Earthquake Spectra

The Sichuan (China) and L'Aquila (Italy) earthquakes have again highlighted the question of our preparedness for natural hazards. Within a few seconds, an earthquake can demolish many buildings, destroy infrastructure, and kill and injure thousands of people. In order to reduce the impact of earthquakes on human life and to prepare hospitals to cope with future disasters, this paper discusses earthquake-related damage to healthcare facilities. It investigates the damage to 34 healthcare facilities in seven countries caused by nine earthquakes between 1994 and 2004, in order to determine common and specific issues. The investigation shows that structural and architectural damage tended to be different and specific to the situation, while utility supplies and equipment damage were similar in most cases and some common trends emerged.

Three different buildings built according to the same design have experienced three different near-field strong ground motions over a period of 11 years in three different cities in Turkey. The input motion was known for each because strong-motion sensors were located adjacent or close to the buildings. We examine the performance of the five-story, reinforced concrete-frame buildings. Bidirectional nonlinear time history and nonlinear static analyses on 3-D analytical models are performed. The principal focus is to assess whether the analytical model of the buildings could indicate column-beam damage consistent with that observed at the sites after the earthquakes. Results illustrate that nonlinear time history analyses are capable of indicating the occurrence of shear failure in captive columns; however, they overestimate the global damage. The overestimation is greater where the building sustained a pulse-type motion without significant distress. It appears that difference between visual observations and analytical results persists.

The structural reliability in terms of maximum interstory drift—and, alternatively, in terms of plastic hysteretic energy—is evaluated for six regular moment-resisting steel frames designed according to the Mexico City Building Code, and located in the Lake Zone of that city. While the maximum interstory drift was used because of its relevance within the format of current seismic design codes, the plastic hysteretic energy was considered due to its importance for the performance of structures when subjected to severe cumulative plastic deformation demands. The demand hazard curves of the frames in terms of drift and energy are compared to provide a general idea of the reliability levels associated to the models, and to provide insights into which response parameter dominates their dynamic behavior and structural performance. In some cases, large differences are observed in the reliabilities computed by measure of the two different response parameters under consideration.

Fragility functions are generated for bridges in liquefied and laterally spreading ground using equivalent static global nonlinear finite element analyses. Bridges are classified based on structural configurations and vintage. Probability density functions are assigned to both structural and geotechnical properties of bridges. Nonlinear equivalent static analyses are conducted with inputs sampled randomly using the Monte Carlo simulation method. Cumulative distribution functions are fitted to the simulated data, and define the probability of exceeding various engineering demand parameters (pier column curvature ductility, pile cap displacement, abutment displacement, etc.) conditioned on the maximum free-field lateral spreading ground surface displacement. Correlations among EDPs are presented to facilitate risk assessment based on a vector of EDPs. The derived fragility functions, combined with seismic hazard analysis, liquefaction potential, and lateral spreading estimation, are useful in the context of performance-based earthquake engineering and risk assessment of current bridge inventory in California.

There has been widespread interest in the development and use of self-centering (SC) lateral resisting systems that eliminate residual drifts after large earthquakes. SC systems often include a restoring force component and a component that dissipates seismic energy. Typically, it is assumed that the criterion for self-centering is satisfied if the restoring force is proportioned to be greater than the force required to yield the energy dissipating component. A parametric SDOF study was conducted using time-history analyses on several prototype buildings to quantify the effect of varying system parameters on structural response including residual drifts. The ambient resistance of the rest of the building was considered, as well as proportioning the system with less restoring force than the yield capacity of the dissipative component. In addition, the probabilistic mechanism that creates a propensity for reducing residual drifts in systems with little or no restoring force is explored and quantified. It was found that a restoring force that is at least one-half of the force required to yield the dissipative component will still reliably eliminate residual drifts in a non-softening system.

Despite their importance to successful port operations, container cranes have received little attention in the context of seismic behavior. This paper presents the results of scale testing and analysis of a typical jumbo container crane subjected to earthquake loading. A series of 1:20 scale shake table tests of the model structure is performed under increasing levels of acceleration to assess and characterize the critical responses. Complimentary detailed nonlinear finite element time history analyses are also carried out. It is found that the portal frame response dominates elastic behavior and is closely coupled with an uplift and derailment rocking-type response at higher excitation levels. Proposed analytical models can capture this coupled response well, while a simple pseudostatic analysis can accurately estimate the onset of uplift and derailment.

Building collapse is the dominant cause of casualties during earthquakes. In order to better predict human fatalities, the U.S. Geological Survey's Prompt Assessment of Global Earthquakes for Response (PAGER) program requires collapse fragility functions for global building types. The collapse fragility is expressed as the probability of collapse at discrete levels of the input hazard defined in terms of macroseismic intensity. This article provides a simple procedure for quantifying collapse fragility using vulnerability criteria based on the European Macroseismic Scale (1998) for selected European building types. In addition, the collapse fragility functions are developed for global building types by fitting the beta distribution to the multiple experts’ estimates for the same building type (obtained from EERI's World Housing Encyclopedia (WHE)-PAGER survey). Finally, using the collapse probability distributions at each shaking intensity level as a prior and field-based collapse-rate observations as likelihood, it is possible to update the collapse fragility functions for global building types using the Bayesian procedure.

