Earthquake Spectra

The seismic response and performance of pile-supported wharves on sloping ground is not well documented due to an historical lack of instrumentation on port structures. Although general surface observations have been made at numerous ports following recent earthquakes, much more specific soil foundation-structure-interaction data could have been obtained with the more widespread employment of instrumentation. This paper presents the results of empirical and numerical analyses of recorded strong-motion data (SMD) from an array of instruments located on a pile-supported wharf and in the adjacent free field. Data were recorded with an instrumentation array at Berth 24/25 at the Port of Oakland, California, during the M7.0 Loma Prieta earthquake. The primary objectives of this project were to evaluate the SMD and identify the limitations inherent in capturing the complete dynamic character, including soil structure interaction, of a pier or wharf with a structural model. The project is expected to serve the professional engineering community by providing guidance in selecting appropriate techniques for seismic analysis and subsequent upgrade of existing port facilities.

In the Sion area of Switzerland, part of a deep, embanked sediment-filled valley, investigations on soil site effects have been conducted using two independent methods. Two-dimensional (2-D) modeling was performed with a program based on the Indirect Boundary Element Method (Pedersen et. al. 1994). Numerical simulations allow taking into account the subsurface geometry of the valley and its peculiar characteristics, such as a variable shape ratio and a high shear-wave velocity contrast. The H/V method has been applied on ambient seismic noise measurements recorded on sites as close as possible to the 2-D modeling. This technique allows capturing the fundamental resonant frequency of the deepest sediments as well as identifying the existence of a surficial deposit. Both approaches agree on the fact that the fundamental resonant frequency of the valley is below 1 Hz. The amplification level of the predominant frequency obtained with numerical simulation is up to two times higher than the one given by the H/V ratio. These results provide the basis for further investigations in order to resolve differences.

The objective of this investigation is to evaluate the FEMA-356 Nonlinear Static Procedure (NSP) and a recently developed Modal Pushover Analysis (MPA) procedure using recorded motions of four buildings that were damaged during the 1994 Northridge earthquake. For this purpose, displacements and drifts from the FEMA-356 NSP and the MPA procedures are compared with the values “derived” from the recorded motions. It is found that the FEMA-356 NSP typically underestimates the drifts in upper stories and overestimates them in lower stories when compared to the recorded motions. Among the four FEMA-356 distributions considered, the “Uniform” distribution led to the most excessive underestimation or overestimation indicating that the need to carefully reevaluate the usefulness of this distribution in the FEMA-356 NSP. Furthermore, FEMA-356 distributions failed to provide accurate estimates of story drifts for a building that satisfied the FEMA-356 criterion for detecting the presence of higher mode effects indicating the need to carefully re-examine this criterion. The MPA procedure, in general, provides estimates of the response that are much closer to the values from the recorded motion compared to those from the FEMA-356 NSP. In particular, the MPA procedure, unlike the FEMA-356 NSP, is able to capture the effects of higher modes. For a building that exhibits dominant effects of “soft” first story, however, neither the MPA procedure nor the FEMA-356 NSP led to reasonable estimate of the response.

This study addresses the question of selection and amplitude scaling of accelerograms for predicting the nonlinear seismic response of structures. Despite the current practices of record selection according to a specific magnitude-distance scenario and scaling to a common level, neither aspect of this process has received significant research attention to ascertain the benefits or effects of these practices on the conclusions. This paper hypothesizes that neither these usual principal seismological characteristics nor scaling of records matters to the nonlinear response of structures. It then investigates under what conditions this hypothesis may not be sustainable. Two classes of records sets are compared in several case studies: one class is carefully chosen to represent a specific magnitude and distance scenario, the other is chosen randomly from a large catalog. Results of time-history analyses are formally compared by a simple statistical hypothesis test to assess the difference, if any, between nonlinear demands of the two classes of records. The effect of the degree of scaling (by first-mode spectral acceleration level) is investigated in the same way. Results here show (1) little evidence to support the need for a careful site-specific process of record selection by magnitude and distance, and (2) that concern over scenario-to-scenario record scaling, at least within the limits tested, may not be justified.

Dynamic property measurements of the moment-resisting steel-frame University of California, Los Angeles, Factor building are being made to assess how forces are distributed over the building. Fourier amplitude spectra have been calculated from several intervals of ambient vibrations, a 24-hour period of strong winds, and from the 28 March 2003 Encino, California ( _ML =2.9), the 3 September 2002 Yorba Linda, California ( _ML =4.7), and the 3 November 2002 Central Alaska ( _Mw =7.9) earthquakes. Measurements made from the ambient vibration records show that the first-mode frequency of horizontal vibration is between 0.55 and 0.6 Hz. The second horizontal mode has a frequency between 1.6 and 1.9 Hz. In contrast, the first-mode frequencies measured from earthquake data are about 0.05 to 0.1 Hz lower than those corresponding to ambient vibration recordings indicating softening of the soil-structure system as amplitudes become larger. The frequencies revert to pre-earthquake levels within five minutes of the Yorba Linda earthquake. Shaking due to strong winds that occurred during the Encino earthquake dominates the frequency decrease, which correlates in time with the duration of the strong winds. The first shear wave recorded from the Encino and Yorba Linda earthquakes takes about 0.4 sec to travel up the 17-story building.

