Applications of Stress Fields to Assess the Behavior and Strength of Coupling Beams subjected to Seismic Actions

Horizontal loading due to earthquake or wind is usually the governing action for design of reinforced concrete cores in medium to tall buildings. Building cores designed to resist these actions must be dimensioned to resist tributary vertical loads and large shear forces of varying direction and sign. Because wall cores are commonly placed around elevator shafts, openings must be left in walls to allow passage to the elevator areas. The large shears induced in the core by lateral forces have to be transmitted by beams connecting adjacent portions of the core (coupling beams). These beams usually govern both the strength and deformation capacity of the core and their structural response depends primarily on the geometry and reinforcing details chosen. In this paper, the behavior and strength of coupling beams is discussed through analysis of four large-scale specimens tested at the University of Massachusetts Amherst. The specimens (2.03 × 1.65 m), representing two walls joined by a coupling beam, exhibited different failure modes depending on their slenderness and shear-to-flexural reinforcement ratio within the coupling beam region. The behavior of the specimens is modeled using the stress field method to obtain realistic shear force-drift envelopes. Stress fields, in combination with strut-and-tie models, have been applied for design and detailing of members of unusual geometries subjected to monotonic loading. In this paper, a series of guidelines for applying the stress field method to members subjected to reverse cyclic loading are presented and discussed. Comparisons to test results show this technique to be a promising approach for consistent modeling of coupling beams.