UCLA

Reinforced concrete (RC) structural walls (also known as shear walls) have commonly been used as lateral force-resisting elements in buildings in regions of moderate-to-high seismic hazard because they provide substantial lateral strength and stiffness to buildings when subjected to strong ground shaking. Although relatively few wall tests were reported in the literature prior to 1990, a substantial number of tests have since been reported, primarily to assess the role of various parameters on wall deformation capacity, failure mode, strength, and stiffness. However, a comprehensive database that summarizes information and results from these tests does not exist. To address this issue, a comprehensive experimental wall database, referred to as the UCLA- RCWalls database, was created. The database currently contains detailed and parameterized information on more than 1100 wall tests surveyed from more than 260 programs reported in literature, and enables assessment of a spectrum of issues related to the behavior and performance of structural walls. The database was developed using software that enabled use of an engineering database structure with a user-friendly interface to manipulate data, i.e., filter, import, export, and review, and a secure background to store the data.

The underlying premise of the ASCE 7-10 and ACI 318-14 provisions is that special structural walls satisfying the provisions of ACI 318-14 �18.10.6.2 through �18.10.6.4 possess adequate deformation capacity to exceed the expected deformation demand determined using ASCE 7-10 analysis procedures. However, observations from recent laboratory tests and reconnaissance efforts following strong earthquakes, where significant damage occurred at boundary regions of thin walls due to concrete crushing, rebar buckling, and lateral instability, have raised concerns that current design provisions are inadequate. To address this concern, the database was filtered to identify and analyze a dataset of 164 tests on well-detailed walls generally satisfying ACI 318-14 provisions for special structural walls. The study revealed that wall lateral deformation capacity is primarily a function of the ratio of wall neutral axis depth-to-width of flexural compression zone (c/b), the ratio of wall length-to- width of flexural compression zone (lw/b), wall shear stress, and the configuration of boundary transverse reinforcement (e.g., use of overlapping hoops versus a single perimeter hoop with intermediate crossties), and that, in some cases, the provisions of ACI 318-14 may not result in buildings that meet the stated performance objectives. Based on these observations, an expression is developed to predict wall drift capacity associated with 20% lateral strength loss with low coefficient of variation, and a new reliability-based design methodology for structural walls is proposed. The approach has been adopted for ACI 318-19, where a drift demand- to-capacity ratio check is performed to provide a low probability that roof drift demands exceed roof drift capacity at strength loss for Design Earthquake hazard level.

A large number of RC buildings constructed prior to the mid-1970s in earthquake-prone regions rely on lightly reinforced or perforated, perimeter structural walls to resist earthquake-induced lateral loads. These walls are susceptible to damage when subjected to moderate-to-strong shaking; a number of such cases were observed in 1999 Chi-Chi and Kocaeli Earthquakes, and more recently in 2010 Maule and 2011 Christchurch earthquakes. Despite these observations, limited studies have been reported in the literature to investigate the loss of axial (gravity) load carrying capacity of damaged walls and wall piers, primarily due to the lack of experimental data. To study axial failure of structural walls, the database was filtered to identify and analyze datasets of tests on shear- and flexure-controlled walls. Based on the results, expressions were derived to predict lateral drift capacity at axial failure of RC walls and piers.

Furthermore, the ASCE/SEI 41 standard (and other similar standards or guidelines, e.g., ACI 369) represents a major advance in structural and earthquake engineering to address the seismic hazards posed by existing buildings and mitigate those hazards through retrofit. For nonlinear seismic evaluation of existing buildings, these standards provide modeling parameters (e.g., effective stiffness values, deformation capacities, and strengths) to construct backbone relations, as well as acceptance criteria to determine adequacy for a given hazard level. The modeling parameters and acceptance criteria for structural walls were developed based on limited experimental data and knowledge available in the late 1990s (FEMA 273/274-1997), with minor revisions since, especially for flexure-controlled walls. As a result, the wall provisions tend to be, in many cases, inaccurate and conservative, and can result in uneconomical retrofit schemes. Therefore, one of the objectives of this study involved utilizing the available experimental data in the UCLA- RCWalls database and new information on performance of structural walls to develop updated modeling parameters and acceptance criteria for flexure-controlled walls. The updated provisions include a new approach to identify expected wall dominant behavior (failure mode), cracked and uncracked flexural and shear stiffness values of flexure-controlled walls, and updated modeling parameters (backbone relations) and acceptance criteria for flexure-controlled walls. The updates are expected to be significant contributions to the practice of seismic evaluation and retrofit of wall buildings.