Analysis and design of gusset plates in buckling restrained braced frames. (2020)
Type of ContentTheses / Dissertations
Thesis DisciplineCivil Engineering
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
Gusset plates (GP) are commonly used to connect buckling restrained braces (BRBs) to frame elements and must be designed to ensure BRBs perform as expected during realistic earthquake shaking.
This study focuses on the analysis and design of corner GPs without edge stiffeners in buckling restrained braced frames (BRBFs). The GP axial compressive strength is studied by means of: i) cyclic bidirectional testing of single story BRBF with a diagonal BRB, ii) numerical analysis of isolated GPs and the whole BRBF, and iii) analytical study of GPs and the BRB-GP system.
During the experimental program, three pairs of GP specimens with same thickness but different loading were tested to measure the effects of out-of-plane (OOP) frame drifts on the GP axial compressive strength. It was found from experimental studies under in plane (INP) loading only that smaller GP thickness resulted in lower buckling capacity. When a bidirectional loading was app lied, the GP strength ranged between 88% to 97% that obtained when only INP displacements were applied. In all cases, GP yielding was observed prior failure, thus GPs always buckle inelastically.
A parametric study was conducted using finite element analysis (FEA). The numerical models matched the experimental results to an accuracy better then 12% for most cases, and the buckling mode was the same as that in experiments. The models were used to further study the buckling behaviour of GPs under bidirectional loading. For a reference OOP drift of 2%, the GP strength reduced between 9% to 40% due to bidirectional loading. A simple model is proposed to conservatively estimate the strength reduction for different levels of OOP drifts.
A new concept is proposed to analyse GPs considering a realistic geometry and boundary conditions. Here, the GPs are divided into sub-elements and recommendations are presented to calculate their equivalent stiff ness. This approach provided conservative estimations of the GP buckling strength when compared with the results from the tests conducted and those reported in literature.
Using the GP subdivision approach, a design method named the System Stability Method (SSM) was developed. It is proposed to create an analytical model of the BRB GP system using t he GP sub-elements and the stiffness of all other elements in t he system including the flexibility of t he beam-column joints and the BRB restrainer end. Using t he system model, guidance for design is proposed based on the assumptions that significant yielding does not occur under combined INP and OOP loading.
A numerical tool was developed to facilitate the application of t he proposed method in practice. Finally, using this tool, a n example is provided with recommendations for each significant design consideration.
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