A PROPOSAL FOR DEVELOPING RISK-CONSISTENT BEHAVIOR FACTORS FOR CODE-COMPATIBLE SEISMIC DESIGN

Abstract eng:
The application of linear design procedures for seismic loading is based on the approximation of the nonlinear dynamic response of the structure via a linear structural model. To account for the beneficial effects of ductility, which allows trading off damage for lower design forces, EN 1998-1 adopts the behavior factor q to directly reduce (i.e. divide) the elastic design response spectrum. The same factor is also used to scale up the resulting deformations to approximate their actual value due to nonlinearity. Still, EN 1998-1 only provides values of the q-factor for a very limited number of systems without any guidance on quantifying it for others. In order to introduce new and innovative lateral load resisting systems into the code, researchers have at times proposed corresponding q-values, yet without much consensus: Each proposal comes with its own definition of a safety target and seismic performance assessment method, the latter often reflecting the limited resources available to the researchers. Overall, this uneven process lends little confidence to the proposed q-factors, vis-à-vis the target of achieving a uniform risk level across different systems and sites in Europe. Unlike in the US, where the well-received FEMA P-695 standard has settled this debate, Europe has not formulated a standard methodology (barring some recommendations) to define and validate the q-factors. As a direct remedy, a novel procedure is proposed for obtaining consistent values for q based on the definition of a set of structures to represent each class of buildings, the use of nonlinear static and dynamic analysis methods and the incorporation of the effect of aleatory and epistemic uncertainty on the actual systems’ performance to reach a uniform level of safety across the entire building population. The value added goes beyond the current state of art, offering a consistent risk basis for the seismic design of different systems that remain compatible with current uniform hazard design spectra while allowing an extension to future risk-targeted hazard maps.

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