Case Study: Heat-Straightening of a Collision-Damaged Bridge Beam With Multiple Cover Plates
Location: Mercury Boulevard Bridge over Warwick Boulevard
City of Newport News, Virginia
Shannon M. B. Turner, E.I.T., Member, ASCE
Philip D. Quillin II, P.E., Member, ASCE
What this is: A new technology.
What this is not: Mechanical bending.
Often, municipalities are faced with the reality of repairing a bridge beam after an over-height vehicle has crashed into it. Currently, there is little information available and even fewer past experiences to rely on to make safe, cost-effective repairs. This article was written with the intent to convey to others interested the concept of heat-straightening damaged. There are no known resources describing the process to heat-straighten the type of beam described in this article. This is not a technical review but includes enough information to generally describe the process and its advantages.
Bridge engineers are constantly encouraged to produce cost-effective solutions for various repair projects. With federal, state and local transportation agencies facing historically large budget cuts, any new work is under intense scrutiny. In addition, projects initiated due to damage or unscheduled maintenance are typically not included in budgets, further taxing strained funding resources. Money that was ear-marked for sorely needed planned work now has to be shifted to pay for repairs.
The following case study examines how an emerging technology (heat straightening) was utilized to save the client a great deal of money over an industry-standard repair method (beam and deck replacement) and reduce the long-term concerns developed from previously acceptable solutions (hot mechanical bending).
In April of 2002 a mechanical equipment hauler heading southbound on Warwick Blvd. impacted the first interior beam on the south side of the Mercury Blvd. bridge in Newport News, Virginia. This collision resulted in a moderately severe bend in the bottom of the beam, approximately 9 inches out from its original position, with a well-defined horizontal yield line along the mid-depth of the 36 inch deep beam. The length of damage was in excess of 12 feet and involved tearing and bending of auxiliary steel framing and reverse bending in adjacent areas of the beam.
Immediately after the incident a professional team of engineers from MMM Design Group in Norfolk, Virginia, at the direction of the City of Newport News, was on site assessing the damage, evaluating the short and long-term concerns and developing repair alternatives. It was determined that the damage posed no immediate risk to the public and traffic was diverted from the southern side of the bridge. The next step was to choose a repair alternative.
Previous to this project, the Commonwealth of Virginia, through the Virginia Department of Transportation (VDOT) along with many other states, accepted mechanically bending the beam by heating it to a maximum temperature and physically bending it to its original position by applying heavy jacking forces. This method, although conducted quickly and accurately, potentially introduced long-term hidden damage to the beam from excessive jacking forces which could lead to a sudden brittle failure of the beam.
Another alternative was to remove the beam and portions of the deck and auxiliary framing, as required, and replace them in kind. The majority of beams struck in a vehicle collision are the fascia (exterior) beams. This solution is costly for fascia beams and often more so for an interior beam, as was the case for this project, requiring temporary support of exterior portions of the deck and large, long-term delays to traffic.
Before considering the next alternative, more details of the bridge need to be discussed. During the labor-rich, material-poor era of the mid to late twentieth century, bridge designers sought to use as little steel as possible to effectively carry traffic with no excess material. A method often used was to weld steel plates to the bottom of the main load carrying steel beams in critical areas. They often would not run the full length of the beam, as this would not be required for strength. Typically, they were centered in the middle of the span, adding extra strength where there was the greatest stress. Usually, a single cover plate was placed on the bottom of the beam. Occasionally, a second smaller cover plate was placed under the first to reach the optimal design. This was the case for this project. Since welding and ironwork in general was relatively inexpensive, the use of cover plates was the norm.
Often, beams designed with cover plates were built during a time when compositely constructing the deck was not considered. Therefore the beam could be manipulated (or replaced) without a great deal of consideration of the concrete deck. In fact, a beam could simply be replaced by temporarily removing traffic and shoring the deck. This is often the most favorable solution for a non-composite superstructure.
In the case where beams are compositely connected to the concrete deck slab by steel studs, an emerging technology, heat-straightening the beam in place, has become a viable solution and was chosen for this project. A potential hindrance to the complete success of this process was the presence of the second bottom flange cover plate in the damage yield zone.
