Built-Up Plate Girder WPS: Controlling Camber and Sweep

A built-up plate girder is not self-correcting. Every weld pass deposits heat, and heat causes metal to try to expand and then contract as it cools. The net result of those shrinkage cycles across the full length of a girder is a predictable shape change: the flanges pull slightly toward the web, the web may bow, and the overall member can develop sweep or camber that departs from the theoretical geometry. Whether those deviations are controlled or chaotic depends almost entirely on the welding sequence and the parameters written into the WPS.

AWS D1.1:2025 doesn't prescribe a specific welding sequence for plate girders, but it governs the weld quality and workmanship requirements that the sequence must satisfy. The distortion control is the fabricator's problem, solved through a combination of jig design, assembly sequence, and WPS parameters — particularly heat input range, pass count, and stringer vs. weave bead requirements.

The Mechanics of Weld Shrinkage in a Built-Up Section

Before looking at sequence strategies, it helps to understand which shrinkage forces actually act on a plate girder during fabrication.

Longitudinal shrinkage occurs along the weld run. Each weld pass running along the flange-to-web junction shrinks slightly as it cools, creating a compressive force on the web and a tensile force trying to shorten the outer fiber at the flange. If both flanges are welded symmetrically, the longitudinal forces balance and net camber remains near zero. If one flange is welded first and fully completed before the opposite flange, the asymmetric shrinkage can introduce significant hogging or sagging camber in the finished member.

Transverse (angular) shrinkage acts across the joint. As the fillet weld or groove weld cools, it pulls the flange plate toward the web, inducing angular rotation. In a symmetrical double-sided fillet weld, simultaneous welding from both sides minimizes angular distortion. Sequential welding — one side fully completed, then the other — allows the first weld's angular distortion to lock in before the second weld can oppose it.

Sweep is lateral bowing of the girder out of the planned vertical plane. Sweep typically results from unequal heat input on one side of the web along the length, or from asymmetric fillet weld sizes between the two flanges. It is the hardest distortion to correct after fabrication and the most critical to prevent through sequence control.

Rule library based on AWS D1.1:2025; verify against your governing edition.

How WPS Heat Input Parameters Drive Distortion

Heat input is the primary WPS parameter that controls how much the base metal moves. AWS D1.1:2025 defines heat input as:

Heat Input (kJ/in) = (Amps × Volts × 60) / (Travel Speed, in/min × 1000)

Higher heat input means more heat deposited per unit length, which means more expansion and more contraction on cooling — more shrinkage, more distortion. The WPS specifies a heat input range (or, equivalently, specifies the amperage, voltage, and travel speed ranges from which heat input is calculated). A WPS with a wide heat input range gives welders latitude that can be helpful for out-of-position work but dangerous for distortion-sensitive applications.

For built-up plate girders where geometry control is critical, the WPS should specify:

  • Tight amperage and voltage ranges rather than broad bands
  • A minimum travel speed that caps maximum heat input
  • Stringer bead requirements rather than weave bead permission for flange-to-web fillets
  • Maximum pass width (typically no more than 3× electrode diameter for stringers)

Stringer beads matter because they deposit less heat per unit length at a given travel speed than weave beads, allow adjacent stringers to cool between passes, and distribute residual stress over a wider area with less concentrated shrinkage per pass. A plate girder WPS that allows wide weave beads on the flange-to-web fillets is writing distortion into the procedure.

Assembly Sequence Strategy: How to Write It Into the WPS

A well-engineered plate girder WPS specifies not just the welding parameters but the assembly and sequence logic that controls distortion. The major sequence decisions are:

1. Tack and fit-up before any production welding begins. The web must be squared to both flanges, tacked at sufficient intervals to prevent movement during welding, and verified for straightness and sweep before any root passes are run. The WPS should specify tack weld size, spacing, and the qualified tack welders required. See tack weld requirements under AWS D1.1 for the specific code provisions.

2. Balance welds across the neutral axis. Alternate welding passes between the top flange-to-web joint and the bottom flange-to-web joint rather than completing one side fully before starting the other. This is the single most effective sequence strategy for controlling vertical camber. If two welders are available, simultaneous welding on both sides of the web in the same longitudinal direction produces the most balanced heat input.

3. Maintain consistent travel direction across the full length. All passes on a given joint should travel in the same direction, or follow a defined backstep pattern if the full length is too long for a single run. Reversing direction mid-length creates asymmetric residual stress patterns that produce irregular sweep or S-camber.

