| Built-up bracing members have been built as structural members in trusses throughout the United States. These members have been designed to resist wind forces, but not earthquakes. Seismic evaluation of steel built-up members is difficult due to the lack of knowledge on the cyclic inelastic behavior of these members. Although some testing of laced members has been conducted by other researchers, results to date have mostly been project specific; consequently, it is difficult to draw general conclusions from those studies. The objective of this research is to provide additional knowledge on the seismic behavior of laced members, supported by experimental results, and that can be broadly applicable to many structures that share similar structural characteristics. To achieve a general understanding of the hysteretic behavior of built-up bracing members, hysteretic behavior of normal bracing members under repeated cyclic loading is reviewed. To understand how laced built-up bracing members were originally designed, design provisions and steel design textbooks published at the time when laced bracing were being built, are reviewed, along with recent research work conducted for built-up bracing members. From the survey of some of commonly used shapes and details that have been historically used for built-up members, a range of parameters typically encountered for built-up bracing members, including typical built-up member configurations and lacing geometry, typical b/t and KL/r ratios for the built-up members and their lacings, connection details, and other lacing characteristics, are extracted. Based on this information, a limited experimental program is designed and conducted to investigate the hysteretic behavior of built-up compression members, including design and quasi-static testing of the specimens. Strength capacity of the built-up bracing specimens obtained from the testing is correlated with the strength predicted by the AISC LRFD Specification. Assessments of hysteretic properties such as ductility capacity, energy dissipation capacity, and strength degradation after buckling of the specimen are performed. Non-linear pushover analyses are also conducted and correlated with test results. The low cycle fatigue life of the built-up bracing members is quantified and compared with that from the proposed models by other researchers. |