For this project, a specific "yield zone" was determined and marked on the beam with chalk. This area of the beam is where the steel was permanently bent due to the collision. It is also known as plastic deformation. Other areas of the beam experienced elastic deformation: where bending occurred but the steel returned to its original position. The starting and ending points where the bottom of the beam no longer was straight was determined by pulling a string along the straight portions of the beam (or from bearing to bearing) and measuring the extreme distance away from the string.
Often the web bend is elastic, with residual stresses developed that are relieved once the bottom flange is straightened. Residual stresses are localized induced internal stresses that remain until external restraints are removed. A simplified example is a rubber band pistol. When the rubber band is stretched, stresses are developed. Once the trigger is released, these stresses are relieved. The bend in the beam follows a curve along the surface of the web, much like bending a sheet of paper.
Occasionally, the web will develop a plastic bend where a hinge point develops along a line at a certain depth of the beam, establishing a permanent deformation. This occurred in the subject bridge beam, creating an obvious yield zone. It is important to define the yield area because heating areas out of the yield zone is ineffective and possibly counter-productive.
Once the damaged areas of the beam are determined, heating patterns are predicted and repair scheduling is conducted. Generally, heating begins around the impact area to acclimate the beam to the process and relieve any residual stresses. To straighten the bottom flange, a restraining jacking force was calculated to only resist any outward (worsening) movement of the beam. Vee shapes, described later, are then chalked on the flange. In the case of one cover plate, the chalked vee is replicated on the top of the bottom flange and on the bottom cover plate.
Properties of Steel
Steel can be heated to roughly 1300 degrees Fahrenheit with no detrimental long-term effects. This temperature is known as the lower phase transition where, depending on carbon content and other miscellaneous alloys, heating steel above this amount will begin to change the properties of the steel, changing the strength characteristics and degree of brittleness.
For this process, a limit of 1200o F was used and temperature crayons were utilized to verify that this temperature was not exceeded. (Temperature crayons will melt at specified temperatures, indicating a measured "heat" of the steel.) Naturally, steel expands when heated and contracts when cooled, which is true for any temperature range.
In general, heat straightening employs the natural tendency of steel to contract while it cools. If steel is heated using the shape of a vee and expansion is resisted by jacks and the surrounding cool steel, the open end of the vee will tend to contract as it cools and the steel will bend in the direction of the closed end of the vee. For any project, specific heat patterns are determined and restraining jacking forces are calculated to resist expansion. If properly performed, during the cooling time between heat cycles the beam will return to its original position with no excess jacking required.
An option for this project was to remove the bottom cover plate to ensure a successful operation, however in order to proceed as economically as possible, it was first attempted to straighten the beam with the second cover plate attached. This proved to be a hindrance, as the space between plates acted as a heat sink: not enough heat reached the interior plate to be effective.
At this point provisions were made to remove the bottom cover plate in the yield zone by first placing false-work on top of the deck to alleviate the dead load deflection from the bottom of the beam. False-work could have been utilized below the bridge under the right circumstances, however with heavy traffic conditions it is advisable to attempt to support the weight carried by the beam from above. This method only requires the closure of one lane of traffic, and more commonly this is the shoulder or weaving lane. Once the plate was removed, the heat straightening resumed and significant movement was noted. Accepted FHWA laboratory standards were followed and the entire operation was a success, saving dollars for the City and public time spent in traffic.
Although a similar method is used to create curves in new beams, as a newer technology, only a handful of active bridges in a few States have had beams straightened by this method. According to Federal Highway Administration officials, heat-straightening a bridge beam with two cover plates had never before been attempted. No prior experience was available as a guideline, so lessons were learned in the field.
- Eliminates excessive jacking forces (which may lead to brittle cracks).
- Reduces traffic delays.
- Saves money.
This case study is but one example of the limitless number of applications for the steel heat-straightening process. For further information as related to straightening bridge beams, the latest information can be found through the United States Department of Transportation Federal Highway Administration in publications such as Heat-Straightening Repairs of Damaged Steel Bridges: A Technical Guide and Manual of Practice, which was employed in this case study.
1. Strong-back beam support on bridge deck
2. Welder's setup below bridge deck
3. Weak axis yield of bottom flange
4. Yield zone heat application area
5. View of Beam BEFORE Repair
6. View of Beam AFTER Repair