Developing replaceable members for steel lateral load resisting systems

Bozkurt, Mehmet Bakır
Steel structures utilize lateral load resisting systems to provide sufficient strength, stiffness and ductility. Damaged structures need to be either demolished or retrofitted to recover their initial properties after a major earthquake. In steel structures, damage is concentrated to predefined fuse members and most other members are designed to behave elastic under seismic events. In buckling restrained braced frames (BRBFs) and eccentrically braced frames (EBFs), the fuse members are well defined and can be conveniently repaired. In the literature, experimented studies were conducted to develop fuse members for BRBFs and EBFs. This thesis reports findings of a three-phase experimental research program on steel encased buckling-restrained braces (BRBs) and a two-phase experimental research program on eccentrically braced frames with replaceable links. The first experimental research program investigated the potential use of steel encased BRBs using subassemblage testing. Because steel encasements can provide lighter solutions, they are more advantageous compared to concrete or mortar filled encasements in terms of replacement of BRBs. Pursuant to this goal, a three-phase experimental research program consisting of thirteen tests was conducted where BRBs were investigated under subassemblage testing. The first phase of the program aimed at studying the performance of steel encased BRBs which utilize constant width core plates. Test results indicated that these braces develop unacceptably high compression and tension resistances and the behaviors of these BRBs under uniaxial testing and subassemblage testing are markedly different. In second phase of the research program, a new type of BRB core, which utilizes a welded overlap, was developed to improve the cyclic performance observed in the first phase. Experimental results showed that the braces sustain axial strains that vary between 2.0 and 2.5% and resistances in tension and compression were found to improve significantly when compared with the findings of the first phase. Welded overlap core steel encased BRBs were found to sustain cumulative axial strains that are 419 times the yield strain when properly detailed. The third phase focused on connections of welded overlap steel encased BRBs. Two typical connection details, namely the pin connection and gusseted connection, were experimented by considering the collar detail as the prime variable. Test results indicate that the gusseted detail does not require collars to be used while the pinned detail mandates the use of collars for acceptable performance. The second experimental research program concentrated on developing replaceable links for steel eccentrically braced frames. A replaceable link detail, which is based on splicing braces and the beam outside the link, was proposed. This detail eliminates the need to use hydraulic jacks and flame cutting operations for replacement purposes. The first phase of the research program concentrated on replaceable links with direct brace attachments while the second phase concentrated on links with gusset plate connected brace attachments. Performance of these proposed replaceable links was studied by conducting eight full-scale EBF tests with directly attached braces and eleven full-scale EBF tests with gusset plate connected braces under quasi-static cyclic loading. The link length ratio, stiffening of the link, loading protocol, connection type, bolt pretension, gap size of splice connections, and demand-to-capacity ratios of members were considered as the prime variables. The specimens primarily showed two types of failure modes: link web fracture and fracture of the flange at the link-to-brace connection. No failures were observed at the splice connections indicating that the proposed replaceable link details provide excellent response. The inelastic rotation capacity provided by the replaceable links satisfied the requirements of the AISC Seismic Provisions for Structural Steel Buildings (AISC341-10). The overstrength factor of the links exceeded 2.0 which is larger than the value assumed for EBF links by design provisions. The high level of overstrength resulted in brace buckling in one of the specimens with direct connected brace and one of specimens with gusset plate connected brace which demonstrated the importance of overstrength factor used for EBF links.
Citation Formats
M. B. Bozkurt, “Developing replaceable members for steel lateral load resisting systems,” Ph.D. - Doctoral Program, Middle East Technical University, 2017.