Heat Straightening of steel beams 7

[mattybhoy]

Trying to find some useful information on the limitations, timescales involved and the actual process of using heat straightening to repair steel beams struck by high vehicle in lieu of cutting out and replacing damaged rolled steel sections?

Any clues/tips??

 

[hippo11]

I've heard that "vee-straightening" is quite effective, where they sweep a heat source in a vee shape, with the side you want to shrink most being heated more, if that makes sense.

 

[fuchsjr]

You may want to look at:

http://www.fhwa.dot.gov/bridge/hs17007.htm

Hope this helps.

 

[Rich2001]

Here is a typical procedure that you might find helpful.

Recommended Practice for the Bending and Cambering of Steel using Heat


1.0 Scope:

To provide a guideline of the methodology for an engineering assessment of the heat assisted cambering of steel shapes and structures. This guide addresses engineering issues related to the analysis and design of heat-cambered structural steel. This guide provides recommendations related to the implementation of Heat Cambering of Steel Shapes.

Details associated with implementation of the use of heat shaping are included only to the extent necessary for engineering consideration. The intent is to provide the Structural or Manufacturing Engineer with tools for the analysis and design of heat shaping in a format similar to current procedures associated with traditional structural design for new construction.

1.1 Heat:

These guidelines are for the use of localized Heat not exceeding 1,200ºF. to induce thermal stress resulting in desired controlled metal bending.

1.2 Material Selection:

This process is typically limited to the low-mid carbon structural steels as defined by AWS in the appropriate related welding code.

1.3 Structural Analysis:

The knowledge of and determination of internal stress forms a basic part of the successful use of heat to shape structural steel. Additionally this knowledge is vital to leaving the shaped steel item with the lowest possible residual stress levels.

1.4 Heat Configuration:

This term includes the selection of heating patterns, number of specific heats, and constraining force patterns.

1.5 Loading during Heat:

While gravity may be used with heating to induce deformation no other additional force is specified in these guidelines to avoid delayed crack formation.



1.6 Sizing:

This procedure pertains to only individual items, thus sizing herein pertains to number, dimensions, locations and magnitude of thermal related parameters including Heat Zones, Maximum and Minimum Temperatures and constraining/gravity forces along with the prediction of behavioral bending response with in acceptable limits.

2.0 Heat Cambering of Rolled Beams and Welded Plate Girders

2.1 General:

The project engineer prior to the beginning of work shall approve all Heat Cambering procedures. When heat cambering is approved only deep (<20º) Vee heating patterns will be permitted.

2.2 Heat Cambering of Rolled Beams:

Rolled beams shall be heat cambered to provide the required curvature. Triangular Vee heating patterns shall be spaced through out the length of the member. The material shall be heated in a single pass following the specified pattern and allowed to cool below 250ºF prior to re heating. The Apex of the heating Triangle Vee shall be located in the web at a point not less than 75 percent of the depth of the member from the from the flange that will be concave after heat cambering. Heating shall begin at the apex of the heating pattern and progress slowly with a total included angle not greater than 20 Degrees towards the base of the pattern and across the full width of the flange as described in Section 2.6.

The heating torch shall not be returned to the apex of the Vee after heating has progressed toward the base of the Vee. Same pattern Re Heating or adjacent heating shall wait until the last Vee maximum temperature is less than 250º F. Heating patterns shall be centered upon connecting plates. Simultaneous Vee Heats may be used provided that the clear spacing between Vees is greater than the width of the plate element.

When rolled beams are to be fabricated with cover plates, the rolled beams shall be heat cambered prior to the attachment of the cover plates.

All detail material such as connection plates, bearing stiffeners and gusset plates shall be attached to the rolled beam after the beam has been heat cambered.

2.3 Support of Structural Members for Heat Cambering:

Structural members to be heat cambered shall be supported with the web vertical and with the flange that will be concave after cambering placed upwards. Supports shall be place to take optimum advantage of the gravity load in the member prior to the application of heat.



2.4 Heating Process and Equipment:

Heating shall be performed using large approximately 1” diameter multi-orifice heating torches operating on Propane. Other torches / fuels may be used with prior approval.

Heating shall be confined to the patterns described herein and shall be conducted to bring the steel within the planed pattern to a temperature between 1,000º F and 1,150º F as rapidly as possible with out overheating the steel.

Heating the Steel to a temperature greater than 1,250ºF shall be considered destructive heating and shall automatically be cause for rejection. Steel rejected for destructive heating shall be evaluated for re acceptance, repair or replacement.

2.5 Location of Heating Patterns:

Heating patterns shall be spaced uniformly along the full length of each flange to produce a circular (not Parabolic) curvature. Sufficient heating patterns shall be used in each piece to eliminated unsightly chording effects. Heating patterns shall be adjusted to produce the required curvature, compensating for differences in flange thickness and width as required. Thicker wider plates in general will require wider heating patterns to produce the same degree of curvature as smaller plates. Care should be taken when heating relatively thin, wide plates to guard against flange buckling.

2.6 Heating Patterns and Method of Heat application:

Only truncated Triangular Vee heating patterns shall be used, the base of the triangle shall be the flange edge that will be concave after heat curving. The apex of the triangle shall be truncated to provide a 1-inch width. This truncated Vee end shall be located as follows:

2.6.1 When the required radius is 1,000 feet or less the truncated end of the Vee Triangle shall be located 1/8 of the flange width, but not more than 2 inches beyond the intersection of the web and flange.
2.6.2 When the required radius is greater than 1,000 feet, the heating pattern may be described in 2.6.1 or at the fabricator’s option be modified to locate the truncated Vee end at the junction of web and flange.

