Heat Straightening of Steel Members

Steel structures periodically are damaged. This damage can be from various sources, such as, vehicle damage, wind damage, seismic damage, vessel damage, and construction damage. Unlike timber and concrete, steels commonly used in the structural elements of bridges and buildings have properties that allow the material to be restored to its original design configuration when damaged. This may be accomplished through the controlled application of heat and force with little or no adverse impact on the strength or other properties of the steel. Therefore, when damage occurs, it is often more cost effective to repair the damaged members rather than replace them.

Heat straightening is a method of applying heat to a steel member to make the member change its shape. Steel has particular properties that make heat straightening possible. Steel is a combination of various elements added to iron. These elements can include carbon, manganese, phosphorus, sulfur, silicon, nickel, chromium, molybdenum, copper, columbium, and vanadium. These basic elements combine in a way to form what is called crystals. These crystals are modified by rolling, pulling through a die, and other methods of working the material. This working changes the crystalline structure and the properties of the steel. Many older steels were not worked as much as modern steels. If controlled heat is applied to these older steels, their mechanical properties typically do not change. However, if excessive heat is applied to newer worked steels, they change their structure and thus their mechanical properties.

When a steel member is damaged or bent as shown below, the steel has plastically deformed and has assumed a new shape. The plastic deformation takes place at the area of impact and for a short distance on either side of the point of impact. Outside of the area of plastic deformation, there is typically a zone of non-plastic deformation of the member. Heat is applied in a series of Vee shaped heats beginning at the inside of the curvature with the point of the Vee and progressing outward with the Vee increasing in width toward the outside of the curvature as shown below. The member should be heated to a temperature in the range of 1100 F to 1200 F. As heat is applied, the metal in the Vee expands and since the material is constrained in the longitudinal direction, the material thickness increases. As the metal in the Vee cools, the metal shrinks with the greater shrinkage occurring at the outside of the curvature, thus shortening the stretched metal. This method is applied incrementally over a series of heats to produce a straightened member. In general, the majority of the heats should be applied to the area of plastic deformation with only a few being required to straighten the non-plastic deformation areas. Forces may be applied to the member to aid in the straightening process.
See Figure 1

Heat straightening cannot be used effectively on all steels. As stated before, some steels derive their physical properties, like strength, from working the steel after the compounds are added to the steel. If the steel has been worked to create its properties, then if it is heated, the crystals will change their shape and loose their strength characteristics. Therefore, older steels with little working during manufacture can be heat straightened with less potential for altering the steel properties.

There are only a few contractors in the Northwest that perform heat straightening. There are only a couple of Departments of Transportation that perform their own heat straightening. With a little assistance and guidance, maintenance personnel can easily be trained to perform heat straightening. Charles Mayhan and Willis Whitney from our office have been trained in heat straightening and have worked with the Washington State Department of Transportation in the straightening of bridges.