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Guide for Heat-Straightening of Damaged Steel Bridge Members
Specifications for the Selection of Contractors and the Conduct of Heat-Straightening Repairs
This Appendix contains suggested specifications for contractor selection and the conduct of heat-straightening repairs. The criteria presented here are guidelines only. The Engineer should select the criteria appropriate for the structure's anticipated use, the complexity of the project and to ensure contractor competency.
The selection of a contractor shall be based on one or more of the following criteria: experience, training, certification, and educational background. If there is neither a certification nor established training program currently available, experience and educational background shall be the primary criteria for selecting a heat-straightening contractor. Typical experience criteria are:
The contractor’s organization shall have at least _____ years of experience in conducting heat-straightening repairs for damaged steel structures. During the preceding three year period, the contractor shall have conducted an average of at least _____ heat-straightening projects per year. Experience documentation shall include: date of project, location, bridge owner, number and type of members straightened, and duration of project.
The years of experience and number of projects conducted can be varied at the Engineer’s discretion. Factors which may influence this decision include: criticality of damaged members, urgency of repairs, traffic volume and need to maintain traffic, complexity of damage, degree of damage, accessibility, climatic conditions, and scale of the project.
Educational background and specific training may be considered by the Engineer if the preceding criteria are not satisfied. Licensing as a professional engineer in such fields as metallurgical, structural, mechanical, or welding engineering may also be considered. Typical educational background criteria are:
The contractor (or the contractor’s field supervisor) shall have a baccalaureate degree from an accredited program in one of the following engineering disciplines and be a licensed professional engineer qualified to practice in one of the following disciplines: structural, metallurgical, mechanical, or welding engineering.
The Engineer may require evidence of qualifications for the technicians involved in the conduct of the heat applications. These qualifications may include evidence of similar, prior work on equivalent structures, documented training in heat straightening, and the ability to explain performance of their duties.
For additional quality control, the following technical specifications apply to the conduct of the project.
The following technical specifications are suggested for incorporation into repair contracts. The Engineer should use judgment in selecting the criteria that best fits the specific damage situation. These are only partial guide specifications focusing on the heat straightening aspects of bridge repair. Specifications on general areas of bridge repair such as traffic control, worker/public safety, permitted hours of operation, documentation of final geometry, etc., should be included by the owner
- 1.1 Heating shall be with an oxygen-fuel combination. The fuel may be propane, acetylene or other similar fuel as may be selected by the contractor, subjected to the Engineer’s approval.
- 1.2 Heat application shall be by single or multiple orifice tips only. The size of the tip shall be proportional to the thickness of the heated material. As a guide, the tip sizes shown in table A2 are recommended. No cutting torch heads are permitted.
- 1.3 Jacks, come-alongs or other force application devices shall be gauged and calibrated so that the force exerted by the device may be controlled and measured. No external force shall be applied to the structure by the contractor unless it is measured.
2. Damage Assessment
- 2.1 Suspected areas of cracking shall be called to the attention of the Engineer and shall be inspected by one or more of the following methods as applicable.
- 2.1.1 Visual Inspection
- 2.1.2 Liquid penetrant examination as described in ASTM E165 (1994 or latest edition).
- 2.1.3 Magnetic-Particle testing as described in ASTM E709 (1994 or latest edition).
|Member Type||Recommended Minimum Tolerance 1,2|
|English (in)||SI (mm)|
|Beams, Truss members, or Columns overall at impact point||½ in over 20 ft|
¾ in over 20 ft
13 mm over 6 meters|
19 mm over 6 meters
|Local Web Deviations||d/100 but not less than ¼ in||d/100 but not less than 6 mm|
|Local Flange Deviations||b/100 but not less than ¼ in||b/100 but not less than 6 mm|
|1 Units of member depth, d, and flange width, b, are inches and millimeters, respectively, for English and SI units|
2 Tolerances for curved or cambered members should account for the original shape of the member
|Steel Thickness (in)||Orifice Type||Size|
- 2.1.4 Ultrasonic examination as described in section 6, part C of the ANSI/AASHTO/AWS Bridge Welding Code D1.5, American Welding Society (1996 or latest edition).
