Cyclic loading test of steel coupling beams with mid-span friction dampers and RC slabs

In coupled wall systems, coupling beams distributed along the structural height are intended to yield and dissipate seismic energy under moderate and severe earthquakes. The use of specifically designed dampers in coupling beams can greatly enhance the seismic performance of a structure. In the present study, a friction damper incorporating brake pad-to-mild steel friction interfaces is proposed for installation in a steel coupling beam at the mid-span. Cyclic loading tests were conducted on the friction coupons and subassemblages of steel coupling beams with mid-span friction dampers. An RC slab, which rested on the top flange of the steel coupling beam without shear connectors, was included in one of the subassemblage specimens. The test results show that the friction dampers exhibited stable and full hysteretic responses with up to 8% chord rotation of the coupling beams. The friction coefficients were gradually increased and the clamping forces of the dampers relaxed during the cyclic loading. The average friction coefficient was close to the nominal value with a small deviation. However, the unintended out-of-plane bending of the steel teeth of the friction dampers reduced the effective normal force on the friction interfaces and, thus, led to a lower-than-expected shear strength of the damper. Though not composited with the steel coupling beam, the RC slab sustained severe flexural cracks as wide as 6 mm at 4% chord rotation of the coupling beam and, thus, might influence the postquake recovery of a building. A simple numerical model was established to investigate the respective contribution of each part of the coupling beam. The results show that the post-sliding stiffness introduced by the RC slab was less than 1% of the initial stiffness of the coupling beam and decreased rapidly with increasing deformation amplitudes. The shear force in the slab exceeded 10% of the strength of the friction damper at 4% chord rotation and is deemed to be a potential source of overstrength for the design of a coupling beam and the surrounding elements.