Influence of the new LRFD seismic guidelines on the design of bridges in Virginia

The Virginia Department of Transportation is currently using the AASHTO Standard Specifications for Highway Bridges, with some modifications, for its seismic highway bridge design. In April 2001, the Recommended LRFD Guidelines for the Seismic Design of Highway Bridges were published. The influence of the LRFD Guidelines on Virginia bridges was investigated by analyzing two existing bridges. The first bridge has prestressed concrete girders and is located in the Richmond area. The second bridge has steel girders and is located in the Bristol area. Both bridges were two-span overpass structures with integral abutments. The bridges were analyzed using the methods prescribed in the guidelines. Then, the combined effects of the dead, live, and earthquake loads were compared to the strengths of the columns and the pier caps. The details of the bridge designs were also checked against the corresponding seismic design requirement. Results indicate that typical column spiral reinforcement is not adequate to satisfy the requirements of the new seismic guidelines. For the bridge in the Richmond area, spiral reinforcement was increased from a No. 5 at a 5-in pitch to a No. 5 at a 4-in pitch. For the bridge in Bristol, the increase was greater, from a No. 3 at 10.5 in to a No. 5 at 4 in. In addition to the increase in spiral reinforcement, other details, such as beam-column joint reinforcing and splice locations, require modifications. The calculated cost increases for the two bridges were 0.1 and 0.3 percent. An associated parametric study explored the effects on substructure design of different column heights, superstructure lengths, and soil classifications in different parts of Virginia. The study indicated that for bridges located on good soil (Class B), typical column longitudinal reinforcing ratios (about 1.5%) provide adequate strength to resist seismic forces. For bridges on poor soils (Class D) in regions of low to moderate seismic activity, column longitudinal reinforcing may need to be increased, particularly in bridges with short columns, long spans, and sliding bearings at the abutments. For bridges on poor soils in regions of higher seismic risk (Southwestern Virginia), column sizes may need to be increased. For columns designed as spiral columns, the increases in transverse column reinforcement will not be great, but for columns designed as tied columns, the increases will be significant.