|FHWA > Bridge > Guide for Heat-Straightening of Damaged Steel Bridge Members > Chapter 3: Assessing, Planning and Conducting Successful Repairs|
Guide for Heat-Straightening of Damaged Steel Bridge Members
Chapter 3: Assessing, Planning and Conducting Successful Repairs
As with other types of repair, a successful heat-straightening repair requires assessment, planning and design. Several procedures should be considered as part of the process. Critical aspects include: determination of degree of damage, location of yield zones and regions of maximum strain, limitations for heat-straightening repair, selection of heating patterns, and selection of jacking restraints. Each requires the exercise of engineering judgment. Outlined in this chapter are some key aspects of assessing, planning and designing a repair. One of the primary keys is ongoing coordination between the engineer, field supervisor or inspector, and the contractor conducting the repair.
The engineer is responsible for selecting the most appropriate repair technique for the specific damage. Alternatives must be evaluated and the most effective solution determined. The key considerations include: cost, implementability, adequate restoration of strength, longevity of repair, time to complete repair, aesthetics, and impact on traffic. These aspects constitute the concept referred to as design.
Although frequently overlooked, repairs should be designed in a similar manner to new structures. The typical process includes: selecting a trial repair scheme, conducting a structural analysis (which may require assumptions of certain geometric or material properties), defining the parameters of the repair (or verifying the capacity after repair), possibly re-analyzing and re-designing, evaluating alternate repair or replacement schemes, and finally, providing complete details and specifications for the system selected.
Heat-straightening repair is not the solution for every damage situation. The engineer's role is to assess its specific applicability. Aspects to consider are: current condition of the rest of the structure and other anticipated repairs, degree of damage, presence of fractures, cause of damage and likelihood of repetitive damage, accessibility, and the repair method’s impact on material properties. Once the heat straightening alternative is selected, then the repair parameters such as traffic control, contractor access and work areas, permitted hours of work, typical heating patterns, maximum restraining forces and locations, and maximum heating temperature must be chosen. Finally, plans and specifications should be developed which generally define how the repair is to be accomplished.
Since most heat-straightening repairs are conducted by contractors, the field inspector, representing the bridge owner, has major responsibilities to insure that the repair is being conducted according to plans and specifications. Of particular importance is insuring that procedures are followed which are not detrimental to the steel.
The third member of the team is the contractor who actually executes the repair. The ultimate success of the project hinges on the skills and understanding for the project by the contractor’s personnel. While others may have designed the repair plan, the details of execution lie with the contractor. Important considerations may include: (1) scaffolding arrangements; (2) selection of proper heating equipment; (3) implementing the restraint plan with appropriate jacks and come-alongs; (4) placing the heats in proper patterns and sequences; and (5) analyzing the progress of the repair. The contractor must be alert to the response of the structure and be prepared to suggest changes to ensure stability and expedite the process. In spite of our current knowledge and analytical capabilities, movements during heat straightening cannot always be predicted accurately.
The primary reasons for this difficulty are that: (1) damage patterns are often a complex mixture of the idealized cases and require experience to determine the details of the heating process; and (2) residual stresses and moments which may have been locked into the structure during both original construction and also the damage phase are difficult to predict and may prevent or increase the expected movement. The contractor must be able to assess the reaction of the structure to the planned repair and suggest modifications if the structure is not performing properly. These modifications may range from changes in heating patterns and jacking arrangements to decisions on whether to remove secondary or bracing members during the repair. Perhaps most important is that the engineer, the inspector and the contractor maintain open and clear channels of communication. This interaction of the three key players in a heat-straightening repair will go a long ways toward insuring a successful project.
A successful repair requires the control and selection of certain specific parameters. The first key is the selection of the heating patterns and sequences. The combination of vee, line and strip heats must be chosen to fit the damage patterns. Heat should only be applied in the vicinity of those regions in which yielding of the material have occurred. Typically, vee heats should be relatively narrow. A good rule of thumb is to limit the open end of the vee to 250 mm (10 in) for one inch thick plates. However, a smaller limit should be considered for progressively thinner plates. These limits will minimize distortion which might occur due to local buckling of the plate element.