Increase in strength due to aging of sands is reflected in higher blow counts and tip resistance values in penetration resistance measurements. This affects the magnitudes and peak ground acceleration estimates of prehistoric earthquakes obtained from an analysis of geotechnical observations at paleoliquefaction sites in the South Carolina Coastal Plain. In this study, corrections were made to account for the effects of soil aging, which were neglected in earlier estimates. The results show that when the effects of aging of soils on their geotechnical properties are incorporated, the resulting back-calculations reduced earlier magnitude estimates of prehistoric earthquakes by about 0.9 units. The peak ground acceleration estimates were reduced by about 15% for those earthquakes originally estimated at approximately 0.15 g. For those earthquakes whose original estimates were greater than 0.2 g, there was no noticeable change when a correction was made for the aging of soils.

This paper is concerned with the design of friction dampers designed to slip at a predetermined level and dissipate a substantial portion of the seismic energy, leaving the structure practically intact without its members having to yield or buckle. They are appropriate for use in seismic design of new buildings and in retrofitting existing structures. By choosing a practical design requirement rather than minimizing some energy criterion, a novel design procedure attains the stiffness of the individual braces and their displacements at the threshold of activation. The procedure is a two-phase process that uses in Phase 1 an equivalent single-degree-of-freedom (SDOF) system to obtain an optimal natural period of the structure by performing a full nonlinear dynamic analysis for a set of earthquake records. Phase 2 then enforces the same first mode on both the braced and unbraced frames, with the aim of ensuring simultaneous slippage. The procedure was applied to a 10-story steel frame. It yielded a rather technically attractive design of the braces since for close to mean plus standard deviation of the records, the resulting maximum roof displacements fell within the allowable design, as initially constrained to, and simultaneous slip of all braces occurred for most records. This procedure is rather simple in that the main computational effort, i.e., nonlinear analysis needed for Phase 1, is performed on an equivalent SDOF system only, whereas analysis of the multi-degree-of-freedom (MDOF) system is a linear eigenvalue analysis.

This paper presents a classification and prioritization of nonstructural systems, including medical equipment, in hospitals based upon the results of extensive surveys of effects of major seismic events. Surveys included damage surveys, interviews of medical and administrative personnel, and solicitation of expert opinion. As part of a larger study on nonstructural mitigation in hospitals, this effort sought to identify the importance and interdependence of various nonstructural systems subjected to earthquakes and other extreme events. Focused information was obtained for the 1994 Northridge earthquake. Additional information was obtained from experiences in the 1995 Kobe earthquake, the 1999 Chi-Chi earthquake, and the 1999 Kocaeli earthquake. Survey results led to a prioritized list of hospital nonstructural systems that can aid mitigation efforts in maximizing the continued functionality of essential medical facilities when exposed to extreme events.

The structural properties of a structure deteriorate when deformations reach the range of inelastic behavior. A possible consequence of deterioration of the hysteretic behavior of a structure is failure of critical elements at deformation levels that are significantly smaller than its ultimate deformation capacity. Seismic design methodologies that account for low cycle fatigue can be formulated using the concept of target ductility. The practical use of one such methodology requires the consideration of simple low cycle fatigue models that consider the severity of repeated loading through a normalized plastic energy parameter. The inconsistencies inherent to the use of such indices can be corrected through simple empirical rules derived from an understanding of the effect of the history of energy dissipation in the assessment of the level of structural damage.

An analytical study is performed to propose a simple expression for the fundamental period of vibration of concentrically braced steel frames. This work was carried out in the context of the development of the 2005 National Building Code of Canada (NBCC) and is mainly applicable to structures located in moderate and low seismic regions. Available field and test measurements of building periods are compared to analytical predictions. An empirical period formula is developed based on a survey of analytically computed building periods published in the literature. The influence of the 2005 NBCC design provisions on braced steel-frame periods is examined through a closed-form period solution and an extensive parametric study. Building periods are found to vary significantly with seismic hazard levels and soil conditions. An expression is tentatively proposed that provides reasonable lower-bound period estimates for most situations in Canada.

Concepts of uncertainty modeling and fragility assessment in earthquake engineering and their applications in consequence-based engineering (CBE) of buildings in a region of low-to-moderate seismicity are presented. Efficient treatment of uncertainties in components and systems that are major contributors to building vulnerability is an essential ingredient of risk assessment and decision making. Current methodologies are reviewed first. A framework for fragility analysis then is demonstrated using an unreinforced masonry building as an example. The importance of fragility analysis in various stages of risk assessment, loss estimation, and decision making in CBE to achieve the desirable long-term objective in reduction of loss and consequence with the most efficient intervention measures is clearly indicated. Current research issues related to vulnerability assessment are identified.

Complete probabilistic seismic hazard analyses incorporate epistemic uncertainties in assumptions, models, and parameters, and lead to a distribution of annual frequency of exceedance versus ground motion amplitude (the “seismic hazard”). For decision making, if a single representation of the seismic hazard is required, it is always preferable to use the mean of this distribution, rather than some other representation, such as a particular fractile. Use of the mean is consistent with modern interpretations of probability and with precedents of safety goals and cost-benefit analysis.