Design ground motions for seismic retrofit evaluation of large bridges in the Eastern U.S.

Beginning in the mid 1990's, the first seismic retrofit evaluation projects were carried out for large highway bridges in the eastern U.S. We summarize experiences gained from these projects. A major findings is, that while the seismicity rates are often low, and the seismic hazard is moderate, the seismic risk potential can be high in large cities with a large stock of aged structural inventory.First results indicate that retrofit of some portions of some of the investigated bridges would be desirable in order to meet considered seismic perfomance criteria.(AU)

Quantifying seismic hazard and providing realistic ground motions for engineering applications primarily in the Eastern United States

NCEER's research program on Seismic Hazard and Ground Motions is a comprehensive and systematic program to produce quantitative hazard estimates and ground motion predictions for engineering research and practical applications. In eight years this program has successfully contributed to NCEER's goal of mitigating the risk from earthquakes in several innovative ways. One important contribution is the development of a fundamentally new way of collecting strongmotion data and redistributing these data to a diverse user community.(AU)

Seismic hazard assessment of the Queensboro bridge

In this demonstration project, a team of NCEER researches and peer reviewers has provided the technical support and expertise necessary to permit a comprehensive evaluation to be made of the seismic vulnerability of the Queensboro Bridge. The results of this study are now being used by Steinman Engineers in the vulnerability assessment of the bridge. Preliminary results from this structural phase indicate that several critical elements in the bridge may require strengthening if it is to meet the seismic perfomance criteria previously defined. These elements include the connections between the superstructure and the masonry piers. The decision to retrofit the bridge has not yet been made but the process will probably extend beyond the seismological and engineering issues described above. Such a decision will likely include the socioeconomic considerations of retrofitting as compared to the consequences of closure and/or the repair/replacement of this critical lifeline for New York City, should an earthquake occur at a future date.(AU)

Low - level dynamic characteristics of four tall flat - plate buildings in New York City

Many reinforced concrete structures utilize flat plates to provide lateral resistance when architectural constraints prevent wide-spread use of shear-walls. Understanding the resistance of flat - plate frames to large lateral loads is important for serviceability as well as seismic vulnerability assessment of hundreds of buildings on the East Coast. In February of 1991, the authors collected ambient vibration measurements to study the behavior of four flat plate, reinforced concrete structures in Manhattan ranging from 27 to 52 stories. All but one structure rests on a rock supported foundation. This report presents the development of ambient vibration analysis software based on the fast Fourier transform. Long ambient vibration records allow accurate power spectrum estimation. A robust peak picking method, also tailored for ambient vibration data, facilitates the interpretation of autopower and phase spectra. Damping estimates based on spectral peak band- widths are very sensitive to bias and leakage errors in the spectrum estimate. However, root-mean-square statistics of acceleration as well as of velocity and displacements can reasonably be estimated from the auto-power spectrum of response acceleration. Measured fundamental periods are shorter than those calculated from the 1982 UBC formula, but are longer than those calculated from the 1988 UBC Code or the proposed NYC Seismic Code. The results presented herein provide a base-line set of periods, deflections, and damping ratios to be compared to results expected to be obtained during strong winds. Periods estimated from ambient, mostly windinduced vibration measurements are used to calibrate finite element models of the structures, and are compared to values calculated from code design rules. A future goal of this on-going research is to relate effective flat plate width to lateral load level at service load levels, i.e., between ambient conditions and strong winds. Estimating dynamic parameters under various loading conditions coupled with ultimate load tests on model structures is intended to reveal the load-dependent stiffness of these structures (AU)