Case Study: Cable Tray Seismic Fragility Evaluation Involving Ductile Failure of a Cold-Formed Steel Section in Flexure

This paper presents a case study for a recent seismic fragility evaluation of cable trays at a nuclear power plant in the United States. The evaluation was initiated due to the presence of non-serrated strut nuts in cable tray supports throughout the plant, making the cable trays potentially vulnerable in a beyond design basis seismic event. The fragility methodology included a bounding case selection process developed specifically for this evaluation. The evaluation incorporated and applied plant-specific, full-scale, cable tray shake table test results, as well as pull tests on the non-serrated strut nuts. The test results were used to develop seismic capacities for the selected bounding cable tray support configurations, including shear slip capacities of the strut nuts.

A few load path elements in the as-built configuration, including several cold-formed steel sections, were not represented in the tested configuration. Therefore, the shake table test results were supplemented by performing new ultimate strength tests on flexural failure of cold-formed sections similar to those in the field. This empirical approach proved to be a cost-effective and reliable alternative to the more theoretical analytical design methods available from the American Iron and Steel Institute (2013) for evaluating the post-yield behavior of these cold-formed steel sections. Finally, the flexural test results were used to judge applicable plastic limit states, realistic ultimate capacities, and corresponding variability for the cold-formed steel sections. The seismic fragility was governed by flexural failure of the cold-formed steel support, although the capacity of the non-serrated strut nuts was only slightly higher. In the future, the ultimate strength test results may be used directly or further extended to calculate inelastic energy absorption factors for fragility evaluations involving flexural failure of cold-formed steel sections.

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