Design strategies to achieve target collapse risks for reinforced concrete wall buildings in sedimentary basins

Abstract

Studies of recorded ground motions and simulations have shown that deep sedimentary basins can greatly increase the damage expected during earthquakes. Unlike past earthquake design provisions, future ones are likely to consider basin effects, but the consequences of accounting for these effects are uncertain. This article quantifies the impacts of basin amplification on the collapse risk of 4- to 24-story reinforced concrete wall building archetypes in the uncoupled direction. These buildings were designed for the seismic hazard level in Seattle according to the ASCE 7-16 design provisions, which neglect basin effects. For ground motion map frameworks that do consider basin effects (2018 USGS National Seismic Hazard Model), the average collapse risk for these structures would be 2.1% in 50 years, which exceeds the target value of 1%. It is shown that this 1% target could be achieved by: (1) increasing the design forces by 25%, (2) decreasing the drift limits from 2.0% to 1.25%, or (3) increasing the median drift capacity of the gravity systems to exceed 9%. The implications for these design changes are quantified in terms of the cross-sectional area of the walls, longitudinal reinforcement, and usable floor space. It is also shown that the collapse risk increases to 2.8% when the results of physics-based ground motion simulations are used for the large-magnitude Cascadia subduction interface earthquake contribution to the hazard. In this case, it is necessary to combine large changes in the drift capacities, design forces, and/or drift limits to meet the collapse risk target.

Estimating economic losses of midrise reinforced concrete shear wall buildings in sedimentary basins by combining empirical and simulated seismic hazard characterizations

“…As a result, the spectral accelerations from the 2014 to the 2018 NSHM Uniform Hazard Spectrum (UHS) for a hazard level with a 2% probability of exceedance in 50‐year is approximately 50% larger for periods in the range of 1–4 s. Increases in spectral acceleration due to basin amplification are larger at longer periods. For instance, the increase in spectral acceleration at a period of 0.2 s is only 15%, whereas for a 2.0 s period, the increase is 66% 12 . Increases in spectral acceleration from the 2014 NSHM to the 2018 NSHM are negligible for out‐of‐basin sites within Western Washington State.…”

“…37 The resulting synthetics highlight the considerable amplification of spectral accelerations, ranging from factors of 2 to 5, for periods of 1 to 10 s, for sites within the Seattle basin. Marafi et al 12 showed that inside the basin the spectral accelerations of the simulated M9 CSZ ground motions at periods of 1.5 to 4 s were greater than the spectral accelerations of the ASCE 7-16 risk-targeted maximum considered earthquake (MCE R ), which has a return period of around 2000 years in Seattle. Table 2 summarizes the CMS spectral acceleration, at the fundamental period of each REF archetype, using the 2014 and 2018 NSHMs for intensity levels with return periods of 975 and 2475 years.…”

“…Seven design variations were considered for all archetypes as summarized in Table 1. All buildings were designed and detailed by Marafi et al 12 as special RC shear walls, per ACI 318‐14 29 requirements, with a seismic force‐reduction factor (R) of 6, following a modal response spectrum analysis (MRSA) procedure. The reference (REF) archetypes were designed to barely meet the minimum code‐prescriptive requirements of ASCE 7‐16, with a 2% maximum allowable seismic drift, and a 1.0 flexural demand‐to‐capacity ratio at the ground floor.…”

“…In contrast, stiffer archetypes (e.g., DL1.25%) resulted in lower steel tonnages because these archetypes had steel bars placed further apart (i.e., larger moment arms) which reduced the overall steel area to achieve a similar design strength from the REF archetype. Additional details of the RC shear wall building designs can be found in Marafi et al 12 …”

“…The nonlinear response history analyses of the archetype buildings have been published in other papers. 12,13 The novelty in this paper lies in the assessment of the earthquake impacts, expressed in terms of economic losses, which can be used to inform planners and policymakers of underlying risks, and the engineering design community of the tradeoffs of adopting different design strategies.…”