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Road Management Journal
Copyright © 1997 by TranSafety, Inc.
October 1, 1997
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Iowa Field Study Documented Successful Heat-Straightening Repair of a Steel Bridge by In-House Personnel
Sealers Shown to Lengthen the Service Life of Concrete Bridges Exposed to Chloride
Large Trucks a Significant Factor in Major Freeway Incidents in Houston, Texas
Warnings Combined with Enforcement Can Reduce Speeding

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Iowa Field Study Documented Successful Heat-Straightening Repair of a Steel Bridge by In-House Personnel

When steel bridges require heat-straightening repair, state departments of transportation typically hire contractors to do the repair. This can be costly and time-consuming. In an effort to reduce both the cost and the time involved, the Iowa Department of Transportation (IDOT) recently trained its own personnel to do heat-straightening repairs. The five-day training program consisted of classroom and laboratory instruction. Following the class sessions, trainees performed heat-straightening repair of an actual bridge girder, under the supervision of IDOT personnel.

The project provided a valuable case study in which recent advances in heat-straightening research could be put into practice. In addition, according to the authors, "this project represent[ed] one of the few cases where the response of the damaged girder to the heat-straightening process has been carefully measured and documented for comparison with theoretical models." The successful bridge repair took four days to complete and realized a substantial cost savings. A contractor's price for similar repairs would be about $20,000. By doing repairs in-house, the IDOT could expect to save approximately $11,000. R. Richard Avent and Bruce L. Brakke reported on the project program and its results in "Anatomy of Steel Bridge Heat-Straightening Project" (Transportation Research Record 1561).

METHODS

The bridge chosen for repair (the IA 130 overpass over I-80 near Davenport) represented a type of damage that commonly occurs in steel bridges (see Figure 1). When one or more girders are hit at the bottom flange, the result is significant lateral plastic deformations. Because the impact force is lateral, the bottom flange will act as a continuous beam, and the diaphragms will act as interior supports. Here, only one girder was involved. The northeast exterior girder over the eastbound lanes, continuous over the supports and spanning 57.9 meters (190 feet), was struck as the vehicle left the underpass. The lower flange, which had sustained minor damage from a previous impact, had deflected outward approximately 18 cm (7 inches). The impact occurred adjacent to the fifth diaphragm and fractured 14 of the 18 connecting bolts.

FIGURE 1: Damage configuration of IDOT bridge girder: cross sections of diaphragms


Heat-straightening repair requires two primary actions: maintaining a maximum heating temperature of 650 degrees C (1200 degrees F) and "limiting the jacking/restraining forces so that internal moments will not exceed approximately one-third the initial yield moment." Research has established that "the appropriate heating pattern . . . consists of vee heats on the bottom flange and line heats on the web." A structural analysis of the girder must be completed to decide the appropriate level of jacking forces, "since overjacking can result in a brittle fracture of the beam during heat-straightening." (Please refer to the article for a complete description of the structural analysis.)

Beyond the benefits already mentioned, this project "presented the opportunity to obtain additional experimental data related to jacking force effects on the lower flange." While the use of jacking forces speeds the heat-straightening process, "these forces should be passive and their magnitude limited" to prevent fracture.

Because the exterior girder on the bridge was deformed outward, the following actions were taken:

Since the jacking force should be applied in the direction of desired movement, a chain come-along system was used with an instrumented hydraulic load cell. The system was connected between the bottom flanges of adjacent girders, with the interior girder serving as a reaction point. The jacking force could be carefully controlled and monitored during heating. The system was set at a specified load level before initiation of heating. As the heating cycle produced movement, the jacking force decreased.

Given that the degree of damage at the point of impact (joint 5) was approximately twice that of the two adjacent plastic hinges (joints 4 and 6), joint 5 required twice as many heats. The basic heating pattern of the bottom flange for each cycle consisted of placing single vees at joints 4 and 6 and a pair of vees at joint 5. On successive heats, the vees were shifted across the plastic zone to guarantee uniform straightening over the zones, and no heats were made in the elastic zones. Line heats were used on the web, with a single cycle typically consisting of one or more web line heats, followed by vee heats on the bottom flange. Crew personnel heated the vees at the three joints simultaneously, and heat did not exceed 650 degrees C (1200 degrees F).

The heat-straightening was divided into two phases, the first with the damaged diaphragm in place. This first phase involved eight heats in three different heating patterns, along with fairly low initial jacking forces. Fourteen heating cycles took place in the second phase, with six additional line heating patterns. Researchers carefully documented the behavior of the bridge during the entire course of repair.

RESULTS

During the first heating phase (shown in Figure 2), the flange began to straighten, but after six cycles the movements were too small to measure. As a result, they removed the diaphragm at joint 5 and began the second heating phase. During this second phase, the girder straightened consistently until it was essentially straight. Measurements were taken with the jacking force in place, "thus requiring additional cycles past zero displacement to allow for the elastic rebound when the jacks were removed."

FIGURE 2: Heating patterns of first stage (diaphragm in place) ncluding two vees on the lower flange

Once the flange straightening was completed, a small web bulge remained at diaphragm 5. Crews subsequently heated this bulge with a star vee pattern, which closed all the vees in the star pattern and reduced the bulge. Figure 3 illustrates the vee pattern of heating.

FIGURE 3: Heating pattern 1 at end of diaphragms--single vee heat zone, lower flange (looking up)

As the bulge flattened, higher jacking forces were required. Instead of increasing the jacking force over the capacity of the jacking system, the heating was stopped when the movements remained static at 1.3 cm (1/2 inch) maximum bulge.

Figures 4 and 5 show the bridge girder before and after repair, respectively.

FIGURE 4: Bridge girder before repair

FIGURE 5: Bridge girder after repair

CONCLUSIONS

This project had two major goals: to develop the in-house capability to perform heat-straightening repair of damaged bridges and to repair a damaged bridge actually using personnel recently trained in the latest heat-straightening technology. The project was successful on both counts and resulted in significant cost savings. The cost of the equipment purchased for the project was $5,320, the cost of rental equipment was $735, and the cost of technical guidance, the training course, on-site assistance, and travel and expenses was $4,800. Costs from contracted repair for a similar project would be about $20,000. As such, the IDOT would save approximately $11,000, assuming the cost of the purchased equipment was depreciated over four projects. Based on the success of this project, a similarly damaged bridge beam could be straightened as a complete in-house project requiring approximately six IDOT personnel.

The project also resulted in significant benefits beyond its training and economic successes. It was one of the few cases to allow researchers to gather detailed field data. The data suggested "that additional theoretical modeling is needed to more accurately predict" the behavior of bridges under similar repair conditions. However, "the measured response to heat-straightening obtained here can serve as an experimental benchmark for future models."

Copyright © 1997 by TranSafety, Inc.


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