Earthquake Spectra

A novel modal identification approach for the use of a wireless sensor network (WSN) for structural health monitoring is presented, in which the computational task is distributed among remote nodes to reduce the communication burden of the network and, as a result, optimize the time and energy consumption of the monitoring system. Considering the need for having an agile system to capture the earthquake response and also the limited energy resource in WSN, such algorithms for speeding the analysis time and preserving energy are essential. The algorithm of this study, called iterative modal identification (IMID), relies on an iterative estimation method that solves for unknown parameters in the absence of complete information about the system. Applying IMID in WSN-based monitoring systems results in significant savings in time and energy. Validation through implementation of the algorithm on numerically simulated and experimental data illustrates the superior performance of this approach.

Much research on people's seismic adjustment activity in highly seismic areas has assumed that low levels of adjustment are attributable to insufficient awareness of seismic risk. Empirical evidence for this assumption is weak, and there is growing appreciation of the role played by sociocultural and emotional variables in risk perception and behavior. This study explored these socio-cultural and emotional dimensions via 144 interviews and questionnaires, with matched samples of locals in Seattle (United States), Osaka (Japan), and Izmir (Turkey). The data showed that high awareness of possible seismic adjustment measures was not translated into behavior, with all sites demonstrating low adjustment uptake, though the North Americans adopted significantly more adjustments than the other cultures. Thematic analysis of the interview data suggested that adjustment behavior was undermined by anxiety, distrust, distancing self from earthquake risk and fatalistic beliefs. The paper concludes by recommending how culture-specific disaster mitigation plans may be developed to address these factors.

A thorough four-step performance-based seismic evaluation for a six-story unreinforced masonry building is conducted. Incremental dynamic analysis is carried out using the applied element method to take advantage of its ability to simulate progressive collapse of the masonry structure including out-of-plane failure of the walls. The distribution of the structural responses and inters-tory drifts from the incremental dynamic analysis curves are used to develop both spectral-based (Sa) and displacement-based (interstory drift) fragility curves at three structural performance levels. The curves resulting from three-dimensional (3-D) analyses using unidirectional ground motions are combined using the weakest link theory to propose combined fragility curves. Finally, the mean annual frequencies of exceeding the three performance levels are calculated using the spectral acceleration values at four probability levels 2%, 5%, 10%, and 40% in 50 years. The method is shown to be useful for seismic vulnerability evaluations in regions where little observed damage data exists.

Two-dimensional computational models of bridge abutment-soil systems are developed using the OpenSees framework. Nonlinear hysteretic behavior of backfill and foundation soil is simulated through a pressure-dependent nested yield surface plasticity model. Sliding and debonding at the soil-structure interfaces are simulated using node-to-node contact elements. Cantilever abutments and retaining walls of 6 m and 12 m height are analyzed using two recorded free-field accelerograms scaled for peak ground acceleration values of 0.12 g, 0.24 g, 0.36 g, 0.48 g, 0.60 g, and 0.72 g. Further, pseudo-static analysis using Mononobe-Okabe theory is carried out with varying seismic coefficients, and the results are compared with those of the dynamic analyses. Based on this comparison, recommendations are made for the value of seismic coefficient to be used in pseudo-static analysis so as to obtain results that are comparable to those obtained through a sophisticated nonlinear analysis.

In order to evaluate the capability of building damage detection from optical satellite images, a procedure for digital image analysis is examined and applied to images captured before and after the 2006 Central Java, Indonesia, earthquake. In the image analysis, the pixels of the images are classified into vegetation, bare ground, and built-up areas. The damage areas are detected by the differential of the digital numbers in the built-up areas. The estimated damage distribution is validated by comparing it with the GIS data on building damage obtained from a field survey. The results show that the severely damaged areas were well detected by the analysis. In the densely vegetated areas, however, the damage was underestimated because many of the buildings were obscured by trees. For assessing quantitative damage information, the relationship between the number of collapsed buildings and the areas detected by the image analysis is evaluated.

An iterative linear-equivalent procedure to take into account nonlinear soil-structure interaction effects in the displacement-based seismic design is presented for the case of shallow foundations. The procedure is based on the use of empirical curves to evaluate the stiffness degradation and the increase of damping ratio as a function of foundation rotation. Iterations are performed to ensure that admissible values of foundation rotations are complied with, in addition to the standard checks on structural displacements and drifts. Some examples of application of the approach to the design of bridge piers are provided. Design results are checked by means of nonlinear dynamic time-history analyses performed by a macro-element-based numerical tool, assuming nonlinear behavior of both structure and soil-foundation system.

This paper concerns the analysis of the site amplification that significantly influenced the non-uniform damage distribution observed at San Giuliano di Puglia (Italy) after the 2002 Molise earthquake (M_W = 5.7). In fact, the historical core of the town, settled on outcropping rock, received less damage than the more recent buildings, founded on a clayey subsoil. Comprehensive geotechnical and geophysical investigations allowed a detailed definition of the subsoil model. The seismic response of the subsoil was analyzed through 2-D finite-element and 3-D spectral-element methods. The accuracy of such models was verified by comparing the numerical predictions to the aftershocks recorded by a temporary seismic network. After calibration, the seismic response to a synthetic input motion reproducing the main shock was simulated. The influence of site amplification on the damage distribution observed was finally interpreted by combining the predicted variation of ground motion parameters with the structural vulnerability of the buildings.

