Non-Linear Time History Analysis of Tall Steel Moment Frame Buildings in LS-DYNA

Non-linear time history analyses were carried out in LS-DYNA in order to assess the seismic performance of existing tall steel moment resisting framed buildings. Ground motion earthquake records representative of the Maximum Considered Earthquake (MCE) hazard level defined in current building codes were utilized in the analysis. A number of different component models were utilized to capture the complex non-linear elements of the structure: beams, columns, panel zones and splices. Both beam and column elements were modeled using the Belytschko-Schwer element formulation with lumped plasticity at both ends of the resultant beam. Columns elements captured interaction between bi-axial bending moment and axial force, buckling in compression and degradation parameters for response under cyclic loads calibrated to match experimental tests results. Beams elements captured implicit degradation in bending and random fracture at the connections. The random fracture was modeled such that plastic rotation at fracture occurred as a random variable characterized by a truncated normal distribution following results from experimental testing. Panel zones and column splices were modeled with discrete elements and general nonlinear translational and rotational springs. Panel zones were modeled using the Krawinkler model by means of an assembly of rigid links and rotational springs to capture the tri-linear shear force-deformation relationship of the joint. Column splices were modeled as non-linear springs capable of reaching their nominal capacity with a sudden brittle failure in axial tension and/or bending and full capacity in compression as observed in experiments. The paper briefly discusses the advantages and limitations of utilizing an explicit solver in simulating the non-linear dynamic response of structural systems and components

Seismic Assessment and Retrofit Recommendations for Tall Steel Moment Frame Buildings in San Francisco

Most of the tall building stock in San Francisco was designed according to prescriptive guidelines that are now believed to be inadequate for tall building design. Furthermore, the lateral resisting system most commonly utilised in such buildings has shown significant vulnerability in past earthquake events. This document summarises the background and work conducted to date as part of a larger study, currently in progress, aimed at assessing seismic performance and providing retrofit recommendations where required for typical 1970s tall steel moment frame buildings in San Francisco

A comparative study on the seismic vulnerability of 1970s vs modern tall steel moment-resisting frame buildings

This paper outlines the methodology followed in the risk-based assessment of two archetype tall buildings in downtown San Francisco: a 50 story steel moment resisting frame (MRF) office building designed following the requirements of the Uniform Building Code of 1973 and a 50 story steel MRF office building designed following modern code requirements (International Building Code 2012). The methodology enables the development of the vulnerability function for the archetype buildings under consideration, highlighting loss contribution from (1) collapse, (2) irreparable damage from excessive residual deformations and (3) reparable damage. The goal of this study is to benchmark the performance of older existing steel MRF buildings against modern designs, providing an overall comparison of their seismic vulnerabilities. The results illustrate that existing tall steel MRF buildings from the 1970s are drastically more vulnerable to earthquakes than tall steel MRF buildings designed to modern standards. The vulnerability function for the 1970s archetype building highlights that collapse potential is the highest contributor to the losses, with a collapse fragility characterized by a relatively low median spectral acceleration. The resulting vulnerability function of the modern archetype building indicates that: i) at low ground motion intensities of shaking, losses are influenced by repairable damage; ii) at medium intensities of shaking losses are equally dominated by repairable damage and residual drift rendering the building irreparable; iii) collapse only starts contributing to the loss at large spectral amplitudes, but even then losses are largely dominated by residual drifts. The collapse fragility of the modern archetype building is in agreement with the design objective of modern building codes, which is to produce designs with low probability of collapse under a Maximum Considered Earthquake

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

Seismic Loss and Downtime Assessment of Existing Tall Steel-Framed Buildings and Strategies for Increased Resilience

