Analytical fragility assessment using unscaled ground motion records

Abstract

Summary It is desirable that nonlinear dynamic analyses for structural fragility assessment are performed using unscaled ground motions. The widespread use of a simple dynamic analysis procedure known as Cloud Analysis, which uses unscaled records and linear regression, has been impeded by its alleged inaccuracies. This paper investigates fragility assessment based on Cloud Analysis by adopting, as the performance variable, a scalar demand to capacity ratio that is equal to unity at the onset of limit state. It is shown that the Cloud Analysis, performed based on a careful choice of records, leads to reasonable and efficient fragility estimates. There are 2 main rules to keep in mind for record selection: to make sure that a good portion of the records leads to a demand to capacity ratio greater than unity and that the dispersion in records' seismic intensity is considerable. An inevitable consequence of implementing these rules is that one often needs to deal with the so‐called collapse cases. To formally consider the collapse cases, a 5‐parameter fragility model is proposed that mixes the simple regression in the logarithmic scale with logistic regression. The joint distribution of fragility parameters can be obtained by adopting a Markov Chain Monte Carlo simulation scheme leading directly to the fragility and its confidence intervals. The resulting fragility curves compare reasonably with those obtained from the Incremental Dynamic Analysis and Multiple Stripe Analysis with (variable) conditional spectrum–compatible suites of records at different intensity levels for 3 older reinforced concrete frames with shear‐, shear‐flexure‐, and flexure‐dominant behavior.

Effects of the long-term behaviour of isolation devices on the seismic response of base-isolated buildings

“…The cloud method is adopted for the fragility analysis, based on fitting a linear regression model in the logarithmic scale to the pairs of an engineering demand parameter (EDP), representative of the structural response, versus an intensity measure for a suite of unscaled ground motions. In Figures and are reported cloud data of the EBI and ESBI structures, respectively, obtained by combining a scalar demand to capacity ratio of the EDP, that is equal to unity at the onset of the ultimate limit state, with an intensity measure …”

“…In order to estimate the probability that a given level of damage will occur in the EBI and ESBI structures, undergoing a given ageing time at different air temperatures, fragility analysis needs to be developed. To this end, a robust fragility software based on the cloud data above described is adopted for an efficient fragility assessment . Fragility curves of the EBI and ESBI structures are reported in Figures and , respectively, with reference to the superstructure and base‐isolation system.…”

Soil‐Structure Interaction Effect on Fragility Curve of 3D Models of Concrete Moment‐Resisting Buildings

“…Jalayer and Cornell used IDA concept to estimate total dynamic instability capacity of a regular RC structure [8]. Robust fragility assessment for Cloud Analysis considering the global dynamic instability and IDA can also be found in provided detail by Jalayer et al [9]. In addition, their research includes details about how a modified version of the Cloud Analysis can officially handle the existence of records that trigger collapse.…”

Cloud to IDA: Efficient fragility assessment with limited scaling

“…On the other hand, Baker and Cornell and Lin and Baker suggest that excessive scaling of records within the IDA procedure may lead to ground motion waveforms whose frequency content and duration might not represent the corresponding intensity level. This might manifest itself in terms of a bias in the IDA‐based fragility curve with respect to fragility curves obtained based on no scaling or spectral‐shape‐compatible scaling …”