The second key is to control the heating temperature and rate. Temperatures should be limited to 650°C (1200°F) for non-quenched and tempered steels, 590°C (1100°F) for A514 and A709 Grade 100 and 100W quenched and tempered steels and 565°C (1050°F) for A709 Grade 70W quenched and tempered steel. Higher heats may adversely affect the material properties of the steel and lead to a weaker structure.
The third key is to control the applied restraining forces during repair. Research has shown that the use of jacks to apply restraint can greatly shorten the number of heating cycles required. However, over-jacking can result in buckling or a brittle fracture during or shortly after heat straightening. To prevent such a sudden fracture, as illustrated in Figure 21, jacking forces should be limited. The recommended procedure is to calculate the plastic moment capacity of the damaged member and limit the moment resulting from the combination of initial jacking forces and dead loads to one-half of this value. If practitioners do not take this precaution, brittle fractures or excess deformation may occur. It is strongly recommended that jacks be gauged and calibrated, then set for the maximum force computed. Of course, the jacking forces should always be applied in the direction tending to straighten the beam.
The execution of a heat-straightening repair that incorporates these keys must begin with the assessment of the damaged structure.
Many incidents resulting in damage to steel bridges produce an emergency situation. The first step in the rehabilitation process is a site investigation to assess the degree of damage and the safety of the existing structure. The purpose of this section is to provide guidelines for damage assessment in the form of steps required for a complete assessment. All aspects may not be required in each case, so judgment must be used when deciding if, and when, to eliminate a part of the process.
The purpose of the inspection is to protect the public, employees of the owner and repair personnel. This inspection is often visual and conducted with special concern for safety. The major aspects of damage are recorded and documented with photographs and measurements. During this inspection, a preliminary list of repair requirements and options should be made. Particular attention should be paid to temporary needs such as shoring, traffic control, access and other short-term considerations. A part of this evaluation may require a review of the design drawings and computations to determine the safety and stability of the bridge. The specific cause of damage may also influence the final decision on repair and should be investigated if possible. Typical damage causes are: (1) over-height or over-wide vehicle impact; (2) overweight vehicles or overloads; (3) out-of-control vehicles or moving systems; (4) mishandling during construction; (5) fire; (6) blast; (7) earthquakes; (8) support or substructure movement; and (9) wind or water-borne debris.
Applicability of a heat-straightening repair depends on the type and degree of damage. Three aspects should be carefully checked: (1) signs of fracture; (2) degree of damage; and (3) material degradation.
While some fractures are quite obvious, others may be too small to visually detect. However, it is important to determine if such cracks exist since they may propagate during the heat-straightening process. When in doubt, one of the following conventional methods can be utilized: (1) dye penetrant, (2) magnetic particle, (3) ultrasonic testing, or (4) radiographic testing.
Degree of damage can be evaluated using two different criteria. One is the angle of damage, φd, which is a measure of the change in curvature. The other is the strain ratio, μ, which is a measure of the maximum strain occurring in the damaged zone. For either case an evaluation of the degree of damage requires measurements to be taken. Two types of damage are quantified by measurements: (1) Overall bending or twisting of a member; and (2) localized bulges or sharp crimps. These measurements can be used to compute the maximum damage-induced strain, m, or to determine the angle of damage, φd.
For determining angle of damage, the usual procedure is to begin by measuring offsets from a taut line, laser beam or straight-edge. A typical layout is shown in Figure 22 showing the definition of φd. This layout may be done by either using the unyielded adjacent regions on either side of the damage as reference lines, since their curvature is small in comparison to the plastic zones, and determining the included angle between them, or by establishing a base line and finding the offsets in the damage zone. For the first case, tangents from the straight portions define the angle or degree of damage between the tangents. If the offsets are taken in the elastic zone on either side of the damage as shown in Figure 22b, the degree of damage, φd, can be computed.
Based on measurements taken at the site, degree of damage can be calculated as follows:
where φd is the angle of damage or angle of permanent deformation at the plastic hinge and yi is a measured offset as shown in Figure 22b.
In some cases direct measurements of φd can be made from a photograph. If a photograph can be taken perpendicular to the plane of curvature, then tangents can be laid out and measured directly. For small zones of damage, two straight edges can be used to produce the tangent intersections. Again, the angle of damage can be measured with a protractor. While this method may seem somewhat crude, a reasonable degree of accuracy can be obtained.
