Twist and Bow in a Welded Box Beam
A 6.0 m long box beam is fabricated from 12 mm flange plates and 10 mm web plates, giving a 300 × 300 mm section. The flanges are connected to the webs with continuous 6 mm fillet welds on both sides, welded by FCAW in one pass per side. The beam is tacked in a strongback and welded from one end to the other with a single operator, completing the four fillets in sequence. After cooling, the beam has an 8 mm twist over its length and a 5 mm bow. The print allows 3 mm maximum twist and 4 mm bow over 6 m. The shop wants to cold-twist the beam with a forklift and chain, then weld a stiffener plate over the worst area. The customer's inspector will arrive tomorrow.
1. Identify the likely causes of the twist and bow, and evaluate the forklift cold-twisting proposal.
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The likely causes are unbalanced heat input and a welding sequence that ran continuously from one end. Welding four fillets in sequence with one operator heats one side and one end first, so shrinkage is not balanced about the neutral axis. The strongback restrains the beam during welding but does not prevent residual stress from redistributing after release. Fit-up, tack placement, and an oversized 6 mm fillet also contribute. Cold-twisting with a forklift and chain is not an acceptable correction. It is uncontrolled, can overstress the fillets or base metal, can initiate cracks, and may leave the beam with unknown residual stress. Welding a stiffener over the worst area may hide a crack or lock in the distortion rather than correct it.
2. Propose a revised welding sequence and distortion-control plan for the next beam.
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For the next beam, balance heat about both axes. Use two welders working opposite each other, or alternate sides and ends in short backstep passes. Weld from the center outward, not continuously from one end. Alternate top and bottom flanges and left and right webs so shrinkage is symmetric. Use the minimum qualified fillet size, control fit-up and gap, and avoid excessive tacks. Preset or pre-camber the beam if the distortion pattern is known. Allow controlled cooling between passes rather than quenching. Measure twist and bow after each stage, not only after completion, so the sequence can be adjusted before the residual stress is locked in.
3. What inspection and acceptance checks are needed before and after any correction?
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Before correction, survey the twist and bow at defined points, record the baseline, and inspect all fillets visually and by magnetic particle or penetrant testing for cracks. Check weld size, undercut, and tack condition. Any correction method should be reviewed by the engineer and qualified. If heat straightening is permitted, use a documented procedure with controlled temperature and cooling. After correction, repeat the dimensional survey and inspect the welds again for cracking, especially at the heat-affected zones and stiffener locations. The beam should not be accepted until twist is within 3 mm, bow is within 4 mm over 6 m, and the welds pass the required inspection.