Reliability of structural wall shear design for tall reinforced-concrete core wall buildings

February 25, 2023

Reinforced-concrete core walls are widely utilized as seismic load resisting systems for high-rise buildings, especially along the west coast of the United States. During strong ground shaking, these systems are expected to dissipate energy by yielding of the coupling beams and then by flexural yielding at the wall base. The structural walls’ behavior is governed by flexure, but the design is often governed by shear, as the walls experience high shear demands. The shear design acceptance criterion per force-controlled action was first introduced in Pacific Earthquake engineering Research Center Tall Buildings Initiative and Los Angeles Tall Buildings Structural Design Council (LATBSDC), and since then, the shear design acceptance criterion has been developing for the last decade.

Although the shear design acceptance criterion equation varies per code or tall building design guidelines, the general format is given by the formula: γF_u ≤ ϕF_n,e, where γ is the demand factor, F_u is the mean shear demand factor, F_n,eis the nominal shear strength and ϕ is the uncertainty in . However, despite the developments in the shear design acceptance criterion, there is still a lack of consensus regarding the recommendations and usage of use of γ and ϕ factors, resulting in different tall building designs.

Considering the significance of structural walls in resisting seismic forces in tall reinforced concrete core wall buildings, Professor Sunai Kim and Professor John Wallace from the University of California, Los Angeles, studied the reliability of the structural wall shear design acceptance criterion using Monte Carlo simulations and closed-form solutions. The study was based on the 2012 International Building Code that was later adopted by the 2013 California Building Code. Reliabilities were computed with various parameters for shear demand and capacity, using 20- and 30-story case study buildings designed and analyzed per the LATBSDC (2014) guidelines. The work is currently published in the journal, Engineering Structures.

The authors showed that for all ranges of concrete strength ƒ_(c) considered, γ =1.25, γ = 1.4 and γ = 1.7 were required to obtain 90%, 94% and 97% reliability, respectively, for a dispersion in shear demand of 0.40. On the other hand, γ =1.3, γ = 1.5 and γ = 1.85 were required to achieve 90%, 94% and 97% reliabilities, respectively, for a dispersion in shear demand of 0.50.

In summary, Kim and Wallace utilized a robust methodology toassess the reliability of structural wall shear design acceptance criterion for case study 20 and 30- story reinforced concrete core wall buildings. In a statement to Advances in Engineering, the authors noted that the useful insights provided in this study would pave the way for an advanced reliability study of tall buildings, including a large population of tall buildings, to facilitate the necessary changes in the building codes.

Sunai Kim, Ph.D., S.E., is a Fulbright specialist and an assistant professor of civil engineering at California State Polytechnic University, Pomona. She specializes in structural and earthquake engineering and has seven years of industry experience working as a design engineer and/or researcher at John A. Martin and Associates (CA), Thornton Tomasetti (NY), Los Angeles Unified School District (CA), and CTE|AECOM (IL). Her research interests involve behavior of structures subjected to earthquake and wind loading, performance-based design, and reliability of tall buildings. She is currently collaborating with University of California, Los Angeles and California State University Fullerton to examine performance and repair of ordinary structural walls subjected to wind and seismic loading protocols. Dr. Kim received her BS in Civil Engineering from the University of Illinois at Urbana Champaign (2006) and MS and PhD (2016) in Civil Engineering from the University of California, Los Angeles.

John Wallace, Ph.D., C.E, F. ACI, F. ASCE, professor of civil engineering at the University of California, Los Angeles (UCLA), is an internationally recognized expert on the seismic behavior of reinforced concrete structures. His research contributions focus on assessing the behavior of structures subjected to earthquake loading, laboratory and field testing of structural components and systems, developing and validating models for structural analysis and design, and utilizing sensors and sensor networks to improve seismic to assess complex interactions and improve seismic safety. He has published nearly 100 peer-reviewed journal papers, four which have been recognized with outstanding paper awards, and has been recently recognized with the ACI Boase and SEAOSC Barnes awards. Professor Wallace is active as a consultant and peer reviewer on high-profile performance-based design projects for seismic retrofit of existing buildings and seismic design of tall buildings. He has actively participated in updates to ASCE 41 and ACI 318, as well as the PEER TBI and LATBSDC Guidelines. Dr. Wallace is a voting member of ACI Committee 318 and Chair of ACI Committee 318H – Seismic Provisions. Dr. Wallace received a BS in Civil Engineering from the University of Vermont (1982) and MS and PhD (1989) in Civil Engineering from the University of California, Berkeley.

Reference

Kim, S., & Wallace, J. (2022). Reliability of structural wall shear design for tall reinforced-concrete core wall buildings. Engineering Structures, 252, 113492.

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