SMIP 2000 Seminar

Empirical relationships are developed to predict amplification factors for 5% damped spectral acceleration as a function of surface geology. Amplification factors are derived for spectral periods T = 0.01 – 5 s by assigning a reference spectrum to > 700 recordings from shallow crustal earthquakes. The reference spectrum is derived from soft rock attenuation relations modified to account for event-specific source/path peculiarities and rupture directivity effects. Strong motion sites are classified according to three geologic classification schemes: age only, age + depositional environment, and age + material gradation. Within each scheme, amplification is regressed against ground motion amplitude, and for one scheme, against amplitude and duration. The material gradation scheme is found to produce the least scatter in the amplification functions. The results of the regression indicate significant nonlinear ground response effects, and pronounced variations in the levels of amplification across geological categories. Amplification is also found to be sensitive to the duration of strong shaking. Due to the soft rock reference spectra used in this study, amplification levels are smaller than had been identified in previous studies employing reference motions from relatively firm rock sites.

Data recorded by downhole arrays with sensors installed at different depths and geologic layers provide critical information for studies of local site amplification effects.

The soft-soil/rock array at Treasure Island near San Francisco was installed by the California Strong Motion Instrumentation Program in cooperation with other agencies. Analysis of the recorded low amplitude data shows that the average amplification factor from the bedrock to the surface of the soft soil reaches factor of 10 at periods of 1.2-1.3 seconds.

Geotechnical arrays at La Cienega in Los Angeles, Meloland in El Centro, in Eureka and the newly instrumented arrays near the Vincent Thomas Bridge in Long Beach represent deep soft alluvium sites. A comparison was made of the average site amplifications calculated for a number of M<5 events with the site amplification for the 7.1 Mw Hector Mine earthquake. The site amplification curves are similar at short periods, but at longer periods the amplification factor is significantly lower for the distant large-event records.

The Tarzana downhole is located on the top of a small hill, and represents a soft-rock site. The downhole data from small events recorded so far demonstrate a significantly higher amplification effect for the component perpendicular to the hill than for the component parallel to the hill.

Large (up to 10 cm) long-period (up to 8 seconds) displacements were recorded at the La Cienega, El Centro, Tarzana and Long Beach arrays during the Hector Mine earthquake at the distances of more than 200 km from the epicenter. In contrast to the small events, the data recorded during the Hector Mine earthquake show that for the displacements and velocity curves there is practically no near-surface site amplification.

A critical evaluation of the four analytical methods recommended in FEMA-273 for the estimation of seismic demands is carried out using measured response characteristics of four instrumented steel buildings. Prior to conducting the FEMA-273 analyses, computer models of each structural system were calibrated to the observed response. Two of the buildings which experienced little or no damage were modeled as fully three-dimensional systems while the remaining two buildings which suffered moderate damage in recent seismic events were further tuned to simplified two-dimensional models to permit detailed inelastic evaluations. Dozens of linear and nonlinear computer analyses under both static and seismic loads were carried out on each building, however, only a few typical results are presented in this paper. Results of the evaluations provide insight into both modeling issues and the validity of the four analytical procedures outlined in the FEMA-273 document. It is found that calibrating structural models to observed response is sensitive to mass and stiffness modeling assumptions. Linear and nonlinear static procedures do not adequately predict inter-story drift estimates, a critical parameter in seismic evaluation and design.

This paper discusses the steps that are necessary in assessing the seismic performance of existing and new transportation structures, including (1) characterizing expected ground motions, (2) setting performance criteria, (3) modelling and dynamic analysis, and (4) interpreting predicted dynamic response behavior. To establish a basis for assessing the performance of older existing bridges, the history of loading criteria over the past 50 years, as specified in the U.S. AASHO/AASHTO national bridge code, is reviewed.

The recorded motions by California Strong Motion Instrumentation Program (CSMIP) for seven different bridge systems are analyzed using parametric and non-parametric system identification methods. The results of these analyses include identification of modal frequencies, mode shapes and damping ratios. An excellent fit of the recorded motion in time domain is obtained using parametric methods. Utilizing the results from the identification study, the paper evaluates commonly used bridge design provisions in California.

The COSMOS virtual data center http://db.cosmos-eq.org/ is a web accessible relational database for strong motion data. This database was designed to make it simple for a user to retrieve strong motion data that are most relevant to the needs of the user. At the same time it gives primary responsibility for quality control of the original data to the agencies that collected and processed the data. The virtual data center has information on 95 earthquakes, 3180 accelerograms, and 570 station descriptions. For each earthquake the data center has tried to include all the available accelerograms.

The TriNet project greatly increases the number of strong motion recordings available after significant earthquakes. A means of communicating that data to the earthquake engineering community in the aftermath of major earthquakes is described, which can be viewed as an Internet based evolution of the customary Quick Reports and full data reports produced after significant earthquakes. A second evolution is that the post-earthquake data collection will be multi- agency. In parallel, the low-amplitude records obtained at modern strong motion stations will routinely be made available through the seismological data center of the Southern California Earthquake Center for seismological analysis.