Sherif El-Tawil, PhD, PE
Professor and Associate Chair
Department of Civil & Environmental Engineering | The University of Michigan
Hybrid Coupled Walls
Hybrid Coupled Wall (HCW) systems are lateral load resisting systems comprised of reinforced concrete walls coupled by steel beams. Our long running research has shown that HCW systems possess the necessary combination of stiffness, strength, and toughness for application in regions of moderate to high seismicity. This research project obtained fundamental information about system behavior under severe seismic loading and synthesized it with existing data into practical recommendations that can be utilized by practitioners for seismic analysis and design of HCW systems.
Reinforced concrete (RC) coupled wall systems, where RC beams couple two or more RC walls in series are frequently used in medium and high-rise construction. The benefits of coupling in such systems are well recognized and understood. The coupling beams provide transfer of vertical forces between adjacent walls, which creates a frame-like coupling action that resists a portion of the total overturning moment induced by lateral loads. This coupling action has three major benefits. First, it reduces the moments that must be resisted by the individual wall piers resulting in a more efficient structural system. Secondly, it provides a means by which seismic energy is dissipated over the entire height of the wall system as the coupling beams undergo inelastic deformations. A final important advantage of a coupled wall system is that it has a lateral stiffness that is significantly greater than the sum of its component wall piers, permitting a reduced footprint for the lateral load resisting system.
The shear force and deformation demands expected on coupling beams during a design-level seismic event, coupled with their low span-to-depth ratio and the degradation of shear resisting mechanisms attributed to concrete under load reversals, have led designers to provide special diagonal reinforcement detailing for and in the vicinity of RC coupling beams. This special reinforcement complicates erection, potentially increasing both construction time and cost. Furthermore, the limited shear capacity of RC coupling beams often requires designers to provide impractically deep members. To mitigate these problems, some engineers have turned to structural steel coupling beams as an alternative to reinforced concrete beams. The resulting structural system is referred to as a hybrid coupled wall (HCW) system.
Hybrid coupled wall systems are often built in conjunction with steel framing systems and the combined structural system may be considered as a dual frame-wall system. For most practical designs, however, the lateral stiffness of the coupled core wall system is much greater than that of the more flexible steel frame. The wall system will therefore attract the majority of the earthquake-induced lateral loads and must be designed accordingly.
In response to seismic excitation, steel coupling beams are expected to dissipate energy in a manner similar to the response of shear links in eccentrically braced frames. Shear links, and coupling beams in turn, fall into three categories: short, intermediate, and long, depending on their structural and geometric properties. When architectural constraints permit, short coupling beams, which dissipate energy primarily through inelastic shear distortion, are preferred to longer coupling beams that dissipate energy through flexural hinge rotation. Mechanisms that involve well-controlled inelastic shear deformation in steel coupling beams are generally more ductile than those involving flexure-related plastic hinge deformations.
Under the auspices of the ASCE Committee on Composite Construction and with funding from an ASCE Special Project, a task group led by Prof. El-Tawil has developed Recommendations for Seismic Design of Hybrid Coupled Wall Systems. The Recommendations (Figure 1) can be used for proportioning HCW systems comprised of two or more reinforced concrete wall piers coupled via steel coupling beams and located in zones of moderate to high seismic risk. Where applicable, the Recommendations draw upon existing specifications for steel and concrete, as well as other research on composite systems.