UCLA

The design of buildings under wind demands has traditionally been based on prescriptive code provisions such as ASCE/SEI 7, which requires the building components to stay essentially linear elastic. Recent developments in wind tunnel testing, structural analysis techniques, and performance-based design procedures led to the publication of the ASCE/SEI Prestandard for Performance-Based Wind Design (ASCE/SEI, 2019). The Prestandard allows limited inelastic behavior in ductile elements of a building’s Main Wind Force Resisting System (MWFRS) under extreme wind events. However, because yielding of some building components has not historically been permitted, there is limited research available to understand the inelastic behavior of structural elements subjected to wind demands. The advantages of the performance-based wind design (PBWD) are considered to be most impactful for the design of tall buildings, where it is very common to use coupled reinforced concrete shear wall systems as the MWFRS. Although some research has been conducted to understand the inelastic behavior of reinforced concrete and steel-reinforced concrete coupling beams, there is no published research that investigates the inelastic behavior of reinforced concrete shear walls under wind demands. To fill this gap, four reinforced concrete C-shaped structural walls have been tested in two phases under quasi-static, biaxial cyclic loading protocols simulating extreme wind events. Following the wind loading protocol, a seismic loading protocol was applied.

The design of the walls was based on the core-wall design of actual buildings that were designed and constructed in high-wind and low-seismic zones in the United States. The 5 in. thick flanges and webs of the walls were 30 in. and 75 in. long, respectively, representing approximately one-third scaled C-shaped walls of the core-walls of these buildings. The test walls were detailed as Ordinary walls based on the provisions of Chapter 11 of ACI 318-19. The test variable for the Phase-I walls (CW-1 and CW-2) was the longitudinal reinforcement ratio (ρl); 0.75% for CW-1 and 1.5% for CW-2. Based on the experimental results of the Phase-I walls and feedback from a Project Advisory Committee (PAC), two more walls (CW-3 and CW-4) were tested during Phase-II. The design of Phase-II walls was based on CW-2 (ρl=1.5%), and the test variables were the amount of confinement provided at the end zones of the flanges (flange edges) and the amount of axial load applied during the biaxial wind loading protocol.

The wind test results indicated that for the wall with the low-to-moderate reinforcement ratio (CW-1, ρl=0.75%), rotational ductility demands of 3.0 can be achieved without any damage (e.g., concrete spalling, bar buckling, or bar fracture) and with very small residual flexural crack widths (around 0.1 mm). Since CW-1 failed at a rotational ductility demand of 20 during the seismic loading protocol, modest inelastic response can be allowed during extreme wind events for the walls with 0.75% or lower longitudinal reinforcement ratios. Concrete spalling was observed at the flange-web corners and the flange edges during the wind loading protocol for the walls with the higher longitudinal reinforcement ratio (1.5%). Depending on the amount of axial load applied during the biaxial load application, concrete crushing and bar buckling were also observed. Phase-II tests showed that the flange edges were more susceptible to damage than the other portions of the C-shaped walls. With moderate confinement provided at the flange edges and reduction in wall axial load during the biaxial loading (when the flange edges were under compression), bar buckling and concrete crushing were not observed during the wind loading protocols. Application of the seismic loading protocol revealed that, for the walls that did not sustain any significant damage during the wind tests, rotational ductility demands of at least 8.5 could be achieved prior to 20% loss in lateral strength.