Heating patterns shall be plainly marked on the flange surfaces prior to heating. Heat shall be applied simultaneously to the opposing flanges at essentially the same location in the member starting at the truncated apex of the Vee. Beginning at the apex the heating should progress slowly in a spreading in a tight horizontal S pattern to fill the marked angle. The heating torches shall not begin to progress toward the Triangle base until the truncated end reaches the required temperature. This progression should take 5-10 minutes without overheating the steel. The base of the heating triangle should not exceed 10 inches in width regardless of flange width and thickness.


When the flange width exceeds 1-¼ inches both surfaces shall be heated simultaneously. Heat shall not be applied to the inside flange surface until the heat being applied to the outside surface has progressed beyond the web and flange junction.

When heating the inside flange surface, the truncated end of the heating triangle shall be just inside the junction of flange and web. The Vee Triangle shall share a common base with the heating pattern on the outside of the flange at that point.

The Heating torches shall be manipulated to guard against general and surface over heating. When heating thick plates it may be necessary to occasionally interrupt the heating process for periods of less than one minute to allow the heat to soak into the flange and avoid surface overheating.


2.7 Heat Measurement

The Contractor shall provide the Inspector with temperature indicating crayons made for 600ºF, 1,000ºF, 1,100ºF, and 1,250ºF. Heat measurements shall be made immediately after the removal of the heating flame from the steel.


2.8 Artificial Cooling

Quenching with water and/or air is not permitted. Cooling with dry compressed air will be permitted after the steel has cooled to 600ºF.

3.0 “Rules of Thumb”, A short Summary:

3.1 Shoring is usually not necessary for single members unless required by the design engineer.

3.2 Hydraulic Jacks and Rigging are for support only and should not be used to force members in to shape. Heat should do all the work. Rigging should only take up the strain on the heated member to help upset conditions in the metal.

3.3 Upsetting the metal causes it to change volume in the direction of least resistance. This is usually in the thickness (long) rolling dimension of shapes and plates. Control of upsetting is caused by design of heat patterns to obtain a desired degree of bending from volumetric changes.

3.4 Use a Vee pattern for most Heats; following the AASHTO/FHWA procedures is a good start to follow in designing and applying each heat.

3.5 Heat the side that is to be shrunk


3.6 Start with a few small controlled Heats at planned locations to develop a “feel” for the amount of curvature gained with each set of heats. Work up to desired camber without going beyond by finishing with small heats. Remember that each type and lot of steel may react differently to the application of Heat.

3.7 Remember that you can reverse a Heat bend after cooling by reversing the next Heat pattern.

3.8 Start your Heat at the truncated to of the Vee waiting and testing the heat to be at 1,050 – 1,100º F before beginning your heating pattern. Measure starting temperature.

3.9 The heating pattern should take 5-10 minutes to complete.

3.10 Measure temperature of the heated steel during Heat with contact Thermometers or Tempil Sticks at 30 % and 70% of flange / member width.

3.11 Use a sequence of Heats that will leave as little residual stress as possible in the member.

3.12 Let Air Cool to 250ºF and below do not use water to hasten cooling.

3.13 Do not Heat during rain.

3.14 Over heating (>1,200ºF) will destroy the steels ability to change shape in the desired directions and will require repair or replacement.

3.15 Welding Repairs may be required for severely damaged members.

3.16 Use only an approved qualified repair welding procedure written by a welding engineer.

3.17 Non Destructive Testing (NDT) beginning with VT followed by MT during the welding operation and followed by UT is recommended for full penetration tension member welds. RT may be used after passing UT, but is not to be considered a substitute for UT. All NDT should conform to the appropriate AWS Code section for acceptance criteria.

 

[HgTX]

A more general page of that FHWA heat-straightening website:

http://www.fhwa.dot.gov/bridge/heat.htm

Note that they're still doing the medicine show, with demos scheduled in Toronto and San Antonio.

There are two publications, the workshop manual and the report. You can also get a CD from them that can help you calculate what you need to do, if you happen to have a cooperative geometry. They also have another CD with their case study illustrated a heat repair to a bridge hit. Krishna Verma can send you the lot.

If the steel turns red, it's too hot. Q&T steel shouldn't go over 1100F.

Hg

 

[mattybhoy]

Thanks to all for replies and the useful links - I'm a first time user and have been pleaseantly surprised at the response. Rich2001, really appreciate time and effort spent on answer. - Very detailed and systematic approach will help me greatly - cheers.
Mattybhoy

 

[Ron]

mattybhoy....notice the temperature ranges in Rich2001's procedure. This is critical. Bridges are "fracture critical" structures. Heating above the transition temperature (about 1200F) causes metallurgical changes that can affect many properties of the material (See 3.14 in Rich's procedure), including its fracture toughness. Not only do you have to be careful not to exceed the recommended temperatures during straightening, you must also be considerate of the cooling rate for the material. Treat it similar to pre-heat and post-heat procedures for welding.

 

[arto]

Check out SSC-198; SSC-207; SSC-247 @

http://www.shipstructure.org/reports.shtml

for more info on Heat Straightening

 

[dbuzz]

There are several relevant AISC publications:
- What You Should Know About Heat Straightening Repair of Damaged Steel
- Principles and Practices of Heat-Straightening Repair
- Engineered Heat Straightening
- Designing Heat Straightening Repairs
- Heat Straightening of Steel: From Art to Science