Earthquake Spectra

Analysis of the seismic risk to a structure requires assessment of both the rate of occurrence of future earthquake ground motions (hazard) and the effect of these ground motions on the structure (response). These two pieces are often linked using an intensity measure such as spectral acceleration. However, earth scientists typically use the geometric mean of the spectral accelerations of the two horizontal components of ground motion as the intensity measure for hazard analysis, while structural engineers often use spectral acceleration of a single horizontal component as the intensity measure for response analysis. This inconsistency in definitions is typically not recognized when the two assessments are combined, resulting in unconservative conclusions about the seismic risk to the structure. The source and impact of the problem is examined in this paper, and several potential resolutions are proposed. This discussion is directly applicable to probabilistic analyses, but also has implications for deterministic seismic evaluations.

A primary aim of recent earthquake engineering research has been to integrate analyses of social and technical systems, to take into account the potential consequences of earthquakes for society as a whole. In some cases the resulting decision frameworks treat acceptable consequences and risks as fixed objectives. This paper describes the rationale for abandoning a priori assessment of the acceptability of earthquake consequences as conceived in consequence-based engineering (CBE). Support for an alternative approach comes from behavioral and policy research including an examination of seismic retrofit decision processes for U.S. General Services Administration-owned federal buildings. Additional review of research on earthquake mitigation policy making at the local government level also supports the case. We then suggest dynamic decision structuring that would better support the integration of engineering expertise and decision makers’ preferences.

This paper outlines the last two phases of a joint research study performed by the University of Texas at Austin and the U.S. Army Corp of Engineers, Construction Engineering Research Laboratory, Engineer Research and Development Center (CERL). The study coordinates and synthesizes experimental testing, analytical modeling, practical implementation, and real-world application to enhance FEMA-310, the predominant seismic evaluation methodology for low-rise reinforced masonry buildings with flexible diaphragms. In earlier phases of study, conclusions from shaking-table testing, quasi-static testing, and analytical modeling were used to develop a simple tool for the seismic analysis of these types of buildings. In this paper, the tool is developed in the context of performance-based earthquake engineering into a supplementary evaluation methodology intended to fill a gap in FEMA-310. The tool is applied to four existing buildings and ultimately shown to be simple, useful, and necessary.

Downtime includes the time necessary to plan, finance, and complete repairs on facilities damaged in earthquakes or other disasters. It is an essential component of loss modeling, because it is one measure of operational failure in lifelines and business interruption in buildings. Data from building repairs after the Loma Prieta and Northridge earthquakes, together with institutional risk management projections are used to define the rational and irrational components of downtime for buildings. Rational components include construction costs and time. Irrational situation-specific components take into account the time needed to mobilize for repairs and include financing, relocation of functions, workforce availability, regulatory changes, and economic uncertainty. To quantify these factors, the pace of recovery can be linked to the scale of damage in a stock of buildings in the affected region.

This paper investigates the consequences of well-known characteristics of near-fault ground motions on the seismic response of steel moment frames. Additionally, idealized pulses are utilized in a separate study to gain further insight into the effects of high-amplitude pulses on structural demands. Simple input pulses were also synthesized to simulate artificial fling-step effects in ground motions originally having forward directivity. Findings from the study reveal that median maximum demands and the dispersion in the peak values were higher for near-fault records than far-fault motions. The arrival of the velocity pulse in a near-fault record causes the structure to dissipate considerable input energy in relatively few plastic cycles, whereas cumulative effects from increased cyclic demands are more pronounced in far-fault records. For pulse-type input, the maximum demand is a function of the ratio of the pulse period to the fundamental period of the structure. Records with fling effects were found to excite systems primarily in their fundamental mode while waveforms with forward directivity in the absence of fling caused higher modes to be activated. It is concluded that the acceleration and velocity spectra, when examined collectively, can be utilized to reasonably assess the damage potential of near-fault records.

Gypsum drywall partitions may contribute significantly to the lateral strength and stiffness of woodframe structures, whether or not the walls are explicitly designed for that purpose. This paper proposes analytical models to determine the lateral shear strength and initial elastic stiffness of wood-framed gypsum wall panels, taking into account the effects of wall geometry, door and window openings, connector type and spacing, and wall boundary conditions. The strength and stiffness models are incorporated in a multilinear curve to describe the monotonic lateral shear versus deformation response of the walls. Additional parameters to calibrate the response of a peak-oriented hysteretic cyclic model are also proposed, thus making the models suitable for nonlinear time-history simulations of woodframe buildings. The models are developed and validated using published data from 16 shear tests of full-scale gypsum wall panels.

The requirements of performance-based earthquake engineering place increasing importance on the optimal characterization of earthquake ground motions. With respect to liquefaction hazard evaluation, ground motions have historically been characterized by a combination of peak acceleration and earthquake magnitude, and more recently by Arias intensity. This paper introduces a new ground motion intensity measure, _CAV5, and shows that excess pore pressure generation in potentially liquefiable soils is considerably more closely related to _CAV5 than to other intensity measures, including peak acceleration and Arias intensity. _CAV5 is shown to be an efficient, sufficient, and predictable intensity measure for rock motions used as input to liquefaction hazard evaluations. An attenuation relationship for _CAV5 is presented and used in an example that illustrates the benefits of scaling bedrock motions to a particular value of _CAV5, rather than to the historical intensity measures, for performance-based evaluation of liquefaction hazards.