In areas of high seismicity in the United States, the design of many existing tall buildings followed guidelines that do not provide an explicit understanding of performance during major earthquakes. This paper presents an assessment of the seismic performance of existing tall buildings and strategies for increased resilience for a case study city, San Francisco, where an archetype tall building is designed based on an inventory of the existing tall building stock. A 40-story moment-resisting frame system is selected as a representative tall building. The archetype building is rectangular in plan and represents the state of design and construction practice from the mid-1970s to the mid-1980s. Nonlinear response history analysis (NLRHA) are conducted with ground motions representative of the design earthquake hazard level defined in current building codes, with explicit consideration of near-fault directivity effects. Mean transient interstory drifts and story accelerations under the 10% in 50-year ground motion hazard range from 0.19 to 1.14% and 0.15 to 0.81 g, respectively. In order to influence decision making, performance is reported as the expected consequences in terms of direct economic losses and downtime. Furthermore, to achieve increased levels of resilience, a number of strategies are proposed including seismic improvements to structural and nonstructural systems as well as mitigation measures to minimize impeding factors. Expected direct economic losses for the archetype building are in the order of 34% of building cost and downtime estimates for functional recovery are 87 weeks. The strategies presented in this paper enable up to a 92% reduction in losses and minimize downtime for functional recovery to 1 day or less

Loss assessment of tall buildings from a vulnerability perspective

As the number of tall buildings in seismic areas around the world continues to grow, the ability to perform loss assessments becomes increasingly important. Due to their size, tall buildings house many businesses and/or residents, and any damage to these buildings has the potential to affect a large number of people. Furthermore, these buildings are expensive to build and repair. The financial resources needed to recover from the damage induced by earthquakes are generally not trivial amounts, and thus the ability to realistically model losses in tall buildings becomes essential. The loss assessment of tall buildings presents unique challenges, including the tendency for significant damage to be concentrated in a few stories rather than distributed throughout the building. The presence of excessive residual drifts in one or a few stories can result in the building being declared a total loss and demolished, even when the levels of damage in the rest of the building are relatively low. Accessibility issues can increase repair costs in a tall building relative to a shorter building as, for example, it is much easier to replace the window on the 2nd story of a 5-story building versus on the 20th story of a 50-story building. The long first-mode periods of tall buildings as well as the significant contribution of higher modes means that the ground motions used to assess the structural response must be carefully considered as both the low frequency and high frequency components of the ground motion affect the response. The evolution of building design is also an important factor in the loss assessment of tall buildings. The trend in recent years toward performance-based designs and a growing awareness for designs that reduce expected seismic losses play an important role in differentiating the expected losses of newer versus older tall buildings. This is in addition to the effects of advances in building codes and design practice that are typically seen over time, such as improvements in designing for ductility and reducing the risk of connection fractures in steel moment-resisting frames. This study examines the loss assessment of tall buildings from a vulnerability perspective, drawing on the unique characteristics of tall buildings previously noted. It discusses how the vulnerability characteristics of tall buildings affect the relative seismic risk and uses examples of major cities in North America and New Zealand to illustrate the effects

Seismic Loss and Downtime Estimates of Existing Tall Buildings and Strategies for Increased Resilience

Tall buildings play an important role in the socio-economic activity of major metropolitan areas. The resilience of these structures is critical to ensure a successful recovery after major disasters. Events such as the Canterbury earthquake in 2011 have highlighted the impact of poor performing buildings on the business continuity of downtown districts, where tall buildings are typically clustered together. Following the 2011 earthquake, Christchurch’s Central Business District (CBD) red zone covered a significant area of the city and more than 60% of the businesses were displaced (CERC 2012). Until the introduction of Performance Based Seismic Design (PBSD) in the 1990s, buildings were designed using conventional building codes, which follow a prescriptive force-based approach based on the first mode translational response of the structure (FEMA 2006). Researchers and engineers have raised concerns that the prescriptive approach of building codes is not suitable for tall building design due to the significant contribution of higher mode effects (PEER 2010a). As a result of these shortcomings, several jurisdictions in areas of high seismicity throughout the Unites States (e.g. Los Angeles and San Francisco) have adopted a PBSD approach for the design of new tall buildings. While new designs follow a more adequate approach, little is known about the seismic performance of older existing tall buildings that were designed prior to the adoption of PBSD (Almufti et al. 2012). This paper presents an assessment of the seismic performance of existing tall buildings in a case study city, San Francisco, where an archetype tall building is designed based on an inventory of the existing tall building stock. Non-Linear Response History Analysis (NLRHA) are conducted with ground motions representative of the design earthquake hazard level defined in current building codes, with explicit consideration of near-fault directivity effects. In order to influence decision making, performance is reported as the expected consequences in terms of direct economic losses and downtime. Once the performance of the archetype building is assessed, a range of structural and nonstructural enhancements are explored for enhanced performance as well as mitigation measures for increased resilience. Expected direct economic losses for the archetype building are in the order of 34% of building cost and downtime estimates for functional recovery are 87 weeks. The strategies presented in this paper enable up to a 92% reduction in losses and minimize downtime for functional recovery to 1 day

