Comparative risk-based seismic assessment of 1970s vs modern tall steel moment frames

This study benchmarks the performance of older existing tall steel moment resisting frame buildings designed following historic code-prescriptive requirements (1973 Uniform Building Code) against modern design standards (2015 International Building Code). The comparison is based on seismic risk assessments of alternative designs of a 50-story archetype office building, located at a site in San Francisco, CA. The mean annual frequency collapse risk of the 1973 building is 28 times greater than the equivalent 2015 building (28 × 10^{-4} versus 1 × 10^{-4}), or approximately 13% versus 0.5% probability of collapse in 50 years. The average annual economic loss (based on cost of repair) is 65% higher for the 1973 as compared to the 2015 building (0.66% versus 0.40% of building replacement cost). The average annual downtime to re-occupancy for the 1973 building is 72% longer (8.1 vs 4.7 days) and to functional recovery is about 100% longer (10.4 vs 5.0 days). Building performance evaluations at the design basis earthquake (DBE) and the maximum considered earthquake (MCE) shaking intensities further suggest that 1970s tall steel moment frames have much higher risks of collapse under extreme ground motions and risks of damage and building closure in moderate earthquakes. Furthermore, while modern building code requirements provide acceptable seismic collapse safety, they do not necessarily ensure a level of damage control to assure a swift recovery after a damaging earthquake due to extensive downtime. A set of vulnerability functions are proposed for both archetype buildings considered in the assessment

Physical mechanisms underlying the influence of ground motion duration on structural collapse capacity

This study explores the physical mechanisms by which the duration of strong ground motion influences structural response. While a number of previous studies have found that ground motion duration influences only cumulative damage indices, and not peak structural deformations, a few recent studies that employed realistic, deteriorating structural models were able to demonstrate the effect of duration on peak deformations and structural collapse capacity. These recent studies were, however, empirical in nature and did not fully explore the reasons behind the observed effects of duration. Many of the previous studies qualitatively attributed the effects to the cyclic deterioration of strength and stiffness of the structural components, which represents just one mechanism by which duration exerts its influence. In contrast, the present study shows that the gradual ratcheting of drifts, accentuated by the destabilizing P − ∆ effect, is an equally important mechanism by which duration influences structural response. The relative contributions of the two mechanisms—cyclic deterioration and ratcheting—to the observed influence of duration on the collapse capacity of a five-story steel moment frame building, are quantified by conducting incremental dynamic analysis (IDA) using spectrally equivalent sets of long and short duration ground motions. The use of spectrally equivalent ground motions allows controlling for the effect of response spectral shape. A response parameter called the ratcheting interval is defined and used to explain the larger potential for a long duration ground motion to cause structural collapse, when compared to a spectrally equivalent short duration ground motion scaled to the same intensity level. These findings shed light on the interaction between structural model characteristics and the observed influence of ground motion duration on structural response. In addition, they highlight the importance of using models that capture both cyclic deterioration and the P−∆ effect to reliably account for the effect of ground motion duration when assessing structural collapse risk

Hazard-consistent ground motion duration: Calculation procedure and impact on structural collapse risk

Calculation of structural collapse risk using non-linear response history analysis requires the selection of ground motions at different intensity levels. These selected ground motions should be consistent with the seismic hazard at the site under consideration. Source-specific, conditional distributions of ground motion duration and response spectra are proposed as targets to select hazard-consistent ground motions. Target distributions of duration are computed using a prediction equation for duration and earthquake source characteristics (e.g. source type, magnitude, and distance) obtained from seismic hazard deaggregation calculations, conditional on the exceedance of a spectral acceleration value corresponding to a specific hazard level. The correlation between the residuals (£ values) of response spectral ordinates and duration are accounted for in the calculation procedure. Sample calculations are performed for three sites in Western USA: Seattle (Washington), Eugene (Oregon), and San Francisco (California) to illustrate the contribution of interface earthquakes in subduction zones that are known to produce long duration ground motions. Previous studies have found that long duration ground motions, on average, predict lower collapse capacities than short duration ground motions such as the FEMA P695 far field records and moderate to large amplitude records from the PEER NGA West2 database, which are commonly used for collapse capacity estimation. The examples presented in this paper illustrate that the use of only short duration records for sites where interface earthquakes contribute significantly to the seismic hazard can lead to an over-estimation of collapse capacity and un-conservative structural designs

