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11.19: Seismic fragility assessment of steel frames: Application of advanced reliability analysis

During the application of advanced design procedures (e.g. performance based design) engineers may need to compute the actual risk of the structures. The fragility of structures exposed to seismic effects is often characterized by the fragility curves that show the failure (or collapse) probability under an earthquake characterized by a given intensity measure. Since the accurate calculation of such curves can be computationally demanding and extremely complicated due to the high nonlinearity of the problem and the difficulties in describing the seismic effect, common methodologies often invoke significant simplifications: neglecting certain types of uncertainties, assuming certain distribution model, using simplified analysis procedures to calculate the mechanical response of the structure, etc. It is emphasized that uncertainties in the resistance are often handled in a vague manner in the calculations, since their relevance is thought to be secondary in comparison to the uncertainties in the seismic effect.