Title: Analysis and Design of Double-Beam Coupling Beams
Author(s): Youngjae Choi and Shih-Ho Chao
Publication: Structural Journal
Appears on pages(s): 79-95
Keywords: coupled wall systems; diagonally reinforced coupling beams; double-beam coupling beams (DBCB); interface shear strength
Double-beam coupling beams (DBCBs) are a viable alternative to diagonally reinforced concrete coupling beams (DCBs). While construction time and effort of DBCBs is much less than DCBs, their ability to sustain strong earthquake forces was also experimentally proven to be equivalent to DCBs. DBCBs consist of two slender reinforced concrete beams with an unreinforced concrete strip (UCS) between them. A DBCB gradually splits into two slender beams from small to large displacements, thereby transitioning from a brittle shear mechanism to a ductile flexural mechanism. This paper presents a recommended design procedure for DBCBs based on previous experimental results. An additional specimen was designed based on the design recommendation and tested under the same displacement protocol. The specimen showed satisfactory seismic performance with ductile behavior up to 6% beam chord rotation. One of the major advantages of DBCBs is they allow utility ducts such as polyvinyl chloride (PVC) pipes to pass through the coupling beams at the UCS. Experimental testing and nonlinear finite-element analyses reveal that the location of the utility ducts can have a significant effect on the behavior of DBCBs. Research results suggest that these utility ducts should be placed at the ends of the UCS. Prior experimental testing indicated that when the span-depth ratio ln/h of a DBCB becomes smaller, the load-versus-
beam chord rotation response exhibits a pinched shape due to the greater influence of shear cracks. This pinching effect was investigated by nonlinear time-history (NTH) analyses on a 42-story coupled wall system subjected to both design-basis earthquakes and maximum-considered earthquakes. A hybrid model for DBCBs considering shear, flexure, and pinching was developed. The NTH analyses show that the seismic performance in terms of peak interstory drift ratios is nearly identical between DBCBs and DCBs; therefore, no evidence indicates pinching having an appreciable effect.