For the case where the offsets are taken in the damage zone (see Figure 22a). The radius of curvature, R, can be approximated as
The degree of damage can then be calculated from:
or (Eq. 3.4)
Where Lr-1 = Lr = L
Approximations are involved in using these equations. The assumption is made that the radius of curvature is constant over the entire length of the damage although it usually varies. If the damage curve is smooth, this assumption is fairly accurate. If the curve is irregular, the assumption becomes more approximate. For highly irregular curvatures, measure only the worst portion of the damaged region using the three-point offset procedure and the calculation of radius of curvature from Eq. 3.2. In general, the approaches described here give an adequate estimate of the radius of curvature and angle of damage.
In order to calculate the maximum strain ratio, the maximum curvature should be measured as previously described. Shown in Figure 23 is a damaged beam of uniform curvature. The radius of the bend is defined as radius of curvature, R. Strain is proportional to curvature and curvature can be computed from field measurements, so the radius of curvature to the yield curvature, Ry, may be expressed as
where E = modulus of elasticity, Fy = yield stress, and ymax = the distance from the centroid to the extreme fiber of the element.
The radius of curvature is related to the strain by
where R is the actual radius of curvature in the damaged region.
Since damage measurements are taken at discrete locations, the radius of curvature can be approximated from Eq. 3.2. Once the smallest radius of curvature is determined in the damaged region, the maximum strain can be computed from Eq. 3.6 and compared to the yield strain
From Eqs. 3.6 and 3.7, the strain ratio is
Research data has shown that heat straightening can be successful on steel with plastic strains up to 100 times the yield strain, εy. There is reason to believe that even larger strains can be repaired. However, since no research data exists beyond the 100εy range, engineering judgment is required.
Certain aspects of material degradation will influence the decision to heat straighten. Nicks, gouges and other abrupt discontinuities in the damage zone will be stress risers during the repair when jacking forces and heat are applied. Such discontinuities should be noted and ground to a smooth transition prior to heat straightening.
A second concern is exposure to high temperature (such as a fire) when the damage occurred. As long as the steel temperature did not exceed either the tempering temperature or the lower phase transition temperature, no permanent degradation would be expected from the heating. However, if the damaged steel reached higher temperatures, metallurgical tests should be performed to ensure material integrity before heat straightening is applied. Tests that should be considered include: (1) a chemical analysis; (2) a grain size and micro structure analysis; (3) Brinell hardness tests; (4) Charpy notch toughness tests; and (5) tensile tests to determine yield, ultimate strength, and percent elongation. In-place, non-destructive tests (Brinell, appearance) avoid removing material that must be restored. Charpy and tensile tests require significant removal of material straight enough to machine specimens from damaged and undamaged areas for comparative results.
Several visual signs may suggest exposure to high temperature including: melted mill scale, distortion, black discoloration of steel, and cracking and spalling of adjacent concrete. Tests can then be conducted at suspicious regions. For example, a significant increase in Brinell hardness, in comparison to undamaged areas of the same member, indicates potential heat damage. Or, for the Charpy V Notch test, a significant reduction in values over those from an undamaged specimen may indicate damage. The most definitive test is usually a metallurgical comparison of microstructure between damaged and undamaged areas. Evidence of partial austenization and recrystallization into finer grain size indicates heating above the lower phase transition temperature.
Often the design drawings are available to confirm the structure’s original configuration, design parameters and type(s) of steel. If drawings are not available, then enough measurements should be taken so that a structural analysis can be conducted if required.
Once the damage assessment is complete, the repair can be designed. The following steps may be required as part of this planning and design process:
Each of these will be discussed in the following sections.
Heat-straightening repairs have been conducted for strains up to 100ey, or m=100. Repairs may be successful at even greater strains. But research studies have not focused on such strains so engineers should use judgment in straightening beyond this range.
Fire damage involving high temperature may be an exception to this limit. If the distortion is due to diminished strength at high temperature material properties have probably been detrimentally affected. Repair decisions should then be based on metallurgical analysis and expert opinion as well as the 100ey strain limitation.