This paper sets out the foundations for developing robust models of community recovery from earthquake disasters. Models that anticipate post-disaster trajectories are complementary to loss estimation models that predict damage and loss. Such models can serve as important decision support tools for increasing community resilience and reducing disaster vulnerability. The paper first presents a comprehensive conceptual model of recovery. The conceptual model enumerates important relationships between a community's households, businesses, lifeline networks, and neighborhoods. The conceptual model can be operationalized to create a numerical model of recovery. To demonstrate this, we present a prototype computer simulation model and graphical user interface. As the model is intended for decision support, it is important to involve potential users in model development. We conducted a focus group involving Puget Sound, Washington, area disaster management practitioners to elicit local insight about community recovery and model development needs, using the prototype as stimulus. Important focus group issues included potential model inputs, useful recovery indicators, potential uses of recovery models, and suitable types of software systems.

In advanced earthquake analysis/design methods, the cyclic behavior of reinforced concrete (RC) members, which is characterized by strength, deformability, and the amount of dissipated energy, must be estimated with reasonable precision. However, presently, the amount of dissipated energy is estimated by either empirical equations, which are not sufficiently accurate, or experiments and sophisticated numerical analysis, which are difficult to use in practice. In the present study, nonlinear finite element analysis was performed to investigate the behavioral characteristics of flexure-dominated RC members subject to moderate plastic displacements. The results showed that flexural pinching can occur due to the effects of axial compressive force and asymmetrical rebar arrangement. However, axial force has little effect on the energy dissipation. The arrangement and ratio of reinforcement have substantial effects. Based on the findings, a simplified method to estimate the energy dissipated by flexure-dominated members was developed, and was verified by comparing its results with those of existing experiments on beams, columns, and structural walls.

The performance of an urban electric utility distribution system was evaluated for the February 2001 Nisqually earthquake. The restoration rate of the lifeline following the event was determined; the distribution of outage durations was estimated; and correlations between lifeline damage and instrumental Modified Mercalli intensity, peak ground velocity, and peak ground acceleration values were ascertained using a GIS (geographical information systems) approach. Using a logit regression analysis, a fragility curve was developed for the lifeline in a manner similar to O'Rourke's formulation of water-line performance (O'Rourke et. al. 2000). Extrapolation of the model to the Seattle Fault earthquake scenario was made to demonstrate its feasibility for prediction.

The ground motions of the Chi-Chi, Taiwan, earthquake ( _Mw =7.6) were recorded at 420 strong-motion stations, including 69 near-fault sites. However, the site conditions of many stations are not available. Among 420 strong-motion stations, the site conditions are known for only 87 stations, which were classified into four groups (_S1, _S2, _S3, and _S4) by using borehole data and some surface geology. This paper presents a methodology to estimate the missing site condition information at strong-motion stations in Taiwan. The method is based on the shape of the 5% damped pseudo-acceleration spectrum of the horizontal ground motion component normalized with respect to average PGA, where the classification scheme is developed using the data from the 87 stations for which the site conditions are known. Possible effects of soil nonlinearity, and distance to the fault on the classification are investigated. The results obtained from the proposed methodology are well correlated with the available known site classification information data. The methodology is then applied to estimate the site condition for the other 333 stations without known site classification. Our results are compared to previous results obtained based on interpretation of geologic maps and geomorphologic data. We find that the two approaches agree in 71% of the cases. We also tested the horizontal-to-vertical spectral ratio technique to estimate the site classification of other 333 strong-motion stations. However, this technique resulted in lower accuracy than does the proposed technique based on the spectral shape of normalized response spectra.

The design criteria for a new, low-cost base dissipation technique for simple masonry buildings are presented together with the results of some numerical analyses. The proposed apparatus, termed the “reinforced cut wall” (RCW), is made up of a layer of weak mortar (50 mm) and a 4-mm-thick elastomer membrane sheath laid between the foundation and base of the wall, which is further reinforced by a series of vertical steel rods (8–12 mm in diameter) connecting it to the foundation. Experimental trials have been performed on a sample set of dissipators in order to establish their mechanical properties and define suitable assembly techniques. Numerical simulations, consisting of incremental nonlinear static analyses, of several types of masonry buildings highlight the system's efficient performance in terms of force-displacement law. For moderate horizontal actions, the RCW does not cause relevant foundation-base displacement. However, when a prescribed force limit is exceeded, friction behavior of the weak layers of the RCW permit inelastic deformation along both horizontal directions, while the vertical steel rods ensure an elastic restoring force for rocking movements as well. Furthermore, the RCW provides the added benefit of isolating the structure from ground humidity.

The economic equivalent value of deaths and injuries in the 1994 Northridge earthquake has not previously been calculated, although number of injuries by category of treatment has. Using dollar-equivalent values for injuries accepted and used by the U.S. government for evaluating the cost-effectiveness of risk-mitigation efforts, the value of injuries in the 1994 Northridge earthquake is estimated to be $1.3 to 2.2 billion in 1994 (90% confidence bounds, equivalent to $1.8 to 2.9 billion in 2005). This is equivalent to 3–4% of the estimated $50 billion (in 1994) estimated direct capital losses and direct business interruption losses. If injuries in the 1994 Northridge earthquake are representative of injuries in future U.S. events, then the economic value of future earthquake injuries—the amount that the U.S. government would deem appropriate to expend to prevent all such injuries—is on the order of $200 million per year (in 2005 constant dollars). Of this figure, 96% is associated with nonfatal injuries, an issue overlooked by current experimental research. Given the apparently high cost of this type of loss, this appears to represent an important gap in the present earthquake research agenda.