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An efficient structural system for tall buildings to resist earthquake loads consists of reinforced concrete (RC) shear walls connected by coupling beams. Construction of coupling beams that satisfy the strength and detailing requirements set forth in ACI 318-05 for diagonally reinforced coupling beams is cumbersome and costly; therefore, ACI 318-08 provides a new detailing option that aims to improve the constructibility while maintaining adequate strength and ductility. Eight 1/2-scale specimens were tested to compare the performance of beams constructed using new and old detailing options, to compare beams with diagonal reinforcement to beams with straight bars at higher aspect ratios, and to assess the impact of reinforced and post-tensioned (PT) slabs. Test results indicate that the new detailing approach provides equal, if not improved, behavior as compared to the alternative detailing approach and that including a slab had only a modest impact on strength, stiffness, ductility, and observed damage.
Keywords: confinement; constructibility; coupling beam; diagonal reinforcement; experiment.
INTRODUCTION
Tall building construction is common in metropolitan areas and it has become increasingly important to provide methods of construction that both improve seismic performance and constructibility. Core walls, with coupling beams above openings to accommodate doorways, are an efficient lateral-force-resisting system for tall buildings. When subjected to strong shaking, the coupling beams act as fuses and typically undergo large inelastic rotations.
Various testing programs have been carried out to assess the load-deformation behavior of coupling beams.1-5 The primary test variables in these studies were the beam clear span to total depth ratio (commonly referred to as the beam aspect ratio) and the arrangement of the beam reinforcement. In a majority of these studies, the load-deformation behavior of low-aspect-ratio beams (1.0 to 1.5) constructed with beam top and bottom longitudinal reinforcement were compared with beams constructed with diagonal reinforcement. Concrete compressive strengths for most tests were approximately 4 ksi (27.6 MPa). Although these tests provided valuable information, they do not address issues for current tall building construction, where beam aspect ratios are typically between 2.0 and 3.5 and concrete strengths are in the range of 6 to 8 ksi (40 to 55 MPa). In addition, in none of the prior studies was a slab included as part of the test specimen, whereas the slab might restrain axial...