International Concrete Abstracts Portal

Performance-Based Seismic Design (PBSD) of reinforced concrete buildings has rapidly become a widely used alternative to the prescriptive requirements of building code requirements for seismic design. The use of PBSD for new construction is expanding, as evidenced by the design guidelines that are available and the stock of building projects completed using this approach. In support of this, the mission of ACI Committee 374, Performance-Based Seismic Design of Concrete Buildings, is to “Develop and report information on performance-based seismic analysis and design of concrete buildings.” During the ACI Concrete Convention, October 15-19, 2017, in Anaheim, CA, Committee 374 sponsored three technical sessions titled “Performance-Based Seismic Design of Concrete Buildings: State of the Practice.” The sessions presented the state of practice for the PBSD of reinforced concrete buildings. These presentations brought together the implementation of PBSD through state-of-the-art project examples, analysis observations, design guidelines, and research that supports PBSD. This special publication reflects the presentations in Anaheim. Consistent with the presentation order at the special sessions in Anaheim, the papers in this special publication are ordered in four broad categories: state-of-the-art project examples (papers 1-5), lateral system demands (papers 6-8), design guidelines (papers 9-10), and research and observed behavior (papers 11-13). On behalf of Committee 374, we wish to thank each of the authors for sharing their experience and expertise with the session attendees and for their contributions to this special publication.

The use of a Performance-Based Seismic Design (PBSD) approach to design buildings whose heights exceed 240 ft (73 m) has become common in many West Coast cities. This paper studies trends across 14 special reinforced concrete shear wall PBSD towers designed within the last 5 years. The primary purpose of evaluating these trends is to compare demands calculated using a linear elastic design approach (i.e. for Design Earthquake or Service Level shaking) to the demands (average results from 7 or 11 ground motions) determined through nonlinear analysis (i.e. for Maximum Considered Earthquake shaking). The specific demands evaluated include core wall shears and foundation overturning moments. The paper also demonstrates that shear and moment amplification are significant phenomena for concrete buildings, and are believed to be primarily due to nonlinear behavior, material over-strength, higher mode effects, and damping and stiffness assumptions. The results present a useful range of trends to provide an engineer guidance on the expected demands and the level of variability between projects. The paper highlights some of the reasons for the variability in these trends, and provides general proportioning recommendations.

A 160-foot (≈ 49 m) tall 12–story reinforced concrete special moment frame building is designed following ASCE 7-16 and ACI 318-14, and assessed using three Performance-Based Seismic Engineering (PBSE) standards and guidelines including ASCE/SEI 41, the Tall Buildings Initiative (TBI) guidelines for performance-based design of tall buildings, and the Los Angeles Tall Buildings Structural Design Council (LATBSDC) procedures. The assessments are performed at the combination of two performance and hazard levels including Collapse Prevention (CP) at the risk-targeted maximum considered earthquake (MCER) hazard level and Immediate Occupancy (IO) at a frequent ground motion level with 50 percent probability of exceedance in 30 years, i.e. serviceability performance level. Based on the recommendations of each of the three PBSE documents, nonlinear finite element models are implemented in OpenSees. Through nonlinear time-history response analyses, the finite element models are subjected to eleven ground motions that are selected following the ground motion selection recommendations in ASCE 7-16. Assessment results indicate that for the serviceability performance level, the code-compliant building meets the design requirements of the three PBSE documents for the inter-story drift ratio and inelastic deformation of the structural components. At the MCER hazard level, although the building essentially satisfies the design requirements for the peak inter-story drift ratios and inelastic deformation, the mean of the residual inter-story drift ratios as well as the envelope of the residual drift ratios do not meet the limits of the TBI and LATBSDC guidelines. The results indicate that the newly designed building meets the ASCE 41 acceptance criteria but does not meet the design requirements set in TBI and LATBSDC guidelines.

The U.S. Nuclear Regulatory Commission (NRC) defines seismic Category 1 structures as the structures (buildings) that should be designed and built to withstand the maximum potential earthquake stresses for the particular region where a nuclear plant is sited. Seismic Category 1 structures have been designed for ground-shaking intensity associated with a safe-shutdown earthquake (SSE) – the intensity of the ground motion that will trigger the process of automatic shutdown of the reactor in operation. The SSE generates floor response spectra at different floor elevations in a building, and these spectra and their associated forces are used for the design of piping and piping anchors and equipment and equipment anchors at their floor locations. The NRC policy requires that the seismic Category 1 structures whose collapse could cause early or/and large release of radioactive materials into the atmosphere to be analyzed/designed for “no collapse” during the ground-shaking intensity of a review-level earthquake (RLE), which is 1.67 times that of an SSE. Most seismic Category 1 concrete structures, such as containment and shield buildings (curved cylindrical wall; see Figs. 1 and 2 in the next section) and containment internal structures (straight wall; see Fig. 1), use walls to resist earthquakes. This paper presents guidelines for the performance-based seismic design for these wall-typed structures that could meet the NRC policy. The method consists of (1) proportioning wall thickness based on shear stress of 6√fc’ (0.5√fc’ megapascals (MPa)) generated by SSE ground motions, (2) limiting vertical compressive stress in walls to less than 0.35 fc’, (3) providing minimum percentage of reinforcement of 1.0 percent to prevent steel reinforcing bar fracture, (4) subjecting the building design to nonlinear dynamic response analyses under RLE ground motions, (5) identifying any members and their connections in the building that have failed or collapsed during the RLE ground motions, (6) increasing reinforcement or wall thickness, or both, to provide additional strength or/and ductility for the failed or collapsed members and their connections, and (7) resubjecting the revised building design to the nonlinear dynamic response analyses as stated in step (4) until no collapse of the building and its members and their connections. This performance-based seismic design method is a direct, transparent, and scientific answer to whether these important seismic Category 1 structures meet the NRC’s policy that they will not collapse during the RLE ground motions. Examples of using the nonlinear dynamic response analyses are cited and described. Guidelines for the performance-based seismic design of seismic Category 1 concrete Structures are listed at the end of this paper.

The PERFORM-3D software package is used commonly in engineering practice to conduct nonlinear dynamic analyses of reinforced concrete walled buildings to their seismic response. However, few studies have evaluated or improved on common modeling approaches for structural concrete walls. The research presented here was conducted to establish best practices for modeling the full nonlinear response of walls exhibiting common flexural failure modes. First, an experimental data set consisting of eight planar concrete walls was collected; these walls were spanned a range of length-to-thickness ratios, shear stress demands, axial load ratios, and longitudinal reinforcement configurations. For each wall specimen, a reference numerical model was created using typical modeling methods as proposed by Powell. Comparison of simulated and measured cyclic response histories show that typical modeling techniques result in relatively inaccurate simulation of cyclic response and very inaccurate simulation of drift capacity. To improve the model accuracy, experimental data were used to determine appropriate values for the steel and concrete material model cyclic response parameters. Experimental data and mathematical definitions for the concrete compressive energy were used to develop recommendations for defining concrete post-peak stress-strain response to achieve accurate, mesh-independent simulation of drift capacity. Finally, recommendations for the minimum number of elements were examined. Comparison of simulated and measured cyclic response histories show that the new modeling recommendation result in accurate, mesh independent simulation of cyclic response, including drift capacity. Future work will evaluate the proposed modeling approach for asymmetric and flanged walls.