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Guide for Heat-Straightening of Damaged Steel Bridge Members
Chapter 7: Heat Straightening Repair of Localized Damage
Damage in steel members can be broadly classified as global and local damage. Different methods are required for the heat-straightening repair of these types of damage. Global damage entails deformation of both primary and stiffening elements well beyond the point of impact. Local damage is characterized by plastic strain occurring only in the region of impact. It includes small bulges, bends or crimps in single elements of the cross section. The two most frequently encountered patterns can be categorized as flange bulges and web buckles as shown in Figure 48. Flange bulges are associated with local damage to unstiffened cross section elements such as a flange of a girder. Web buckles are associated with local damage to stiffened cross section elements such as the web of a girder. All are classified as Category L damage, but two sub-classifications will be used: Category L/U for local damage to unstiffened elements, and Category L/S for damage to stiffened elements.
The focus of past heat-straightening research has been on various aspects of repairing global damage, but localized damage usually occurs concurrently with global damage. Yet, little published information has been available on heat straightening local damage. As a result, localized damage is often repaired improperly by various combinations of cold mechanical straightening and hot mechanical straightening, as well as heat straightening.
Local damage patterns display common characteristics: large plastic strains (usually tensile) in the damaged zone, and bending of plate elements about their weak axes. If the local damage is to be repaired, shortening must be induced in the damaged area equal to the elongation caused when the element was damaged. In addition, the distortion along the yield lines must be removed as part of the repair process. Studies on global damage repair have shown that vee heated regions shorten significantly during cooling and that line heats can be used to induce bending about the yield lines. Thus a combination of line and vee heats can be used to repair localized damage.
An example of local damage to an unstiffened element is shown in Figure 49. This type of damage was observed during a heat-straightening project executed on the Mississippi River Bridge at Greenville, Mississippi. Three sway struts of the through truss had been damaged by a passing vehicle.
Category L/U local damage is typical in cases with the impact on a plate element with one free edge such as a flange of a beam. Figure 50 shows the typical flange bulge pattern. Often, distinct yield lines form as well as some zones of flexural yielding where curvature is highest. The impacted side of the damaged flange will be referred to as the near side (N). The non-impacted side of the same flange will also typically incur damage. This damage on the far side (F) of the flange has a geometry similar to N, but usually of lower magnitude. The damaged flange typically undergoes rotation about a clearly defined yield-line near the rolled fillet of the web (depth "k" in AISC diagrams). The impacted side (N) of the flange usually deforms in a folded plate pattern, as shown deforming toward the web in Figure 50b. The deformation usually results in strains significantly higher than yield lines which define the edges of the folded plate (Figure 50c).
In some cases, particularly in regions of high curvature, the deformation pattern may be one of a flexural yield surfaces rather than a series of yield lines. These surfaces result from plate element flexure and tend to spread over the surface as the degree of damage increases. Such zones will be referred to here as yield surfaces. The other half of the same flange usually deforms in a similar pattern in the opposite direction, even if not directly impacted.
The pattern, Figure 50d, tends to have smaller deformations, thus δn > δf . Because the web is thinner than the flange, a yield line often forms in the web near the fillet. The section shown in Figure 50b illustrates this behavior. The tee section at the flange/web juncture remains close to a right angle. The yield line forming in the web fillet allows this tee to rotate through an angle θw. The yield line at the flange fillet on the impacted side of the flange (side N) results from the additional rotation, θn, thus the total rotation of the N flange is θw + θn. The other half of the flange (side F) tends to resist rotation thus a second flange yield line may form at the F side fillet. The angle formed by this yield line is θf and the rotation of the F flange is θw - θf. The identification of these yield lines is important in the repair procedure.
The specific heating pattern depends on the details of the damage geometry. The typical damaged cross section is shown in Figure 50a. There are three components of rotation: (1) the web/flange juncture, which remains at right angles, and has a rotation θw resulting from rotation about the web yield line; (2) the near side flange, N, which has a maximum rotation θn, resulting from additional rotation about the flange yield line; and (3) the far side flange, which has a reduced rotation, θw - θf, resulting from the resistance of flange F to rotation caused by forces applied to flange N. The heating/jacking pattern to straighten this damage will depend on how the geometry changes as heat straightening progresses. The following steps outline a typical procedure. However, because there are so many possible damage shapes, exact procedures cannot be established.
This phase is most effective with jacking forces on both the near and far sides of the flange. However, it can be conducted with jacking only on the near (impacted) side. The specific steps are:
Place jacking forces on both the near and far sides of the damaged flange in the direction tending to restore the flange to its original condition. As shown in Figure 51a, a convenient arrangement on the near side is to place a jack, Pn, between the top and bottom flange. The far side jack, Pf, requires a clamping type force which is often more difficult to arrange in field applications. If the clamping force cannot be anchored from the opposite flange, a spreader beam arrangement can be used, as shown in Figure 51d, to anchor the reaction to the straight portions of the far side flange. An alternative is to only jack from the near side. However, the average movement per cycle tends to be lower than similar cases jacked on both sides. In certain cases, Pf should be reversed (see following sections).
Although vee heats may not be necessary, a limited number may be used to assist in the flange shortening effort. The vees should be approximately half depth and applied to both the near and far sides of the flange to eliminate global curving of the member. The vee should be narrow with an angle of 20° or less and the open end of the vees should be at the flange tips. It is best to place the vee heats in regions where no line heats are required. No more than two vees should be used (preferably only one) in one heating cycle. The location should be shifted with each heating cycle so the same location is not re-heated for at least three cycles. A typical arrangement is shown in Figure 52b.