Quantifying the influence of ground motion duration on structural collapse capacity using spectrally equivalent records

© 2016, Earthquake Engineering Research Institute. This study examines the influence of ground motion duration on the collapse capacities of a modern five-story steel moment frame and a reinforced concrete bridge pier. The effect of duration is isolated from the effects of ground motion amplitude and response spectral shape by assembling sets of "spectrally equivalent" long and short duration records and employing them in comparative nonlinear dynamic analyses. For the modern steel moment frame, the estimated median collapse capacity is 29% lower when using the long duration set, as compared to the short duration set. For the concrete bridge pier, the collapse capacity is 17% lower. A comparison of commonly used duration metrics indicates that significant duration is the most suitable metric to characterize ground motion duration for structural analysis. Sensitivity analyses to structural model parameters indicate that structures with high deformation capacities and rapid rates of cyclic deterioration are the most sensitive to duration

Impact of hazard-consistent ground motion duration in structural collapse risk assessment

© 2016 John Wiley & Sons, Ltd. This study evaluates the effect of considering ground motion duration when selecting hazard-consistent ground motions for structural collapse risk assessment. A procedure to compute source-specific probability distributions of the durations of ground motions anticipated at a site, based on the generalized conditional intensity measure framework, is developed. Targets are computed for three sites in Western USA, located in distinct tectonic settings: Seattle, Eugene, and San Francisco. The effect of considering duration when estimating the collapse risk of a ductile reinforced concrete moment frame building, designed for a site in Seattle, is quantified by conducting multiple stripe analyses using groups of ground motions selected using different procedures. The mean annual frequency of collapse (λ collapse ) in Seattle is found to be underestimated by 29% when using typical-duration ground motions from the PEER NGA-West2 database. The effect of duration is even more important in sites like Eugene (λ collapse underestimated by 59%), where the seismic hazard is dominated by large magnitude interface earthquakes, and less important in sites like San Francisco (λ collapse underestimated by 7%), where the seismic hazard is dominated by crustal earthquakes. Ground motion selection procedures that employ causal parameters like magnitude, distance, and Vs 30 as surrogates for ground motion duration are also evaluated. These procedures are found to produce poor fits to the duration and response spectrum targets because of the limited number of records that satisfy typical constraints imposed on the ranges of the causal parameters. As a consequence, ground motions selected based on causal parameters are found to overestimate λ collapse by 53%

Accounting for the influence of ground motion response spectral shape and duration in the equivalent lateral force design procedure

A framework is proposed to explicitly account for the influence of ground motion response spectral shape and duration in the ASCE 7-16 equivalent lateral force design procedure, which currently considers only ground motion intensity, as quantified by Sa(T1). The scalar, dimensionless parameter SaRatio is used to characterise response spectral shape, while significant duration, Ds, is used to quantify duration. Design base shear adjustment factors are computed based on (i) the extended seismic hazard at a site, expressed in terms of the SaRatio and Ds values of the anticipated ground motions; and (ii) the sensitivity of the structure to the effects of response spectral shape and duration. Since these factors account for the influence of additional ground motion characteristics on structural collapse risk, their use in structural design should help achieve a more uniform distribution of collapse risk over different geographical regions and structural systems, in line with the objective of using risk-targeted seismic design maps. Sample calculations using the extended seismic hazard in Los Angeles as a benchmark indicate, for example, that a reinforced concrete moment frame building in Eugene with fundamental elastic modal period 1.0 s would need to be designed to a base shear 67 % higher than the current standard, while a similar structure in San Francisco would need to be designed to a base shear 43 % higher