Welding Distortion in Long Steel Girders: Causes, Checks and Controls

Welding Distortion in Long Steel Girders: Causes, Checks and Controls

Welding heats steel locally, and the heated zone wants to shrink as it cools. In a short plate, the movement is small and easy to ignore. In a long plate or box girder, the same shrinkage adds up along the length, pulling flanges out of line, bowing the web and twisting the section. Distortion is not a defect in the weld. It is a dimensional problem that the shop has to plan for, and it is one of the most common reasons a long girder needs straightening or is held up before dispatch.

Why long girders move

Weld shrinkage pulls on the plate on the side of the weld. On a girder with welds on one side of the web, that pull bends the member toward the weld. Welds placed on both sides balance each other, but only if they are sized and sequenced evenly. An unbalanced weld pattern is the main cause of the sweep and camber errors that appear on the shop floor.

Three factors make the problem worse:

• Large fillet welds, which put more heat into the member than the joint strictly needs
• Welds placed in one long run rather than in short, alternating sections
• Restraint that is removed before the member has cooled

Plan the sequence before the first weld

Distortion control starts on the drawing and the welding sequence sheet, not after the girder bends. The sequence should show which welds are made first, which side of the member each weld is on, and how the work moves between welders so that heat is spread out. A common approach is to alternate between flange-to-web welds on opposite sides and to weld in short segments with rest between them.

The welding procedure sets the heat input. Within the approved procedure, the sequence and the travel speed are the main controls available to the welder, so they should be written down rather than left to the individual.

Use fixturing, but do not rely on it alone

Jigs and fixtures hold a girder straight while it cools. They are useful, but they are not a complete answer. A member clamped rigidly can build stress that is released as soon as it is unclamped, causing movement later. The fixture should hold the member in the position the sequence intends, and the release point should be planned, not left to chance.

Check at the stages that matter

Measure the member at these points rather than only at the end:

• After tack welding and before any production welding, to confirm the starting geometry
• At intervals during welding of long runs, so that a developing sweep is caught early
• After the final weld, once the member has cooled to ambient temperature
• After any straightening or heat correction, before the member is released for the next operation

Record the measurements against the drawing tolerances. A distortion record that shows the trend along the length is more useful than a single end-of-job reading.

Correct it with control

Where distortion is out of tolerance, correction is possible by mechanical straightening or by controlled heat application. Both can change the material and the stress state, so the correction method should be approved within the quality plan. Heat straightening in particular needs a defined temperature range and a record of where it was applied. Correction should never be done by ad hoc hammering or by overloading the member, which can leave the steel damaged and the dimensions unstable.

After any correction, re-check the affected welds and the dimensions, and confirm that the correction did not create a new out-of-tolerance condition.

What a buyer can ask

• A welding sequence sheet for each long member, or at least for each type of girder in the package
• A distortion check record with the measured values at the stages above
• The approved method for any straightening or heat correction, with the records of where it was used

For how these checks tie into the dimensional tolerances on the drawing, see dimensional tolerances on steel fabrication drawings. For the trial assembly step that confirms the final fit, see trial assembly and camber control for long girders.