Earthquake Spectra

This study investigates whether seismic design constraints reduce the architectural interest of buildings in areas subject to high rather than low seismicity. Sixty-three of the most architecturally interesting low-rise buildings from 20 cities, half of which are located in high-seismicity zones, were assessed aesthetically and analyzed structurally. On average, each group of buildings was found to possess the same level of architectural interest and degree of configuration irregularity. Reinforced concrete shear walls were found to be the predominant lateral force resisting system. A concentration of building types in the low-seismicity set of buildings that used structural walls for physical security and other purposes led to the unexpected result of those buildings possessing significantly greater structural footprints than buildings from high-seismicity areas. This finding serves as a reminder that structural elements play far more roles in architecture than merely resisting lateral forces.

In this study, we present a simple framework for the evaluation of the cost to repair building damage using engineering parameters related to current seismic building codes. Specifically, we envision relationships for building damage repair cost as a function of ground shaking spectral acceleration (Sa at structural period, T), design base shear coefficient (V/W), response modification factor (R), height, and framing system. This framework provides a flexible system for collecting and evaluating building damage from ground shaking, improving the building code, and predicting earthquake damage repair costs to new or existing, or retrofitted buildings, in a wide variety of seismic environments. In the absence of comprehensive statistical earthquake damage data, we resample the ATC-13 damage database to calibrate an initial damage model. We call the model CODA, for Code-Oriented Damage Assessment for Buildings.

Developed herein is a new peak ground acceleration (PGA)-based predictive model for 5% damped pseudospectral acceleration (SA) ordinates of free-field horizontal component of ground motion from shallow-crustal earthquakes. The predictive model of ground motion spectral shape (i.e., normalized spectrum) is generated as a continuous function of few parameters. The proposed model eliminates the classical exhausted matrix of estimator coefficients, and provides significant ease in its implementation. It is structured on the Next Generation Attenuation (NGA) database with a number of additions from recent Californian events including 2003 San Simeon and 2004 Parkfield earthquakes. A unique feature of the model is its new functional form explicitly integrating PGA as a scaling factor. The spectral shape model is parameterized within an approximation function using moment magnitude, closest distance to the fault (fault distance) and _VS30 (average shear-wave velocity in the upper 30 m ) as independent variables. Mean values of its estimator coefficients were computed by fitting an approximation function to spectral shape of each record using robust nonlinear optimization. Proposed spectral shape model is independent of the PGA attenuation, allowing utilization of various PGA attenuation relations to estimate the response spectrum of earthquake recordings.

This paper presents results of shake table tests on pipe systems anchored in a full-scale, seven-story building performed on the Large High-Performance Outdoor Shake Table at the University of California at San Diego on 1 May 2006. The purpose of the tests was to investigate the forces that act on post-installed anchors in buildings during a diverse range of earthquake ground motions. A sound understanding of the force levels and number of cycles is important for developing reliable anchor qualification approaches and seismic design guidelines. The tests also provide data on floor accelerations and acceleration amplification for nonstructural components in buildings during seismic events.

In the design of base-isolated buildings, a critical parameter governing the selection of isolator parameters is the peak displacement of the isolation layer. In this study, a model is developed in order to predict the peak base displacements utilizing multiple ground motion parameters, called intensity measures (IMs), as the inputs. The issue of correlation between various IMs is addressed through principal component analysis (PCA). This method also lends itself to dimensionality reduction, as those components that do not contribute significantly to the variance are discarded. The prediction intervals from the model are compared with the results from nonlinear dynamic analysis. An important conclusion is that by using the PCA based model, the standard errors remain relatively small and constant for a wide range of isolation periods. It is therefore clear that by utilizing multiple IMs and accounting for their correlation effects, it is possible to estimate the responses of base-isolated buildings with good confidence.

Retrofitted bridge fragility curves provide a powerful tool for assessing the effect of retrofit measures on the seismic performance of different bridge types under a range of loading levels. Traditional methods for retrofit assessment typically evaluate the effectiveness of retrofit based on the performance of individual components. However, the use of fragility curves for retrofitted bridges has the ability to capture the impact of retrofit on the bridge system vulnerability. Using three-dimensional nonlinear analysis, fragility curves are developed for four common classes of multispan bridges and five retrofit methods. The results show that the effectiveness of retrofit is a function of bridge type and damage state. General conclusions of the influence of the different retrofit measures on the fragility of each class of typical bridges in the Central and Southeastern United States, as well as the fragility parameters, are presented. The results from this work can be used to enhance regional seismic risk assessment and can form the basis for retrofit cost-benefit studies.

Nonlinear static procedures constitute an important tool in design office application of performance-based earthquake engineering concepts, and for this reason, they have been extensively developed and promoted in the last decade or so. However, these efforts focused predominantly on the assessment of buildings, rather than bridges, and hence there is currently a need to verify the validity in the application of such pushover-based methods for the assessment of bridges or viaducts. In this work, therefore, by considering a wide set of bridge configurations subjected to equally varying seismic input intensity levels, four commonly employed nonlinear static procedures (CSM, N2, MPA, ACSM) are scrutinized and compared, with a view to establish their adequacy for the seismic assessment of existing continuous span bridges. Results seem to indicate that all methods are able to predict displacement response with good accuracy, while force estimation, on the other hand, is reasonably attained only by those approaches where higher modes effects are explicitly accounted for.

Wood shear walls are the most common element in the lateral force-resisting system of residential construction. Recent developments have made the use of staples, as a sheathing-to-stud connection, much more feasible and practical. Dynamic cyclic tests of wood shear walls using staples as connectors of sheathing to the studs were performed to determine load and displacement capacities. Enhanced details from standard construction were used to improve the performance of the global system including a double sill plate, a new refined panel corner detail, double staples along blocked edges, and backup bolts for hold-downs. At the strength level, the stapled wood shear walls with enhanced details performed above the International Building Code-IBC (ICC 2006) regarding peak load capacity; however, peak drift capacity was 88% of the specified capacity. The test results were used to determine acceptance criteria, which were compared to the ASCE 41-06 Standard for Seismic Rehabilitation of Existing Buildings (ASCE 2007).

A displacement-based methodology for the preliminary design of a system of buckling-restrained braces is introduced. The methodology applies to the case of low-rise buildings, whose dynamic response is not significantly influenced by global flexural behavior or higher modes. The methodology is applied to the preliminary design of a five-story building located in the Lake Zone of Mexico City. From the evaluation of the global mechanical characteristics of the building and of its seismic performance when subjected to ground motions generated in that zone, it is concluded that the proposed methodology yields an adequate level of seismic design.