Direct Answer
A built-up section is a structural member fabricated by welding steel plates together — typically two flanges and a web — to create a section with dimensions or properties not available in rolled form. Built-up sections are used for transfer beams, long-span girders, crane runway girders, and sections where specific depth-to-flange proportions are required.
Full Explanation
Built-up section types: (1) Plate girder — two flange plates welded to a web plate; used for large spans or heavy loads where W-shapes are unavailable. (2) Box girder — four plates forming a hollow rectangle; high torsional stiffness, used for crane runway beams and curved members. (3) Hybrid sections — high-strength flanges with standard-strength web to optimize material use. Fabrication: flange plates are cut and straightened, the web-to-flange fillet welds run the full length (typically submerged arc welded for efficiency on long members), and stiffeners are added at load points and web panel intervals. Web-to-flange weld size is designed by the engineer based on vertical shear flow.
What This Means for Your Shop
Built-up section fabrication is labor-intensive — planning the weld sequence to control distortion is critical. Long web-to-flange welds on one side of the web cause the plate to bow toward the weld; balanced welding sequences prevent this.
Common Mistakes
Welding all web-to-flange joints on one side before welding the other side. The single-sided weld shrinkage will bow the web plate before the second-side weld is applied — and the second-side weld won't fully correct the distortion.
Recommended Process
For built-up sections, develop a weld sequence plan before starting: alternate between sides, use balanced heat input per pass, and measure distortion at defined intervals. Have heat straightening capability available if distortion exceeds tolerance.