Strengthening of two continuous-span steel stringer bridges

Abstract The need to upgrade a large number of under-strength bridges in the United States has been well documented in the literature. Through several Iowa Department of Transportation projects, the concept of strengthening simple-span steel-beam composite concrete-deck bridges by post-tensioning has been developed. The purpose of this paper is to describe how this strengthening procedure has been extended for use on continuous-span steel-beam composite concrete-deck bridges. The feasibility of using this strengthening procedure was demonstrated on a laboratory 1/3-scale-model three-span bridge. The laboratory model was subjected to various post-tensioning systems in which the positive-moment regions, negative-moment regions and combinations thereof were post-tensioned. Post-tension strengthening systems were then implemented and tested on two existing three-span bridges; both bridges (Bridges 1 and 2) were 150 ft (45.72 m) in length with a roadway width of 24 ft (7.27 m). From analysis, 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 so that strains and deflections could be measured, were loaded with trucks prior to 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 region of all beams were post-tensioned. In Bridge 2, superimposed trusses were employed over the piers on the exterior beams in addition to the post-tensioning of the positive-moment regions of all beams. In both bridges, considerable end-restraint was measured; also it was determined that the guardrails on both bridges were making a structural contribution. In this paper, the effectiveness of both strengthening systems is presented as well as numerous behavioral characteristics noted in the field. This paper summarizes transverse and longitudinal distributions, behavioral changes noted and the effectiveness of the strengthening systems. A design methodology for use by practicing engineers for designing strengthening systems for continuous-span bridges is also briefly described.