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Embargo it or replace it.Those are generally the only options available to state and county highway departments when a bridge won’t support the weight of the vehicles that use it.But those alternatives don’t fit well into Iowa's transportation or financial picture. The Iowa Department of Transportation and the 99 county road de partments are short of funds and bridges are wearing out faster than they can afford to replace them. Embargoing, or prohibiting the use of certain bridges by heavy vehicles, particularly on the state’s primary highway system, causes inconvenience, lost time and increased transportation costs.A team of Iowa State University researchers focused on a third alternative to the bridge dilemma: Strengthen some of the existing bridges to withstand today's heavier traffic.Many bridges in Iowa and throughout the United States are deteriorating and becoming obsolete faster than their 50-year normal life expectancy principally because legal vehicle load limits are greater today than when the bridges were designed and built. Most bridges built on the primary hiehway systemprior to 1950 were designed for 20 ton loads. Today's legal limit in Iowa is 40tons.The problem is more acute on county roads where a 15-ton limit design was used into the early 1970's. As a result of the increased loads, about 200 bridges on the state’s primary highway system and hundreds more on county roads have been embargoed to large vehicles, fore ing, trucks, tractors pulling grain wag ons, and even school buses to find alternate, usually longer, routes.The ISU researchers received a $59,000 grant from the Iowa DOT Highway Research Board to study the feasibility of strengthening single span, steel beam, concrete deck bridges, one of the most common designs from 1930-60. For the study, civil engineering professors F. Wayne Klaiber and Wallace W. Sanders, Jr., and assistant professor Kenneth F. Dunker constructed a 15-by-25-foot bridge in the Town Engineering Building structural engineering laboratory. Concrete dead weights up to 10,000 pounds each were placed on the bridge to simulate stresses caused by heavy vehicles passing over it. The effects of the weights on the beams and concrete deck were recorded with electrical instruments.The laboratory study was completed last spring. While the study answered some questions, it raised others. The highway research board recently approved the team's recommendations to conduct additional laboratory tests and to strengthen two Inws bridges for study under actual use. This project received a $67,000 grand and work began this fall.The strengthening method under study is called “post-tensioning, a relatively simple and inexpensive procedure that involves bolting high strength steel tendons to the bridge support beams. Tightening the tendons causes upward bending of the beams to counteract part of the downward force exerted on the bridge by heavy vehicles. This decreases the critical stress to the bottom of the beams, Klaiber said.Post tensioning is not a new procedure but the ISU team’s application of it for bridge strengthening is. One of the goals of the project is to develop a method for mass application of the procedure to virtually all bridges of this design. It will allow bridge engineers and inspectors to conduct their own studies of bridges to determine if post-tensioning is feasible, then to proceed with the modifications on their own.One problem the researchers have encountered with post-tensioning is that the upward force usually cracks the bridge's concrete deck so repaving eventually may be necessary.Charles Pestotnik, bridge engineer for the Iowa DOT, considers the state’s investment in this research a wise move. “If this method proves successful, and from the laboratory study it appears to be quite promising, we could save the state and counties hundreds of thousands of dollars in bridge replacement costs, he said.“If, through post-tensioning, we can extend the service life of a bridge by 20 years, thereby attaining the expected 50-year service life, which is not unreasonable considering the research results, we could save 40 percent of the cost of replacing that bridge. At today's construction costs, the savings would be about $72,000 on a typical 150-by 30 foot bridge costing $180,000. Modifying two such bridges - assuming a modification cost of $10,000 each - would just about pay the entire cost of the research, with a potential for similar savings on many more bridges.Pestotnik added that several other state highway departments have expres sed interest in the ISU study.
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Oakland Botna Valley Times
Oakland, Iowa, US
Fri, Oct 23, 1981
Page 10
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IA, USA 02 Apr 2024
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