Seismic Behavior of Cold-Formed Steel-Framed Wall-Line Systems in Mid-Rise Buildings

Cold-formed steel (CFS) framing offers many benefits to buildings in seismically active regions. Amongst the most notable CFS attributes include its low fabrication and maintenance costs, noncombustible and corrosion resistant nature, high durability and ductility. These benefits have made CFS framing a popular choice for construction of low-rise and mid-rise structures. From a seismic performance perspective, the light weight and ductility offered by a CFS-framed structure aligns with system resiliency needs in moderate to high seismic zones. Although experimental data exists documenting the performance of isolated CFS-framed shear walls, the structural lateral force resisting systems (LFRS) in CFS-framed buildings are constructed and integrally attached to non-designated systems, such as gravity walls as well as various nonstructural components. The contribution of the non-designated systems and the nonstructural components towards the response of wall-lines within the building system under high intensity earthquake shaking is not well understood. Moreover, experimental data to support code guidelines in current North American standards for design of CFS-framed shear walls, which meet the seismic demands for mid-rise buildings (>6 stories) are lacking. Indeed, the paucity of full-scale test data documenting the behavior of wall-line systems detailed for mid-rise buildings has been a barrier to bringing the potential benefits of CFS framing to the community. To address these limitations, a two-phased experimental program was undertaken in this dissertation to advance the understanding of CFS-framed steel sheet sheathed shear walls placed in-line with gravity walls. Referred to herein as “wall-lines”, these test specimens were detailed to support the lateral load demands anticipated of mid-rise buildings in high seismic zones. In the first phase, wall-line assemblies were tested at full-scale on a shake table, first under a sequence of increasing amplitude (in-plane) earthquake input motions, and subsequently under slow monotonic pull conditions (for select specimens). In the second phase, wall-line assemblies were tested under quasi-static reverse cyclic displacement-controlled loading using a simulated floor-load imposed via hydraulic actuators. Steel sheet sheathed shear walls offered energy dissipation primarily through structural member-to-sheathing connections and yielding of the steel sheet. All specimens demonstrated a tension field that spread across the entirety of the steel sheet at failure. The impact of different test variables governing the structural and nonstructural detailing on the seismic performance of the CFS-framed wall-line specimens is quantified by careful systematic comparison between different configurations. Wall-line assemblies with interior and exterior finish demonstrated substantially increased strength and stiffness without any decrease in drift capacity or change in failure mode. Specimens with hold-downs offered a larger lateral strength compared to specimens with tension tie-rods. However, hold-downs reached their capacity at higher drift demands whereas tension tie-rods remained linear elastic, even though both wall-lines with the different tie-down systems were designed for same overstrength force levels. The second part of this work involved a comprehensive numerical modeling effort, using prior experimental findings, both of the wall-line experiments discussed herein as well as a previous mid-rise six-story building specimen tested at full-scale using a suite of earthquake excitations. The developed finite element model takes into consideration the major assemblies, beyond just the isolated shear walls, which influence the dynamic response of the system, such as the strength and stiffness contribution from gravity walls as well as nonstructural components such as exterior and interior finishes installed over the shear wall and gravity wall segments. In this phase, as is common in west coast practice in the United States, a continuous tie-rod system is also modeled to capture the cumulative floor displacements caused by the axial elongation in the steel rods. The effect of built-up stud packs on strength, stiffness and drift parameters of a shear wall is also considered in the nonlinear hysteretic material model of shear walls. Very good agreement between numerical predictions and available experimental seismic response data of the six-story test building demonstrates that the proposed numerical model scheme can be employed to predict the seismic response of mid-rise CFS-framed buildings. Development of such a numerical model is an essential tool for enabling performance-based seismic design of cold-formed steel structures in this rapidly growing industry.

Doctoral dissertation, University of California San Diego, La Jolla, CA.