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

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.