- 2.1.5 Radiographic examination as described in section 6, part B of the ANSI/AASHTO/AWS Bridge Welding Code D1.5, American Welding Society (1996 or latest edition).
- 2.2 The cost of the inspections under 2.1 shall be additional to other testing required and costs shall be negotiated between the Engineer and contractor.
- 2.3 Contractor shall identify and document all yield zones, yield lines and associated damage and provide this information to the Engineer prior to initiation of heat straightening by either visual inspection or measurements.
- 2.4 Steel with strains up to 100 times the yield strain may be repaired by heat straightening. For strains greater than this limit, the Engineer shall determine if heat straightening may be used.
- 2.5 Cracks and/or strains exceeding 100 times the yield strain, or other serious defects may require changes in the scope of the contract which shall be negotiated between the Engineer and the contractor.
3. Heat Application
- 3.1 The temperature of the steel during heat straightening shall not exceed the following:
- 3.1.1 650°C (1,200°F) for Carbon Steels.
- 3.1.2 620°C (1,100°F) for A514 and A709 (grades 100 and 100W) steels.
- 3.1.3 565°C (1,050°F) for A709 grade 70W steel.
- 3.2 The Contractor shall use one or more of the following methods for routine, ongoing, documented temperature verification during heat straightening:
- 3.2.1 Temperature sensitive crayons
- 3.2.2 Pyrometer
- 3.2.3 Infrared non-contact thermometer
- 3.3 Material should be heated in a single pass following the specified pattern and allowed to cool to below 120°C (250°F) prior to re-heating.
- 3.4 Heating patterns and sequences shall be selected to match the type of damage and cross section shape.
- 3.5 Vee heats shall be shifted over the yield zone on successive heating cycles.
- 3.6 Simultaneous vee heats may be used provided that the clear spacing between vees is greater than the width of the plate element
- 3.7 Repair of previously heat-straightened members in the same region of damage may be conducted once. Further repairs are not recommended unless approved by the Engineer.
4. Application of Jacking forces
- 4.1 Jacks shall be placed so that forces are relieved as straightening occurs during cooling.
- 4.2 Magnitude of Jacking Forces
- 4.2.1 Jacking shall be limited so that the maximum bending moment in the heated zone shall be less than 50 percent of the plastic moment capacity of the member or major bending element. For local damage, the jacking force shall be limited to 50 percent of initial yield of the element.
- 4.2.2 The jacking force shall be adjusted so that the sum of jacking-induced moments and estimated residual moments shall be less than 50 percent of the plastic moment capacity of the member. As an alternative to considering residual moments, the moment due to jacking forces can be limited to 25 percent of the plastic moment capacity of the member during the first two heating cycles. For additional heating cycles, the limit of 50 percent may again be used.
- 4.3 Control of jacking forces
The contractor shall determine and document the maximum jacking force for each damage location, and the proposed sequence of jacking and heating. Copies of the documentation shall be submitted to the Engineer for acceptance before beginning repairs. Modifications due to changing condition shall be submitted to the Engineer. The maximum jacking force may be controlled by measuring the deflection resulting from the jacking force. The deflection limitation can be computed by one of the following methods.
- 4.4 The calibration of jacks and electronic temperature monitoring equipment shall be performed and documented monthly, and load cells used for calibration must be certified within a two year period.
5. Field Supervision of Repair
- 5.1 Jacking forces shall be monitored to insure that limits are not exceeded.
- 5.2 Heating patterns shall be approved by the Engineer.
- 5.3 Heating temperatures shall be routinely monitored to insure compliance with specified limits.
- 6.1 The dimensions of heat-straightened structural members shall conform to the tolerances specified in table A1 except as noted below.
- 6.2 Tolerance limits may be relaxed at the discretion of the Engineer, based on one or more or the following considerations:
- (a) Type and location of damage in the member.
- (b) Time considerations resulting from the nature of traffic congestion during the repair operation.
- (c) Cost of repair.
- (d) Degree of restoration required to restore structural integrity.
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