Bow and Diamonding in a Welded Box Beam
A 6 m long box beam, 300 mm deep × 200 mm wide, is fabricated from 8 mm mild steel plate with four continuous 6 mm longitudinal fillet welds at the corners. Two welders first completed both top fillets from one end to the other, then completed both bottom fillets. After cooling, the beam has a 22 mm upward bow and the 200 mm width has closed by 8 mm at mid-length, giving a diamond-shaped cross-section. The specified straightness tolerance is 10 mm over the full length. The shop has a 100 kN press, oxy-acetylene heating equipment and a 5 t crane. The customer does not permit hammering or arc gouging on the finished beam. The production engineer suggests adding two stiffeners and welding them continuously to 'pull it back'.
1. Diagnose the causes of the 22 mm bow and the 8 mm closing of the width, relating them to weld shrinkage and the sequence used.
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Each fillet weld shrinks both transverse to its length and along its length. Welding the two top fillets first put a large longitudinal shrinkage force near the top of the section. The top fibres became shorter relative to the bottom, so the beam curved out of straight by 22 mm. The same top welds also pulled the two side plates toward each other at the top by transverse shrinkage. Because the bottom welds were made later and could not fully reverse that movement, the 200 mm width closed by 8 mm and the cross-section became diamond-shaped.
Continuous welding from one end to the other allows heat and shrinkage to accumulate in one direction, which increases bowing. The 22 mm bow exceeds the 10 mm straightness tolerance, so the beam is non-conforming.
2. Propose a revised fit-up and welding sequence that would control both bow and diamonding on the next beam, with reasons.
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Balance the welding about the neutral axis. Use two welders on opposite corners, welding the same length at the same time, with matched current, travel speed and electrode size; or alternate top and bottom passes. Divide the 6 m into 1–1.5 m blocks and use back-step or skip welding, starting at the centre and working outward in a symmetric pattern. Clamp or tack the box to a strongback or fixture, and set a small opposite camber to compensate for expected shrinkage.
Check fit-up before welding: uniform 6 mm leg, no excessive gap, and a square cross-section. Do not over-weld; the specified 6 mm leg is enough. For diamonding, fit temporary diagonal braces or cross-bars and weld opposite sides in a balanced order. Record the sequence on the shop drawing so it is repeated.
3. Evaluate the engineer's stiffener proposal and describe a safer correction method for the finished beam within the stated constraints.
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The engineer's proposal is poor. Adding two stiffeners and welding them continuously adds more weld metal and more shrinkage, usually in an uncontrolled direction, and it does not remove the existing bow. It also adds weight and may create new distortion.
For the finished beam, first check whether 22 mm can be accepted by the customer. If not, use controlled flame straightening: heat a wedge or spot pattern on the convex, long side with oxy-acetylene, then cool in a controlled way, measuring with a string line. For the diamonding, use jacks or strongbacks with local heating. A 100 kN press may help, but point loads can dent 8 mm plate; use pads. Flame straightening changes the steel condition and must be approved by the customer and covered by a procedure. Do not hammer or arc-gouge, as the customer forbids it. After correction, re-inspect straightness and the welds.