16 TR News, November–December 1986

RESEARCH PAYS OFF

Strengthening ofHighway Bridges by Posttensioning

Problem

Many simple-span, steel beam, com-posite concrete slab bridges built be-tween 1940 and 1960 are not in com-plete compliance with today's bridgestandards; this problem not only existsin Iowa but also confronts numerousother states in the United States. InIowa alone, approximately 400 bridgesof this type require strengthening tomeet current standards. More specifi-cally, the live-load carrying capacity ofthe steel stringers of these bridges mustbe increased.

The need for additional strength isthe result of increases in legal load lim-its, changes in AASHTO specificationsthat require exterior stringers to carry agreater load, and added weight fromresurfacing with dense concrete over-lays. The alternative to strengtheningthese bridges is to post them with loadrestrictions.

These bridges are on low-traffic sec-ondary roadways as well as on relativelyhigher type primary roadways where aconsiderable amount of truck traffic isinvolved. Trucks with loads heavier thanthe posted limit are required to take al-ternate routes, and thus must travel ad-ditional miles. Strengthening the bridgesin question instead of posting them couldbe extremely cost-effective.

Solution

A feasibility study was undertaken bythe Engineering Research Institute ofIowa State University (ISU) withfunding from the Highway Divisionof the Iowa Department of Transpor-tation (DOT). Among various poten-tial solutions to the problem, the pro-cedure of posttensioning of the steelstringers appeared to exhibit thegreatest promise. To determine thedistribution characteristics of theposttensioning force in a multiplestringer bridge, a half-scale modelbridge (26 ft × 15 ft 8-3/8 in.) wasfabricated and tested. Steel tendonswere tensioned through end anchorsaffixed to the lower portion of theweb and bottom flange at each end ofsteel stringers. Posttensioning wasapplied to exterior stringers only.When the tendons were fully stressedand anchored, a compressive forcewas exerted in the tension area of thestringers, which reduced the live-loadtension stresses in the stringers.

Based on the results of the feasibilitystudy, a second project—also sponsoredby the Highway Division of the IowaDOT—was undertaken by ISU. In thisphase, two existing bridges werestrengthened by using the posttension-ing scheme developed in the feasibility

Suggestions for "Research Pays Off"articles are welcome. Contact Nancy A.Ackerman, Editor, TR News, Transpor-tation Research Board, 2101 Constitu-tion Avenue, N.W., Washington, D.C.20418 (telephone 202-334-2972).

TR News, November–December 1986 17

study. One of the bridges (Bridge 1: 52ft × 30 ft) was twice the size of theprototype tested in the laboratory. Theother bridge (Bridge 2: 72 ft × 30 ft,skewed 45 degrees) is shown in Figure1; the insert in the lower right corner ofthis figure is a close-up of the postten-sioning bracket used on this bridge.Both of the bridges were instrumentedand tested. After a period of approxi-mately 2 years, during which time thebridges were periodically inspected,they were retested. Their behavior wasfound to be essentially the same as thatof the model bridge previously tested.

From the results of the testing plus a

finite element analysis, a simplifieddesign methodology was developed byISU. The design procedure, along withdesign examples, was published by theIowa DOT in a design manual, makingthe procedures readily accessible topracticing engineers.

Application and BenefitsThe procedure has been used by severalstates in addition to Iowa. This pastFebruary, two bridges (50 ft × 30 ft)were strengthened by using the postten-sioning system, at a cost of $13,400 per

bridge. Assuming strengthening ex-tends the service life of the bridge 20years, thereby attaining the expected50-year service life, a savings of 40percent of the cost of replacing thebridge could be realized. At today'sconstruction costs, the savings wouldbe about $28,000 on a 50 ft × 30 ftbridge costing $70,000, with net sav-ings totaling $14,600. Larger savingswould be realized on longer spans, in-asmuch as replacement costs of thelarger bridges would increase consid-erably faster than strengthening costs.In addition to adding service life,strengthening by posttensioning alsocreates savings by avoiding costly de-tours when weight limits on bridgesmust be reduced. If it is assumed thatthe posting of a given bridge affects 50trucks per day and each truck has todetour 10 miles off of a route at a rateof $1.00 per mile, then the posting ofone bridge would cost the trucking in-dustry $182,500 per year.

Currently, a third bridge-strengthening research project spon-sored by the Iowa DOT Highway Divi-sion is under way at ISU, involving thestrengthening by posttensioning ofcontinuous bridges. A one-third scalemodel, three-span bridge (42 ft × 8 ft 8in.) has been constructed in the labora-tory and testing is currently in progress.

Editor's note: NCHRP Project 12-28(4),Methods of Strengthening Existing High-way Bridges, is currently under way at IowaState University. Posttensioning as well asother techniques to strengthen bridges willbe investigated.

For further information contact F. WayneKlaiber, Professor of Civil Engineering andManager, Bridge Engineering Center, IowaState University, 424 Town EngineeringBuilding, Ames, Iowa 50011; or Vernon J.Marks, Research Engineer, Iowa Depart-ment of Transportation, 800 Lincoln Way,Ames, Iowa 50010.