on collapse safety. In particular, changes to the minimum seismic base shear requirement between ASCE 7 2002 and 2005 editions and variations in ACI 318 strong-column weak-beam

A state-of-the-art seismic performance assessment is illustrated through application to a reinforcedconcrete moment-frame building designed per current (2003) building code provisions. Performance is quantified in terms of economic losses and collapse safety. The assessment includes site-specific seismic hazard analyses, nonlinear dynamic structural response simulations to collapse, damage analyses, and loss estimation. When selecting ground motion records for nonlinear dynamic analyses that are consistent with a target hazard level expressed in terms of a response spectral value at the building's fundamental period, it is important to consider the response spectral shape, especially when considering higher hazard levels. This was done through the parameter commonly denoted by . Neglecting these effects during record selection is shown to lead to a factor of 5-10 overestimation of mean annual collapse rate. Structural response simulations, which properly account for uncertainties in ground motions and structural modelling, indicate a 2-7% probability of collapse for buildings subjected to motions scaled to a hazard level equivalent to a 2% probability of exceedance in 50 years. The probabilities of component damage and the means and coefficients of variation of the repair costs are calculated using fragility functions and repair-cost probability distributions. The calculated expected annual losses for various building design variants range from 0.6 to 1.1% of the replacement value, where the smaller losses are for above-code design variants and the larger losses are for buildings designed with minimum-code compliance. Sensitivity studies highlight the impact of key modelling assumptions on the accurate calculation of damage and the associated repair costs.

One of the challenges in assessing structural collapse performance is the appropriate selection of ground motions for use in the nonlinear dynamic collapse simulation. The ground motions should represent characteristics of extreme ground motions that exceed the ground motion intensities considered in the original building design. For modern buildings in the western United States, ground motions that cause collapse are expected to be rare high-intensity motions associated with a large magnitude earthquake. Recent research has shown that rare high-intensity ground motions have a peaked spectral shape that should be considered in ground motion selection and scaling. One method to account for this spectral shape effect is through selection of a set of ground motions that is specific to the building's fundamental period and the site hazard characteristics. This selection presents a significant challenge when assessing the collapse capacity of a large number of buildings or for developing systematic procedures, since it implies the need to assemble specific ground motion sets for each building. This paper proposes an alternative method, whereby a general set of farfield ground motions is used for collapse simulation, and the resulting collapse capacity is adjusted to account for spectral shape effects that are not reflected in the ground motion selection. The simplified method is compared with the more direct record selection strategy, and results of the two approaches show good agreement.

No abstract

No abstract