A structural analysis may be necessary to evaluate the damaged structure. This analysis serves one of two purposes: (1) to determine the capacity in its damaged configuration for safety purposes; and (2) to compute residual forces induced by the impact damage which may effect safety and influence the level of applied restraining forces during heat straightening (see ref. 1 for an example of calculating residual moments). The analysis can be based on the undeformed geometry except when the displaced geometry of the frame or truss system (after damage) results in changes in internal forces by more than 20 percent. However, even if undeformed geometry is used in the analysis, the deformed geometry should be used when computing the member stresses. The allowable stresses should be based on the original properties of the material. When a member has a significant change in shape due to damage, the section properties should be modified when calculating stresses. While each specific application must be considered on an individual basis, some general guidelines can be developed. Assuming that no fractures have occurred, bending and compression members are the most critical to evaluate. Forces due to applied loads in tension members tend to straighten out-of-plane damage (and are thus self-correcting), while such forces in bending or compression members tend to magnify the damage.
While the primary consideration for allowing heat-straightening repair is the degree of damage limitation, other criteria may also influence the decision. Of particular importance is the presence of fractures or previously heat straightened members. A fracture may necessitate the replacement of part, or all, of a structural member. In some cases it may be feasible to heat straighten the suspect region and then repair it in-place by mechanical connectors. In other cases a portion of the member may be replaced while the remainder is repaired by heat straightening.
An example of combining heat straightening with replacement is when one or more girders are impacted by an over-height vehicle. This type of accident often displaces the bottom flange. If the impact point is near diaphragms, the diaphragms are often severely damaged. An example is shown in Figure 24. It is usually much more economical to simply replace a diaphragm rather than taking a lengthy time to straighten it. The recommended procedure is to remove the diaphragm (especially if it would restrain desired movement of the member) heat straighten the girder, and then replace the diaphragm with a new one.
In general, heat straightening can be applied to a wide variety of structural members. However, some have cautioned about straightening fracture critical members (Shannafelt and Horn, 1984). Although there is no research data to support a ban on heat straightening fracture critical members, practically no fatigue testing has been conducted. If heating temperature (including the limits imposed by section 12.12 of the AASHTO/AWS D1.5 Bridge Welding Code) is carefully controlled, jacking forces are maintained, and notches and nicks are ground smooth there is no reason to expect unusual problems. Additional care is warranted for fracture critical members to insure that the heat straightening is properly conducted.
The fundamental heating patterns have been described in Chapter 2. Since typical damage is often a combination of these fundamental damage types, a combination of heating patterns is often required. The key is to select the combination of patterns to fit the damage. When in doubt, concentrate on one of the basic heating patterns at a time. For example, remove the Category W damage prior to addressing the Category L damage.
In general, the vee depth should be equal to the width of the plate being straightened. Partial depth vees do not reduce membershortening as some have speculated. The primary application for half depth vees is the repair of local damage.
The angle of the vee is usually limited by practical considerations. It should be as large as practical for the specific application. If the open end of the vee is too wide, out-of-plane distortion often occurs. Likewise the vee area should be small enough to heat quickly so that differential cooling is limited. A good rule of thumb is to limit the open end of the vee to approximately one-third to one-half the plate width but not greater than 254 mm (10 in). These limits translate roughly to 20-30° vee angles. If the width of the open end of the vee, V, is selected, the vee angle is
where W is the plate width.
Simultaneous vee heats may be performed with proper spacing. It is recommended that the vees be spaced at least one plate width, W, apart. Also, if multiple plastic hinges occur, each hinge may be heated simultaneously.
Since jacking forces can expedite repairs, such forces should be utilized. Jacks should be located to produce the maximum effect in the zones of plastic deformation. Jacks must be gauged and calibrated prior to use and properly secured so they will not fall out as pressure subsides during cooling. The loads applied to the structure should be controlled and the limiting values established. A jacking arrangement for a composite girder bridge is shown in Figure 25. Lateral forces are utilized on the lower flanges, Figure 25a, while jacks between flanges are used for local damage, Figure 25b.
For cases where residual moments are small, the jacking moment, Mj, should be limited to
where Mp is the plastic moment capacity of the member or damaged element (such as the lower flange of a composite girder). Methods of computing jacking forces for various member configurations are available (Avent and Mukai, 1998). Any residual moments will be relieved during the first few heats. Rather than computing residual moments, an alternative is to use a jacking moment of only ¼ Mp during the first two cycles.
