Keywords: seismic reliability, braced frames, confidence level.

Buckling-restrained braces (BRBs) have recently become popular for use as primary members of seismic lateral-force-resisting systems. A BRB is a steel brace that does not buckle in compression but instead yields in both tension and compression [1]. Although design guidelines for BRB applications have been developed, systematic procedures for assessing performance and quantifying reliability are still required. This paper presents an analytical framework for assessing the reliability of buckling-restrained braced frames (BRBFs) when subjected to seismic loads.

Recently, the concept of performance-based design (PBD) has become popular for many types of structures subjected to seismic loading. Structural performance in recent earthquakes has revealed the weakness of current design codes and has shown the need for new concepts and methods for performance evaluation and design. However, in earthquake engineering, precisely modelling the true behaviour of a structural system under the action of future seismic excitations for design purposes is very difficult. This is mainly as a result of the uncertain nature of the expected earthquake ground motions on a site, assumptions made in the structural analysis and the lack of knowledge in structural behaviour modelling. A reliability-based framework for analysis and design is most suitable in the evaluation and design process.

In the structural performance evaluation the most important limit state is collapse. Here, the dynamic behaviour of the structures near collapse is complicatedly nonlinear so that the capacity against collapse is very complex and highly dependent on the excitation. Incremental dynamic analysis (IDA) has been used to determine the capacity against incipient collapse of frame structural systems [2].

In this paper, the IDA method is applied to evaluate and quantify the seismic performance of two-dimensional BRB frames within a probabilistic-based framework. The basic procedure consists of the evaluation of demand and capacity of structural responses for the desired performance level. Then, the ratio of demand to capacity is calculated with the uncertainty factors. This ratio is used to evaluate the confidence level of the probability that the structure would experience it in its worst state performance level. During the reliability analyses, the epistemic uncertainty in the capacity predictions was accounted for explicitly, and, as a result, the probabilities of brace failure were calculated in terms of mean probability, 90% confidence level probability, and 95% confidence level probability. The results of the comparison show that the effectiveness of braced frames can vary significantly with ground motions and building rises.

1

B.M. Andrews, J. Song, L.A. Fahnestock, "Assessment of Buckling-Restrained Braced Frame Reliability Using an Experimental Limit-State Model and Stochastic Dynamic Analysis", Earthquake Engineering and Engineering Vibration, 8, 373-385, 2009. doi:10.1007/s11803-009-9013-8

2

D. Vamvatsikos, C.A. Cornell, "Incremental Dynamic Analysis", Earthquake Engineering and Structural Dynamics, 31, 491-514, 2002. doi:10.1002/eqe.141

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