Title: Ductile Reinforced Concrete Coupled Walls: FEMA P695 Study
Author(s): Negin A. Tauberg; Kristijan Kolozvari; John W. Wallace
Appears on pages(s):
Keywords: FEMA P695, lateral system, rc coupled walls, ASCE 7, ACI 318, seismic, coupling beam, shear
The proposed study aims to justify the use of the proposed design parameters by applying the FEMA P695 approach to the newly defined lateral system. The primary objective of the study is to define a subset of conditions (design provisions) for which the proposed building system response parameters may be used for RC coupled walls. Target values for the Response
Modification Factor (R), System Overstrength Factor (Ω0), and Deflection Amplification Factor (Cd) are selected, as noted in the prior paragraph, and verified by application of the FEMA P695 methodology. An independent review panel of practicing engineers and researchers are peer reviewing the effort, and an advisory panel of practicing engineers and researchers active with ASCE 7 and ACI Committee 318 are providing input on the design and collapse assessment results.
Thirty-seven prototype RC coupled wall configurations (Archetypes) are assessed in this study for the Seismic Design Category (SDC) being considered, i.e., SDC Dmax. The Archetypes considered address a range of variables expected to influence the collapse margin ratio, with primary variables of building height (6 to 30 stories), wall cross section (planar and
flanged/core), coupling beam aspect ratio (ln/h = 2.0 to 5.0), and coupling beam reinforcement arrangement (conventionally reinforced (CR) and diagonally reinforced (DR)). Each Archetype is designed using ASCE 7-16 and ACI 318-14 (including 318-19 approved code change proposals). Initially, a set of preliminary Archetypes were designed conforming to the wall shear provisions of ACI 318-14 and were assessed for conformance with the FEMA P695 acceptability criteria. Due to a high number of shear failures experienced during the collapse assessment process, these initial Archetypes had to be revised; the final Archetype designs conform instead to the wall shear provisions of ACI 318-19 code change proposal using amplified wall shear demands accounting for flexural overstrength and higher mode effects.
To assess the potential for collapse, a nonlinear model is created for each Archetype in an open-source computational platform OpenSees (McKenna et al., 2000) and subjected to ground acceleration response histories. New, state-of-the-art, approaches to predict collapse are implemented and evaluated/calibrated using existing test data. The established failure criteria account for flexural compression (concrete crushing, bar buckling, wall lateral instability), flexural tension (bar fracture), shear, and axial failures. For each Archetype, nonlinear static pushover (NSP) and nonlinear incremental dynamic analyses (IDA), subjected to the ATC-63 forty-four far-field ground motion record set, are conducted to obtain the overstrength/ductility and the collapse margin ratio values, respectively. The methodology prescribed in FEMA P695
is followed to establish limits on the adjusted collapse margin ratios (ACMRs) and to observe the vicinity of the obtained ACMRs to these limits in order to investigate the validity of the chosen R-value.
Initial findings indicated that Archetypes using R = 8 and Cd = 8 and designed conforming to current ACI 318-14 shear provisions, do not meet the FEMA P695 acceptability criteria due to a high number of shear failures experienced during incremental dynamic analysis. The Archetype designs were revised using R = 8 and Cd = 8 and amplified shear demands following an approved ACI 318-19 code change proposal that is similar to approaches used for wall shear amplification in CSA A23.3 (2014) and NZS 3101 (2006). Results for the final redesigned Archetypes meet the FEMA P695 acceptability criteria. Therefore, the revised design approach using R = 8 and Cd = 8 and amplified shear demands has been used for all Archetype designs.