Scenic-Bridge Strengthening

Built in 1937, California’s Big Creek Bridge—a double-arch crossing with half-arch side spans—was due for a seismic retrofit. Challenged by the structure’s unique support system, which doesn’t rely on abutment foundations, and its fractured rock base, engineers devised a foundation retrofit that provided the necessary strength and stiffness via modified tie-down anchors that pass through the alluvium and into the fractured rock. Irregular rock bands located at depths beyond the reach of equipment required the use of vertical tie-downs at one pier, which would hit the rock at higher elevations. The retrofitting was completed in late 1999.