STRENGTHENING OF CONTINUOUS-SPAN COMPOSITE STEEL-STRINGER BRIDGES

On the basis of current bridge rating standards, many continuous-span composite steel-stringer concrete deck bridges in the United States are classified as deficient and in need of rehabilitation and strengthening, or replacement. Through several Iowa Department of Transportation research projects, methods of strengthening such bridges have been developed. Ways in which two of these strengthening procedures--post-tensioning and superimposed trusses--have been applied to actual bridges are described, and a design methodology is explained briefly. The strengthening systems were implemented and tested on two existing three-span bridges; both bridges were 45.72 m (150 ft) long and had roadway widths of 7.37 m (24 ft). From two analyses, it was determined that both bridges, when subjected to legal live loads were overstressed in both the positive and negative moment regions. The bridges, instrumented for strain and deflection measurements, were loaded with trucks before and after strengthening to determine the effectiveness of the strengthening systems. To alleviate the flexural overstress in Bridge 1, a post-tensioning scheme was designed in which the positive moment regions of all beams were post-tensioned. This strengthening scheme reduced the overstresses in both the positive and negative moment regions. In Bridge 2, superimposed trusses were employed over the piers on the exterior stringers in addition to the post-tensioning of the positive moment regions of all beams. In both bridges, considerable end restraint was measured; it was also determined that the guardrails were making a structural contribution. The transverse and longitudinal distribution of post-tensioning forces is summarized, behavior changes are noted, and the effectiveness of both strengthening systems is discussed. A design methodology developed for practicing engineers for use in designing a strengthening system for a given continuous-span bridge is also briefly described. Both strengthening schemes were determined to be cost-effective and practical techniques.