Nonlinear Wind Behavior of Reinforced Concrete Walls Coupled With Steel Reinforced Concrete Coupling Beams

Alexander Patrick Hill

doi:10.7273/000005244

Coupling beams are links between structural walls within buildings that provide ductility and strength in an earthquake or windstorm. Although seismic design has often been the primary application of nonlinear analysis, there has been recent momentum to develop nonlinear design under modest wind demands. The steel reinforced concrete (SRC) coupling beam, simply an embedded I-beam beam encased in concrete, has been shown to produce proficient levels of strength and ductility under seismic loading demands compared to either conventionally reinforced or diagonally reinforced coupling beams, with recent research exploring the element’s performance under nonlinear wind loading demands. This study builds on that research, exploring how the longitudinal wall reinforcement ratio around the beam’s embedment affects the behavior of SRC coupling beams. Two half-scale SRC coupling beam specimens were designed for this research using the AISC 341-22 Section H5 provisions. Quasi-static testing was performed, with large amounts of fully reversed cyclic loading applied to the specimen that increased incrementally, with maximum rotations reaching three times the yield rotation. Loading of the beam coincided with shear and moment demands imposed on the wall, along with a constantly applied gravity load to better simulate the actual behavior of a shear wall-coupling beam connection. The wall, too, was designed according to AISC 341 Section H5 while also complying with the requirements of ACI 318-19 Section 18.10.6.5. The amount of wall reinforcement surrounding the steel beam’s embedment was the only tested variable, as the loading and design of all other components remained consistent between the two specimens. Both were designed with less reinforcement than what is required by the AISC 341-22 provisions. One specimen had a reinforcement ratio of 0.012 with a boundary element and transverse reinforcement, while the longitudinal reinforcement ratio of the other was 0.0031 and was a continuation of the wall web reinforcement without any transverse reinforcement. The performance of the two specimens contrasted substantially. The first produced favorable hysteretic behavior as damage concentrated at the beam-wall interface with a shear capacity comparable to previously tested designs with significantly more wall reinforcement. The second specimen experienced heavy damage during low loading cycles at the connection and within the embedment as well as significant stiffness degradation. Noticeable stiffness degradation was observed for each specimen. However, the degradation of the first was more consistent with findings from similar tests previously conducted on SRC coupling beams. Wall behavior differed significantly, with one exhibiting nearly no wall stiffness degradation or rotation and the other experiencing large amounts of ratcheting during repeated cycles, which indicates the embedded beam was prying the wall apart at the connection and yielding the reinforcement. Backbone models were developed for the specimen that produced favorable beam behavior, with comparisons being made between initial, final, and average cycle-based models. The energy dissipation of the first test was comparable to that of specimens previously tested and showed minimal pinching despite reaching a peak chord rotation of 4.65%. Differences in peak capacity of the coupling beam between positive and negative loading during displacement cycles were observed and were larger than in previously tested specimens subject to nonlinear wind demands. Despite the stiffness degradation and asymmetry of load capacity, one of the beams showed positive results regarding strength and ductility behavior that aligned with the results of similarly tested beams, indicating that current wall reinforcement requirements, at least for moderate to low wall demands, are overly conservative.

Nonlinear Wind Behavior of Reinforced Concrete Walls Coupled With Steel Reinforced Concrete Coupling Beams

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