All flange yield lines should be heated (on the convex surface (if practical) after any vee heats used. A typical pattern is shown in Figure 52a. In yield surfaces of continuous plastic strain such as often occurs in regions such as ABC in Figure 52a, line heats should be spaced over the section at a spacing of approximately bf/4 where bf is the flange width. Similarly, line heats may also be used instead of vee heats on section BCDE. The order of heating the yield lines tends to have a minor impact although it is good practice to heat the ones at the largest damage locations first. It is also recommended to heat the near side lines prior to the far side.
The web yield line should be heated last. It is typically located at the fillet as shown in Figure 52c.
These four steps complete the cycle. The cycle should be repeated until the flange is straightened within specific tolerances. Quite often phase I can be used to nearly straighten the section. However, the progress of the movement should be observed to insure that over-straightening does not take place on either side of the flange. If the flange movement progresses too quickly, then θn or θf may become zero prior to θw. This situation is shown in Figure 51b. Should this behavior occur, a modification in the phase I pattern should be made in Step 3 for line heats. Rather than heating all seven lines (Figure 52a), line 4 should not be heated.
If straightening progresses to the point that θf = θw, then the far flange may over-straighten with the continuation of Phase I heating. The pattern should be changed.
The situation is depicted in Figure 51c. The modification is to reverse the direction of the far side jacking force while continuing the phase I patterns including lines. The force Pf will prevent over-straightening while allowing the near flange and web to continue corrective movement.
If the damage is reversed, i.e., side N is pushed away from the opposite flange instead of toward it, the direction of the restraining forces should be reversed. The heating patterns are similar to those previously described.
Localized damage to unstiffened elements can have a wide variety of geometries, so the cases shown establish both the pattern and principals upon which heat straightening can be based. Judgment is needed to apply this methodology for specific cases.
Select the heating patterns for damaged stiffened elements based on an evaluation of the total situation. Treating the regions of sharpest curvature with combinations of lines and/or narrow vees is the most effective approach, heating only in the regions with plastic curvature. As straightening progresses, regions should become smaller. The following line heat methodology is recommended for bulges in stiffened elements. A star vee pattern is sometimes used but has been found to be less effective.
The typical bulge will have reverse curvature bending as shown in Figure 53. The crown region should be heated first with the torch on the convex side. As movement progresses, the heating patterns can be expanded into the reverse curvature region again with the torch on the convex side. The initial heating patterns should consist of radial and ring line heats as illustrated by solid lines in Figure 53. The exact number of ring heats will depend on the size of this region. The diameter of the smallest ring should be no less 50 mm (2 in) with spacing between rings of at least 50 mm (2 in). For large bulges the ring spacing should be larger than 50 mm (2 in). For cases where the curvature is relatively uniform, equally spaced rings may be used, but a ring heat should be centered at each location where sharp changes in curvature occur.
Heat the outer ring of the crown region (solid lines) on the convex side first and work inward. After the rings are heated, the radial lines in the crown region should be heated. Again, work from the outside in but do not run the radial lines inside the last ring. Continue this pattern cyclically until the crown region begins to flatten. Allow the steel to completely cool between heating cycles.
Jacks are typically placed at the crown tending to straighten the bulge. Heating patterns must be adjusted to work around jacks and to avoid heat transfer to the jacks which may damage them.
As the crown section flattens, the heating pattern should be expanded into the reverse curvature regions as shown by the dashed lines. The ring heats should be spaced as described in 7.3.1 and the radial heats extended as shown by dashed lines in Figure 53b. Rings may be repetitively heated or shifted, depending on the degree of plastic curvature. The steel should completely cool before the next heating cycle begins.
Since there is no direct equivalent to plastic moment for this type of plate element, Capacity should be taken as the load at initial yielding. Jacking forces should not produce stresses greater than 50% of yield. However, the determination of these stresses for local damage is quite difficult to determine analytically. One way to determine the jacking force that produces yield is experimentally. One approach is to select an area of low stress due to live loading and jack in this area until small permanent deformations are observed. This procedure will define the yield jacking force without significant damage to the member. One-half of this value should be the maximum jacking force used in the damaged zone. Otherwise, jacking forces must be estimated and judged by the amount of movement after each cycle. It is recommended that movement not exceed 4 mm (1/8 in.) per cycle.
Local damage to can be heat straightened by using jacking forces and a relatively small number of line heats rather than a large number of vee heats. Straightening local damage is usually done in stages in which both jacking forces and heating patterns are varied in response to the progression of movements. As a general rule, apply heat to the convex side of the surface. For shallow configurations without sharp changes in slope, the jacking force may be greatly relieved during the cooling cycle. To increase effective movement, the jacking force may be maintained at the original, pre-heated level during cooling but, never increased above that value. If jack pressure is maintained, take care not to exceed the desired movement.
Local damage often has highly irregular patterns requiring a variety of heating patterns based on the damage and member configuration. The principles discussed in this chapter provide a guide but judgment is needed for individual applications.
A second area requiring judgment relates to degree of damage. For plate elements bent about their weak axis, the strain ratio (ε/εy) may well exceed 100, often considered the upper limit for heat straightening repairs. However, local damage often occurs at locations where design live and dead load stresses are not large, such as secondary bracing members. In such cases, the repair of large strain cases might be undertaken for Category L damage which would not be considered for Categories S, W, or T. In all cases engineering judgment is required.