4. Sequence stiffener welding relative to flange-to-web completion. Transverse stiffeners welded before the longitudinal flange-to-web welds are complete create restraint conditions that can trap distortion. Standard practice is to complete the flange-to-web fillet welds first, measure the member geometry, and then weld transverse stiffeners after confirming the girder is within tolerance. The WPS and shop quality plan should specify this hold point.

5. Control interpass temperature. High interpass temperatures increase heat input per pass and reduce the time for the preceding pass to contract before the next pass is deposited. The WPS maximum interpass temperature (typically 500°F [260°C] for most carbon steels) is not just a metallurgical requirement — it also limits the cumulative heat input that drives distortion. For more on interpass temperature control in structural welding, see interpass temperature limits and CWI verification under AWS D1.1.

Pre-Set Camber: How the Jig Compensates for Predicted Shrinkage

Most fabricators pre-set a built-up plate girder with intentional opposite camber in the jig before welding begins. The theory is straightforward: predicted weld shrinkage will pull the flanges toward each other and induce a certain amount of upward camber from the bottom flange's hotter, longer heat path. By jigging the girder with equivalent downward pre-set (sagging in the jig), the weld-induced camber brings the finished member to the specified camber or flat.

The pre-set calculation requires knowing:

  • The number of passes on each flange-to-web joint
  • The heat input per pass
  • The expected transverse shrinkage per pass (often 0.01–0.02 in per pass for 5/16 in fillets, from shop empirical data)
  • The flange dimensions and section modulus

Most experienced fabricators develop their pre-set values empirically — recording measured camber after welding similar cross-sections and adjusting the jig for the next run. A WPS that changes heat input range significantly (because of a process or filler change, for example) can invalidate an established pre-set value, forcing the shop to re-develop the jig adjustment. This is a practical reason to keep WPS heat input ranges consistent across similar girder programs.

Distortion Measurement and Correction Hold Points

AWS D1.1:2025 and AISC 303 together define the dimensional tolerances that the finished girder must meet: camber limits, sweep limits, web flatness (plate flatness relative to a 48 in [1200 mm] straightedge), and flange tilt. The CWI's responsibility is to verify that the girder meets these tolerances before it ships.

The measurement hold points that belong in the shop quality plan:

  • After flange-to-web welding, before stiffener welding: Check camber and sweep. Correct while the girder is still accessible for straightening if out of tolerance.
  • After stiffener welding: Recheck. Stiffener welding can introduce incremental distortion, especially on thin webs.
  • After any flame straightening: Verify that thermal straightening operations performed to correct out-of-tolerance distortion were conducted per AWS D1.1 temperature requirements and that the base metal properties were not compromised.

Flame straightening for distortion correction is governed by AWS D1.1:2025 provisions covering post-weld thermal treatment. Heat applied for straightening must not exceed the maximum temperature for the base metal grade, and reheat cycles must be tracked and documented. See flame straightening distorted structural steel under AWS D1.1 for full provisions.

Putting It Together: What Belongs in the Plate Girder WPS

A built-up plate girder WPS that actually controls distortion includes these elements beyond the standard essential variables:

  • Process and filler metal per Table 6.6 essential variables
  • Base metal group and thickness range per Table 6.9 and Table 6.6 Row 1
  • Heat input range — with a specific maximum, not just a minimum
  • Bead type specification — stringer beads required for flange-to-web fillets, maximum pass width specified
  • Interpass temperature maximum — with measurement method
  • Assembly and welding sequence — balanced, alternating between flanges, direction of travel
  • Stiffener welding sequence — after flange-to-web completion, with measurement hold point
  • Pre-set camber requirement — documented in the shop quality plan and tied to the WPS

This level of detail goes beyond the minimum content AWS D1.1 requires for WPS documentation, but it reflects what experienced QC managers and CWIs know is necessary to fabricate to specification on a heavy plate girder program. A WPS that only records essential variables but omits the sequence and heat input controls is technically compliant but operationally incomplete.

Managing WPS content, sequence requirements, and PQR coverage across a plate girder program — especially when multiple girder sizes and steel grades are involved — benefits from a systematic WPS library approach. For software tools that organize WPS parameters, track essential variable coverage, and generate AISC-ready documentation, see the options at wpswelding.com/pricing.

For the broader framework of weld distortion control in structural fabrication — including restraint strategies, back-step sequencing, and distortion measurement — the complete approach is covered in weld distortion control and welding sequence documentation.