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
Highway Safety Publications Catalog. Articles on Road Engineering,
Road Maintenance & Management, and Injury Litigation. Information and consulting for the Automobile and Road User,
as well as for law professionals in accident investigations.
TranSafety's free consumer journal for automobile and road users,
three subscription journals on road maintenance, engineering,
and injury litigation, and highway safety publications catalog.
See our free consumer journal for automobile and road users,
three subscription journals on road maintenance, engineering,
and injury litigation, and a highway safety publications catalog.
|
|
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.
Back to the Road Management Journal Index
|