Risk-based seismic performance assessment of existing tall steel-framed buildings in San Francisco

This study presents the results of a risk-based seismic performance assessment of an archetype tall building representative of the existing tall building stock in San Francisco, CA. The archetype tall building, selected based on an inventory of existing tall buildings, is a 40- storey Moment Resisting Frame (MRF) representative of design and construction practice from the 1970-s to the mid-1980s. A Multiple Stripe Analysis (MSA) was conducted at 8 different intensity levels ranging from frequent to very rare seismic events, i.e. from 85% to 1% probability of exceedance in 50 years. Non-Linear Response History Analyses (NLRHA) were conducted with ground motions representative of each intensity level considered. The results of the NLRHA results were used to assess the probability of earthquake losses, considering collapse potential and the probability of the building deemed irreparable due to permanent residual drifts in the structure. Based on the MSA results, the collapse fragility of the structure, assumed to follow a lognormal cumulative distribution expressed as a function of spectral acceleration at the fundamental period of the structure (T=5 seconds), has an estimated median of 0.15g and a dispersion of 0.30. A number of loss metrics were developed for the archetype building including: a loss function, which provides the annual frequency of exceeding a certain value of loss, e.g. the expected 500 year loss equals $53M or 39$0.6M or 0.46% of the building replacement cost; and loss exceedance rates, e.g. a loss of 10% building replacement cost or $13.5M has an exceedance rate of 95 years. The use of these results to benchmark the performance of the archetype tall building against the design intent in current building codes and to assess the impact of structural retrofit or other building enhancements is discussed

Are residents of Seattle ready for ‘the Big One? An intervention study to change earthquake preparedness

Background: Community preparedness for natural hazards remains poor across cultures. In addition, evaluated intervention studies in natural hazard preparedness are scarce and contain methodological problems. This study presents results of an intervention study on earthquake preparedness conducted in Seattle, U.S.A. Methodology: This is a quasi-experimental, longitudinal, community intervention with a pretest-posttest design, focused on improving earthquake readiness at the household level. The sample included 157 adult residents of Seattle. Preparedness measures were assessed at baseline, one week after the intervention, and at three and 12 months after the intervention. This involved both of the groups in a survey and an observation of preparedness levels in their homes. The primary outcome measure was an observational tool of five preparedness items, which was implemented alongside a survey that measured psychological, social, demographic and self-reported preparedness variables. In addition, the intervention group completed a six-hour workshop on earthquake preparedness, divided over two days. The control group did not participate in the workshop. Results: The intervention group significantly improved their earthquake preparedness levels compared to baseline and to controls one week after the intervention. Nonetheless, the effect of the intervention faded at the 3-month followup, where no significant differences in earthquake preparedness were observed in the intervention group compared to baseline. In fact, preparedness appeared to increase for controls at three months compared to baseline and to one week after the intervention, and although not reaching statistical significance, it exceeded the intervention group’s preparedness levels. Anxiety and trust predicted earthquake preparedness for the control group at three months. Discussion: Despite levels of earthquake preparedness improving significantly for the intervention group right after the intervention, this effect disappeared at the 3 month follow-up, stressing the need for the field to develop measures to facilitate the maintenance of behaviour change over time. Interestingly, controls continued to improve their levels of preparedness, suggesting that the home assessments themselves might have acted as an intervention that was sufficiently powerful to trigger behaviour change in controls. Contrary to the emphasis on self-efficacy and other cognitive variables in the literature concerning natural hazard preparedness, these results suggest that emotions such as anxiety and trust might play a more important role in preparedness. Future preparedness interventions should put emotive factors centre stage in targeting preparedness. The findings of this study have implications for national and international policies on the design and delivery of community interventions to increase hazard preparedness in lay people