Gutiérrez-Urzúa, LF; Freddi, F; Di Sarno, L; (2020) Existing Steel Moment Resisting Frame Buildings: a Critical Comparison of Engineering Demand Parameters. In: Proceedings of the 17th World Conference on Earthquake Engineering (17WCEE). World Conference on Earthquake Engineering (WCEE): Sendai, Japan.

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Abstract

Recent seismic events have continued to underline the high earthquake vulnerability of existing buildings structures and the associated direct (e.g., injuries, casualties, repair cost) and indirect (e.g., downtime) losses. Therefore, there is an urgent need for advanced code standards able to effectively assess their seismic vulnerability allowing a careful evaluation of their safety. A reliable assessment is crucial for determining whether the structure performs satisfactorily or not; if it requires of measures to mitigate the effect of the earthquake loads; or the possibility of including a retrofit solution to bring up its behavior to the required levels of performance. Therefore, modern assessment standards, such as the Eurocode 8 Part 3 in Europe, and the ASCE 41 in the United States, have been developed to provide guidelines to study the performance of existing structures. These codes propose methods to gather information from the structure, to deal with the uncertainty attained to the data collecting process and to perform linear and non-linear structural analyses. The European code provides with performance requirements and compliance criteria for different limit states in a prescriptive way. In contrast, the American regulations offer suggestions on the hazard levels related to the different limit states, and provide with acceptance criteria for each of them, allowing the stakeholders to make the final decision on the desired performance level. In particular, for steel moment resisting frames, the Eurocode has requirements for certain components, and provides detailed numerical compliance criteria for beams, columns and connections, similar to those provided by the older versions of the American code. However, in some cases these criteria ignore the interaction between simultaneous effects which may lead to the overestimation of the capacity. On the other hand, the ASCE 41-06 and ASCE 41-13 provide more detailed numerical acceptance criteria based on more recent research, but their results have been catalogued in the past as conservative for certain members. The ASCE 41-17 proposes some more advanced and less conservative criteria, however, its applicability may be limited in some cases due to the complexity of its formulations. This paper evaluates the seismic performance of existing, i.e., low-code, steel moment resisting frame buildings when assessed by using different code standards and considering different components. Two case studies are analyzed and both typologies are evaluated under the framework of the EC8-3 regulation and the three most recent versions of the ASCE 41 standard. Non-linear dynamic analyses have been carried out on a set of ground motions records perform Incremental Dynamic Analyses and hence to investigate the influence of the record-to-record variability and to derive fragility curves for the whole structure and for several local engineering demand parameters conventionally used in deterministic studies. The results confirm what observed in previous researches and show that, for the analyzed structures, the panel zones have a significant influence on their overall behavior and this outcome is common to all the considered codes. Moreover, the study allows to provide some insights on the impact of the inclusion of gravity and overturning axial loads in the definition of the rotation capacity of steel columns. This aspect is of particular interest as it is accounted for in modern code standard. In light of the revision of the Eurocode 8 Part 1, this study provides some preliminary information of the possible deficiencies of the current regulations and focuses on some of the challenges to be addressed in the new versions of the Eurocode 8 Part 3, in particular in those related to low-code SMRFs.

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