Evaluation of Quasi-Static Loading Protocols for Special Concentrically Braced Steel Frame Systems
Quasi-static cyclic testing has been a standard procedure to identify the performance of structural components and frame systems. The loading protocols are standardized in the provisions, where the universal quasi-static loading protocol for steel structures is based on the seismic responses of steel moment frame systems. However, different structural systems have different dynamic properties. The fundamental periods of steel concentrically braced frame (SCBF) system are usually shorter than those of steel moment frame systems, and the failure modes are also different for the two structural systems. As such, applying the universal quasi-static loading protocol to SCBF systems cannot accurately estimate the damage states of SCBF systems under seismic loading. Thus, it is essential to constitute a more representative quasi-static loading protocol for SCBF systems. This research will propose a quasi-static loading protocol for SCBF systems in terms of structural damage parameters, such as the cumulative plastic deformation, strain energy, etc. First, a series of ground motion records will be selected to represent various seismic hazard levels. The dynamic responses of SCBF systems to the ground motions will be summarized from non-linear dynamic analyses. A representative loading protocol will be proposed based on the analysis results, and then examined by component tests and frame tests. Eight brace components tests with two slenderness ratios and four different quasi-static loading protocols, including the proposed one from non-linear dynamic analyses, will be conducted in the first year of the proposed two-year research. The effects of quasi-static loading protocols on the brace components will be investigated. To further examine the complicated damage behavior of SCBF systems, two full-scale SCBF tests will be conducted in the following year. The global and local failure modes of the SCBF systems will be explored. The research will also employ finite-element analysis to help construct the relationship between the quasi-static loading protocols and the damage states of the structural systems. The research will investigate the current quasi-static loading protocols and propose a more appropriate loading protocol to present the failure modes of SCBF systems under seismic excitations properly. With the proposed loading protocol, engineers and researchers can estimate the performance and the capacity of the SCBF systems more accurately when conducting a quasi-static cyclic testing.
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