UCLA

Reinforced concrete structural walls provide an efficient lateral system for resisting seismic and wind loads. Coupling beams are commonly used to connect adjacent collinear structural walls to enhance building lateral strength and stiffness. Steel-Reinforced Concrete (SRC) coupling beams provide an alternative to reinforced concrete coupling beams, diagonally-reinforced for shorter spans and longitudinally-reinforced for longer spans, and offer potential advantages of reduced section depth, reduced congestion at the wall boundary region, improved degree of coupling for a given beam depth, and improved deformation capacity.

Four large-scale, flexure-yielding, cantilever SRC coupling beams embedded into reinforced concrete structural walls were tested by applying quasi-static, reversed-cyclic loading to the coupling beam (shear) and the top of the wall (moment, shear, and constant axial load) to create cyclic tension and compression fields across the embedment region. The primary test variables were the structural steel section embedment length, beam span length (aspect ratio), quantities of wall boundary longitudinal and transverse reinforcement, and applied wall loading (moment, shear, and axial load). Based on test results, long embedment length, sufficient wall boundary reinforcement, and low-to-moderate wall demands across the embedment region are all associated with favorable coupling beam performance, characterized by minimal pinching and strength degradation in the load-deformation response and plastic hinge formation at the beam-wall interface with a lack of damage (plasticity) in the embedment region. The variation in aspect ratio was not found to significantly affect performance.

Detailed design and modeling recommendations for steel reinforced concrete (SRC) coupling beams are provided for both code-based (prescriptive) design and alternative (non-prescriptive) design. For both code-based and alternative design, modeling a rigid beam for flexure and shear deformations with rotational springs at the beam-wall interfaces is recommended for stiffness, as test results indicate that the majority of the coupling beam deformations were associated with interface slip/extension. Alternative stiffness modeling recommendations are provided, in which an effective bending stiffness that accounts for the aspect ratio or beam length is used instead of interface rotational springs. A capacity design approach, in which the provided embedment strength exceeds the expected beam strength, is recommended for determining the required embedment length of the steel section into the structural wall. Recommendations for computing the nominal and expected (upper bound) flexure and shear strengths are provided. For alternative design, additional parameters are provided to define the strength and deformation capacity (to complete the backbone relations) and to address cyclic degradation for each of the test beams.