The influence of out-of-straightness imperfection in Physical-Theory models of bracing members on seismic performance assessment of concentric braced structures

In physical theory models, the hysteretic response of concentric braces is simulated by modelling the bracings with two nonlinear fibre-section beam–column elements connected together with an initial out-ofstraightness imperfection (Δ0). The width of such imperfection is the parameter governing the numerical prediction of the brace buckling. In this paper, the accuracy of different formulations of Δ0 is investigated and validated against monotonic and cyclic experimental results from literature carried out on different bracing configurations. Correlations and measures of scatter between the predicted response and experimental performance are evaluated. A statistical analysis on both X-braced and inverted-V-braced frames based on Monte Carlo simulation is presented and discussed for the following reasons: (a) in order to account for the dispersion of the nonlinear dynamic-evaluated seismic performance due to epistemic uncertainties associated with examined formulations for camber width and (b) to provide modelling correction factors of simulated interstorey drift demand. The analysis shows that the formulations used for Δ0 affect the drift demand, the collapse mechanism and have a noticeably impact on the seismic response parameters at collapse. In particular, inverted-V-braced frames are more influenced than X-braced configurations, because the prediction of brace–beam interaction is very sensitive to Δ0.