Improved Seismic Design Recommendations for Wide-Flange Columns in Ductile Steel Moment-Resisting Frames considering Three-dimensional Response
Creators: Abrar Islam
Publisher: name: University of Alberta Library
Year: 2022

<p>Deep wide-flange columns are commonly used in the construction of steel moment-resisting frames (MRFs) to resist lateral seismic loads in high seismic regions in North America. Past studies on deep columns with base plastic hinges located at the first storey of MRFs have indicated that such sections can be prone to significant axial shortening and out-of-plane instability under design-level seismic excitation. Despite significant advancement in the seismic performance of steel wide-flange columns, the effect of limit states observed in the past studies on the member seismic stability response has not been quantified in the framework of the Canadian design practice. Moreover, the influence of three-dimensional response of steel MRFs on the stability of the first-storey columns with base plastic hinging using more representative loading protocols expected under seismic loads, e.g., earthquake accelerations, has not been well comprehended yet. Finally, new supporting data is needed 1) to evaluate the current stability design requirements; and 2) to propose enhanced seismic stability recommendations to improve the design of steel MRFs in Canada. Therefore, this study aims to evaluate the stability response of wide-flange columns of Ductile steel MRFs under seismic loading and propose enhanced stability design recommendations in the framework of the Canadian steel design standard (CSA S16).</p>

<p>A prototype five-storey Ductile MRF was selected and designed following the CSA S16 seismic design provisions. Various column design scenarios were considered. The concentrated plasticity-based numerical model of the MRF was then developed, which was used to perform nonlinear response history analyses under the ground motion records representing three predominant seismic actions in western Canada. The global and local responses of the alternative designs were evaluated using nonlinear response history analysis (NLRHA) results. A continuum-based finite element<br>
model of the interior and exterior columns isolated from the prototype MRF designs was developed and subjected to the displacements and axial load histories obtained from the NLRHA. The results of the NLRHA of the frame and those from the CFEM of the isolated columns were used to evaluate the CSA S16 stability design requirements.</p>

<p>A continuum-based finite element model (CFEM) of an MRF subassembly consisting of the exterior bay plus half of the adjacent interior bay was also created to study the three-dimensional response of steel MRFs. A weak-axis bending moment loading protocol was created using the results of the NLRHA of the MRF subassembly, which in combination with in-plane cyclic displacement history and a constant gravity-induced axial compression load were used to perform a parameter study on column stability response using a refined CFEM representing isolated interior first-storey MRF columns. A total of 52 columns were analyzed by varying the section size, unbraced length, and axial load ratios.</p>

<p>The results of this study showed that the equivalent moment factor κ = 0.45 can be used for first-storey columns, which results in a lateral bracing limit of Lb/ry = 70. The CFEM of the MRF subassembly is a good tool to understand the three-dimensional demands of first-storey columns under seismic loading. Moreover, four strength and deformation response parameters, including base moment, axial shortening, out-of-plane displacement, and cross-section twist angle, can be used to determine column instability modes, including out-of-plane buckling at the base and member buckling. Column stability was detrimentally affected by the level of the constant gravity-induced axial load. Finally, a simple, coupled empirical equation as a function of the global slenderness ratio (Lb/ry), cross-section aspect ratio (d/bf), and axial load ratio (Cf/AFy) was proposed to predict the stability response of wide-flange columns with base plastic hinging in the framework of the Canadian steel design standard.</p>