On occasion, a hairline fracture will occur or become visible during heat-straightening repair. The causes are believed to be: (1) excessive restraining forces being applied during the heating process; (2) successive repairs of a re-damaged element; and/or (3) the growth of micro cracks initiated during initial damage. Item (1) is the primary cause, so restraining forces should be specified at safe limits and be monitored during actual repair. For item (2), heat straightening material should be limited to only two damage repairs.
The estimate of number of heats provides a time line for the project. Comparing the estimated movement with the actual movement as it progresses also indicates whether the heating is being properly done. The number of heats, n, can be estimated as
where φp is the predicted plastic rotation per heat and φd is the degree of damage. Formulas for the plastic rotation associated with various structural shapes and damage conditions are provided in a later section of this guide.
The final step is to prepare plans and specifications for the project. These plans will be the inspector’s guide as well as the contractor’s directive. Suggested specifications are given in Appendix I. As noted in 3.1, the owner provides quality assurance inspectors to verify the contractor complies with contract requirements. The contractor is responsible and must provide both quality control and supervisors to satisfy the contract.
Excessive temperatures may cause surface damage or lead to increased brittleness. Temperature can be monitored in several ways. One of the most accurate is to use temperature-sensing crayons. These crayons melt at a specified temperature and are available in increments as small as 14°C (25°F) (Figure 26). By using two crayons that bracket the desired heating temperature, accurate control can be maintained. The crayons and their marks will burn if exposed directly to the flame of the torch, and heat needs a few seconds to penetrate and provide representative readings. Therefore, the torch must have just exited the area tested or be momentarily removed (one to four seconds) before the crayons are struck on the surface. An alternative for thinner material is to strike the crayon on the backside at the point being heated.
Another temperature monitoring method is to use a contact pyrometer. This device is basically a thermocouple connected to a readout device. It can be used in a matter similar to a temperature crayon by placing it on the surface. Because the pyrometer relies on full contact with a smooth surface, the readings vary with position and pressure, typically underestimating the actual temperature. It is recommended that the pyrometer be calibrated with temperature crayons prior to using.
Infrared devices are probably the most convenient to use. These devices record the temperature with a digital readout and can be used from a distance to minimize disruption of the heating process. However, the torch still needs to be beyond the area or momentarily removed while taking the reading.
To complement the crayons, pyrometer, or infrared devices; visually observe the color of the steel at the torch tip. Under ordinary daylight conditions, a halo will form on the steel around the torch tip. At approximately 650°C (1200°F) this halo will have a satiny silver color in daylight or bright lighting. The observation of color is particularly useful for the technician using the torch to maintain a constant temperature. However, this is the least accurate method of monitoring temperature and is approximate at best.
Another concern for the heat-straightening supervisor is the control of restraining forces. Typically hydraulic jacks are used to apply restraining forces (see Figure 27 as an example) and should be calibrated so that the force being exerted can be determined. Mechanical jacks should only be permitted if they are calibrated to control applied loads. The maximum allowable force should be computed as part of the design process and specified in contract documents.
Figure 26. Temperature sensing crayons.
Figure 27. Jacks in place on a Wisconsin bridge.
The inspector should review and accept the heating patterns and torch paths proposed by the contractor. The general patterns can be part of the repair plan. However, as heating progresses there may be a need to modify the patterns. The inspector needs to understand the principles for using various patterns and may allow modifications on site as required.
A significant concern is the tolerance for the completed repair. The contract documents should specify the allowable tolerances and the inspector should verify that these limits either have been met or where (and why) exceptions were accepted. While tolerance levels may be similar to that of new construction, often a greater tolerance is specified to reduce the number of heat cycles required, especially in restricted areas and to minimize the cost of the repair. This decision should be made as part of the design process. Recommended tolerances are given in Appendix I.
The above items relate specifically to heat straightening. The contractor’s supervisor exercises normal control of the job site, as with any construction project, including monitoring of safety procedures.
Remember that the goal is not just to straighten the damage, but to straighten it safely. There are a number of critical items for the supervisor to verify as the repair progresses.