INNOVATIONS IN TECHNOLOGY
Heat-straightening hits the mark
Both quick and economical, the technique of heat-straightening can be a very effective repair option for highway agencies seeki
Both quick and economical, the technique of heat-straightening can be a very effective repair option for highway agencies seeking to restore the original shape of damaged steel bridges. While the technique has been used sporadically for nearly half a century, until recently little existed in the way of guidelines and documentation for the procedure.
To provide guidance to state highway agencies and contractors, the Federal Highway Administration (FHWA) released an interactive two CD-ROM set, Heat-Straightening Repair for Damaged Steel Bridges: An Interactive Guide, in 2000 and is offering specialized training through a series of seminars. Also available is a new manual, "Heat-Straightening Repairs of Damaged Steel Bridges: A Technical Guide and Manual of Practice."
Heat-straightening is conducted by applying a limited amount of heat in specific patterns to the deformed regions of steel in repetitive heating and cooling cycles to produce a gradual straightening of the material. The process relies on internal and external restraints that thicken during the heating phase and contract during the cooling phase. Unlike many other methods, force is not used as the primary instrument of straightening. Rather, the thermal expansion/contraction during the heating process leads to a gradual straightening. When properly done, heat-straightening is a safe and economical way to repair damaged steel.
The three key elements to the heat-straightening process are:
- Select proper heating patterns and sequencing to fit the damage;
- Properly control the heating temperature and rate of heating and cooling; and
- Provide appropriate restraints during the heating cycle.
Two major benefits of using the heat-straightening technique are that the repairs generally dont require the temporary shoring of the bridge and the process can eliminate or greatly curtail the need for traffic detours. The same technique used in heat-straightening also can be applied to the fabrication of new bridges for curving or cambering a girder.
The CD-ROM and Manual of Practice provide information on designing, implementing and managing heat-straightening repair projects, including such details as how to determine both the severity of damage in a steel section and the major components of a repair, common mistakes to watch for on a heat-straightening job, what equipment is needed, how to calculate the number of heats required to straighten a member and what the fundamental heating patterns and typical heating pattern combinations are.
The CD also presents a case study of a heat-straightening repair on a bridge near Lake Charles, La. The two-lane bridge crossing on I-10 had been damaged when equipment on a flatbed tractor-trailer hit a bridge beam. The multi-girder bridge was approximately 33 ft wide and had four spans supported by column bents. While the bridge remained stable, the damage was relatively severe. The repair, which was accomplished in five working days, was designed by a team from Louisiana State University and implemented in conjunction with the Louisiana Department of Transportation and Development.
You can learn more by attending one of the upcoming heat-straightening repair workshops. Seminars will be held in Little Rock, Ark., Nov. 27-28, and in Crawfordsville, Ind., Dec. 10-11. Additional workshops also will be scheduled over the next two years. The training programs will cover:
- An overview of heat-straightening;
- How to conduct various heating patterns;
- How to repair various damage types;
- How to inspect and supervise repairs;
- How to estimate the time required to straighten a beam;
- How to set up technical specifications; and
- A step-by-step description of how the heat-straightening repair procedure was used near Lake Charles, La.
Workshops were previously held this year in Atlanta, Georgia and Topeka, Kan.
More information on the workshops and heat-straightening repair in general, including a link for downloading the heat-straightening manual, can be found on the web (www.fhwa.dot.gov/bridge/heat.htm). You can obtain a copy of the CD-ROM for use in bridge applications by calling 202/366-4601; fax: 202/366-3077; e-mail: krishna.verma@ fhwa.dot.gov.