In analysis of structures subjected to multisupport excitations, generally in addition to the support acceleration time histories as inputs, the velocity and displacement time histories are also required. To identify the cases in which it is sufficient to use just the acceleration histories of supports, this paper attempts to show that there is a relation between multieomponent and multisupport excitation analyses. By considering the ordinary equations of motion as a special case of a more general case in which the motions of all degrees of freedom are taken into account, a matrix transformation is introduced, by which the governing equations of the multisupport excitation case are changed into ordinary equations of the dynamic response analysis of multi-degree-of-freedom (MDOF) systems. It has been shown that the “matrix of earthquake influence factors” cannot be defined for all structures in general state of multisupport excitation.

Experimental studies on the dynamic response of structures comprising soil-foundation systems require an appropriately constructed soil-foundation model below the superstructures in order to properly estimate structural responses. In most studies, applying a small scaling is necessary for constructing the entire structural system, since there is limited space on shaking tables. This constraint has been a hindrance in experimental studies. Thus this study proposes a mechanical interface (MI) that represents the impedance characteristics of a 3 × 5 pile group embedded in a layered soil medium. The MI is constructed on the basis of lumped parameter models with gyro-mass elements. This element is mechanically realized in the MI using a rotational mass in combination with coupling gears. The results show that the MI properly simulates the impedance functions with frequency-dependent oscillations, and shaking table tests using the MI for an inelastic structure are demonstrated.

Despite numerous research efforts in recent years, seismic risk continues to be difficult to perceive and communicate. Although researchers have access to sophisticated tools that can quantify seismic risk, such groups as public authorities, land use and urban planners, stakeholders, end-users, and citizens should also be able to access simple seismic risk information. Thus, SIRIUS was built and mapped into a scale following the Weber and Fechner perception law, with impacts described in a simple yet meaningful language while capturing the two most fundamental dimensions that explain risk variability along the urban space: the reliability deficit and human concentration. With SIRIUS, at-risk places and the reasons why seismic risk is a concern are easy to identify and communicate. To illustrate the potential of this robust indicator, an application of SIRIUS to the city of Lisbon is presented.

The moment-rotation behavior of force-based frame elements is expressed as a function of plastic hinge length and moment-curvature parameters for two types of plastic hinge integration under the representative loading condition of antisymmetric bending. For modified Gauss-Radau hinge integration, there is a unique relationship between the resulting moment-rotation hardening ratio and parameters defining the plastic hinge length and moment-curvature hardening ratio. For two-point Gauss-Radau hinge integration, the spread of yielding across the hinge regions leads to a multilinear moment-rotation response, for which a secant approximation of the hardening stiffness is directed to a target plastic rotation. An example application demonstrates that significantly unconservative assessments of lateral load-carrying capacity can be attained if modeling parameters for plastic hinge length and moment-curvature strain hardening are not calibrated to account for the discrepancy between moment-curvature and moment-rotation behavior of an element.

A systematic study is presented herein on the seismic response of buried pipelines subjected to ground fault rupture in the form of normal faulting. In this study, advanced computational simulations are conducted in parallel with physical testing using a geotechnical centrifuge. For the numerical simulations, the pipeline was modeled using isotropic 3-D shell elements and the soil was modeled using either 1-D spring elements or 3-D solid (continuum) elements. The results from continuum finite-element analyses are compared with those from a Winkler-type model (in which the pipe is supported by a series of discrete springs) and with results from centrifuge tests. In addition, via appropriate modeling of the soil-pipe interaction, the q-z relation of the soil medium is elucidated for normal faulting events. The numerical analysis results demonstrate the potential for continuum modeling of events that induce pipe-soil interaction and results in improved understanding of pipe-soil interaction under normal faulting.

The northern Tehran fault (NTF) is potentially capable of causing large earth-quakes (M_max ~ 7.2) in a very densely populated area of northern Tehran, Iran. Due to the lack of recorded strong motion data for earthquakes on the fault, a hybrid simulation method is used to calculate broadband (0.1–20 Hz) ground-motion time histories at bedrock level for deterministic earthquake scenarios on the NTF. Low-frequency components of motion (0.1–1.0 Hz) are calculated using a deterministic approach and the discrete wave number-finite element method in a regional one-dimensional (1-D) velocity model. High frequencies (1.0–20.0 Hz) are calculated by the stochastic finite fault method based on dynamic corner frequency. The results were validated by comparing the simulated peak values and response spectra with the empirical ground motion models available for the area and the Modified Mercalli intensity (MMI) observations from historical earthquakes of the region.

Engineers usually focus on the performance of structural members, whereas the occupants of a residential building are affected mostly by the performance of infill and partition walls in buildings after a moderate earthquake. This often creates controversy and discussion regarding the post-earthquake use of buildings. Seismic rehabilitation codes for existing buildings offer sophisticated measures in rating the seismic performances of structural components, whereas performance measures suggested for infill and other partition walls are crude by comparison. Furthermore, seismic design codes for new buildings totally disregard such disparity, since their force-based approaches are built on single-level performance targets specified implicitly for the entire building under a design level, that is, a rare earthquake. In this paper, performance levels of buildings after an earthquake of moderate intensity are discussed from the viewpoints of engineers and building occupants. Suggestions are made for achieving uniform performance in structures where the seismic forces are resisted by structural members as well as the infills and partition walls coupling with the structural system although the contribution of such walls to seismic resistance and their performance is not usually considered in design.