FUNDAMENTAL MODAL BEHAVIOR OF AN EARTHQUAKE-EXCITED BRIDGE.

The magnitude 6. 1 Morgan Hill, California, earthquake of April 24, 1984, produced a set of acceleration records at the freeway overpass in San Jose carrying I-280/I-680 across U. S. 101, 12 km from the epicenter. Peak accelerations reached 0. 10 g at ground level and 0. 16 g in the box girder. The integrated displacements at frequencies higher than 3 Hz permit the identification of the fundamental modes and frequencies. The horizontal modes have frequencies of 308 and 3. 9 Hz and include ground level motions up to 90% of peak deck motion. The soil-structure system contributes to these modes.