ABSTRACT
Time histories of hypothetical earthquakes were derived based on the stochastic simulation model proposed by Boore (1983), and recommended by Reiter (1990). This model for deriving the time histories considers many factors, such as the probability of earthquakes from nearby seismic zones, the attenuation of ground motions with respect to distance in the Central United States, and the possibility of a random event occurring outside of the generally recognized zones of seismicity in the area. Using the random-vibration modeling, time histories and response spectra for 50-year, 250-year, and 500-year earthquake events were determined for all of the county seats in Kentucky. Time histories representing the credible earthquake event were generated for the vertical and two orthogonal horizontal directions. The 500-year event is defined as an event having peak horizontal particle acceleration, at the top of bedrock, with a 90% probability of not being exceeded in 500 years (i.e., 10% probability of exceedance). The time histories and associated 0% and 5% damped response spectra were used to establish ground motion zones within Kentucky. Boundaries of the ground motion zones coincide with county lines and are intended to
1 INTRODUCTION
The Commonwealth of Kentucky is influenced by four seismic zones. These zones are comprised of: the New Madrid Zone, Wabash Valley, Giles County (Virginia), and Eastern Tennessee Seismic Zones. Recent observations and seismic measurements indicate that the New Madrid Seismic Zone is still the most hazardous zone located east of the Rocky Mountains (Johnston & Nava 1985). Unfortunately, there are hundreds of bridges in Kentucky that were designed and constructed prior to the application of present-day seismic design codes. Existing, older bridges were not designed to resist seismic loadings and, fortunately, have not yet been subjected to any moderate or strong earthquake. Seismic evaluation and retrofit of these bridges is currently being carried out. The following topics will be presented in the form of case studies on bridges in Kentucky: 1) Development of site specific time histories, response spectra, and seismic acceleration maps for the 120 counties in Kentucky; 2) Field dynamic testing (ambient or free vibration) of long-span bridges; 3) Three-dimensional finite element modeling of long span bridges; 4) Seismic evaluation and retrofit of bridges in Western Kentucky.
