Seismic Performance and Design of Steel Multi-Tiered Buckling-Restrained Braced Frames
Creators: Bani, Moad A
Publisher: name: University of Alberta Library
Year: 2023

<p>Steel Multi-Tiered Buckling-Restrained Braced Frames (MT-BRBFs) are commonly used in <br>
moderate-to-high seismic regions of Canada and United States as the lateral-load resisting systems<br>
of tall single-storey buildings, such as sports facilities, airplane hangars, and warehouses, as well as <br>
in tall stories of multi-storey buildings. MT-BRBFs consist of two or more bracing panels stacked <br>
vertically between column out-of-plane support locations. A multi-tiered configuration is utilized <br>
when the use of a single bracing panel within a storey height is not practical nor economical. <br>
Although MT-BRBFs enjoy robust cyclic performance and large ductility capacity of their BucklingRestrained Braces (BRBs), their seismic response differs from standard multi-storey BRBFs. <br>
Namely, lateral deformation under seismic loads may not evenly distribute along the frame height in <br>
MT-BRBFs as the tiers with tension-acting BRBs tend to deform more than those with compressionacting BRBs. This response may induce in-plane flexural demands on the braced frame columns, <br>
which may lead to plastic hinge formation or even column instability in the presence of a large axial <br>
force induced due to gravity loading and BRB capacity design forces. Furthermore, uneven<br>
distribution of frame lateral deformation can impose excessive strain demands on the BRBs yielding <br>
in tension, which can potentially cause fracture in the BRB core. In Canada, there are no design <br>
guidelines for MT-BRBFs in the 2019 Canadian steel design standard, CSA S16-19. Special design <br>
requirements were introduced for MT-BRBFs in the 2016 edition of AISC Seismic Provisions in the <br>
U.S. to improve column stability response and control tier drift demands. However, very limited <br>
supporting research data is available to verify these requirements. Given the extensive application of <br>
MT-BRBFs, often times in critical structures, there is an urgent need to develop a better <br>
understanding of their seismic response, estimate seismic force and deformation demands on their members, evaluate the current U.S. seismic design provisions and propose potential improvements, <br>
and develop an enhanced design method in the framework of CSA S16.<br>
This M.Sc. research project aims to evaluate the seismic response of steel MT-BRBFs designed to <br>
the Canadian and U.S. provisions and propose enhanced analysis and design methods to better <br>
represent MT-BRBF seismic response with the focus on column force and BRB strain demands. A <br>
combination of mechanics principles, structural analysis techniques, numerical simulation and <br>
experimental testing is used to achieve these objectives. A full-scale test program is conducted on a <br>
two-tiered BRBF to verify experimentally the behaviour of MT-BRBFs under seismic loading. The <br>
test frame is part of a prototype tall single-storey building located in Seattle, WA (AISC 341-10 <br>
design) or in Vancouver, BC (CSA S16 design). The results show that frame lateral deformation is <br>
unevenly distributed between the tiers and that the columns experienced significant in-plane bending <br>
due to this response. Moreover, large deformation demands develop in the BRBs, but no low-cycle <br>
fatigue fracture was observed under the applied loads. The results of the experiment also confirmed<br>
the need for an improved seismic design method for MT-BRBFs. A fibre-based numerical model of <br>
MT-BRBF is then developed and used to perform an extensive nonlinear response history analysis<br>
on a wider range of frames. The analysis results confirm that frame inelastic deformation tends to <br>
concentrate in the tier(s) undergoing tension yielding as they exhibited relatively lower post-yield <br>
stiffnesses and storey shear resistances than the tier(s) undergoing compression yielding. This lateral <br>
deformation pattern induces significant in-plane bending on the columns resulting in yielding or <br>
column buckling in some cases and causes excessive deformations in BRBs. A set of analysis and <br>
design methods is proposed in the framework of Canadian and U.S. seismic provisions. The proposed <br>
methods predict column moment demands and BRB strain with sufficient accuracy resulting in a <br>
safer and more economical design.</p>