Dynamic structural analysis often requires the selection of input ground motions with a target mean response spectrum. The variance of the target response spectrum is usually ignored or accounted for in an ad hoc manner, which can bias the structural response estimates. This manuscript proposes a computationally efficient and theoretically consistent algorithm to select ground motions that match the target response spectrum mean and variance. The selection algorithm probabilistically generates multiple response spectra from a target distribution, and then selects recorded ground motions whose response spectra individually match the simulated response spectra. A greedy optimization technique further improves the match between the target and the sample means and variances. The proposed algorithm is used to select ground motions for the analysis of sample structures in order to assess the impact of considering ground-motion variance on the structural response estimates. The implications for code-based design and performance-based earthquake engineering are discussed.

A two-story, three-bay RC frame with code incompliant seismic design and detailing is tested using continuous pseudodynamic test method for three scale levels of Düzce ground motion. The ground motion produced minimum, significant, and severe damage states on the test structure. Diagonal cracking of the infill wall, column damage in the form of cover spalling and rebar buckling, and complete disintegration of the infill wall were the important observed damage events for the three scale levels, respectively. Nonlinear time history analyses were able to estimate the story displacement response with reasonable accuracy. The importance of element removal for near collapse damage state is unfolded. Tracing the local engineering demand parameters such as strains and curvature was found to be extremely difficult.

Building codes and standards now require seismic qualification of mechanical and electrical equipment and their mounting systems in important buildings to ensure that they remain functional during and after major seismic events. To better understand the seismic behavior of nonstructural building contents and equipment, experimental procedures have been proposed for either displacement or acceleration sensitive nonstructural components, through racking or shake table protocols, respectively. However, certain types of nonstructural systems are sensitive to both accelerations and interstory drifts. An innovative testing protocol is proposed that can subject nonstructural systems to the combined accelerations and interstory drifts expected within multistory buildings during seismic shaking. Moreover, the proposed protocol, when used with equipment such as the University at Buffalo Nonstructural Component Simulator (UB-NCS), allows for the assessment of the seismic performance of distributed nonstructural systems with multiple attachment points, and the evaluation of seismic interactions between components. The versatility and capabilities of the testing protocol are demonstrated through testing of a full-scale hospital emergency room containing typical nonstructural components and life support medical equipment.

An automated performance-based design methodology to optimize structural and nonstructural system performance is outlined and it is shown that it can be used to enhance understanding of structural steel system design for minimum life-cycle costs. Performance is assessed using loss probability with direct economic loss expressed as a percentage of the building replacement cost. Time-based performance assessment is used to compute the expected annual loss of a given steel framing system assuming exposure to three seismic hazard levels. Damage to the structural system, nonstructural displacement-sensitive components, and nonstructural acceleration-sensitive components is characterized using fragility functions. A steel building with three-story, four-bay topology taken from the literature is used to demonstrate application of the algorithm with subsequent comparison of designs obtained using the proposed methodology and others found in the literature.

This paper advances the state of the art in the development and application of a computable general equilibrium model to estimate the business interruption impacts of a Verdugo scenario earthquake on the water system serving Los Angeles. The model has been especially designed to incorporate engineering and spatial aspects of this system in the context of the regional economy to include resilience responses at various water outage levels. The Verdugo earthquake scenario and Monte Carlo simulations show that water outages in LA County could result in business interruption losses of several billion dollars without any resilience adjustment. However, a reduction of these losses by more than 90% is possible through the application of several types of resilience on the customer side, most prominently rescheduling production, in addition to conservation, input substitution, and storage of water. Allowing the price of water to rise to reflect its scarcity would reduce the losses even further.

This paper describes the methodology followed to derive typological seismic risk maps for Italy and then presents the results. In its classical definition, seismic risk is obtained from the convolution of hazard, vulnerability and exposure. Due to the absence of reliable data on exposure for the entire Italian territory, this study proposes typological seismic risk maps, obtained by simply convolving hazard and vulnerability for several building typologies characteristic of the Italian building stock. A specific hazard study in terms of PGA has been carried out. The results have then been convolved with empirical typological fragility curves, that were derived from data collected during post-earthquake surveys after the main Italian events of the last 30 years. Useful applications can be found for the typological seismic risk maps, both for risk mitigation strategies and for purely economical evaluations (e.g., insurance and reinsurance studies).

The 2009 NEHRP Provisions modified the definition of horizontal ground motion from the geometric mean of spectral accelerations for two components to the peak response of a single lumped mass oscillator regardless of direction. These maximum-direction (MD) ground motions operate under the assumption that the dynamic properties of the structure (e.g., stiffness, strength) are identical in all directions. This assumption may be true for some in-plan symmetric structures, however, the response of most structures is dominated by modes of vibration along specific axes (e.g., longitudinal and transverse axes in a building), and often the dynamic properties (especially stiffness) along those axes are distinct. In order to achieve structural designs consistent with the collapse risk level given in the NEHRP documents, we argue that design spectra should be compatible with expected levels of ground motion along those principal response axes. The use of MD ground motions effectively assumes that the azimuth of maximum ground motion coincides with the directions of principal structural response. Because this is unlikely, design ground motions have lower probability of occurrence than intended, with significant societal costs. We recommend adjustments to make design ground motions compatible with target risk levels.

This paper compares base shears computed from floor accelerations (inertial base shear) and column shears (structural base shear) for two mid-rise, multistory buildings due to a suite of 30 earthquake ground motions. The presented results demonstrate that the inertial base shear exceeds the structural base shear in the median by 10% to 20% and may exceed the structural base shear by as much as 70% for individual ground motions. Therefore, it is concluded that the inertial base shear computed from strong motion records should be used with caution to estimate the structural base shear.