When the required section is deeper, heavier or differently proportioned than any rolled section available, which is common on long spans and under heavy concentrated loads. Also when the design benefits from varying the section along the length, which a rolled beam cannot do. Where a rolled section will do the job, it is almost always cheaper, because it involves no fabrication.

That flange thickness, web depth or both can change to follow the bending moment. A simply supported girder needs most capacity at midspan and least at the supports, so thicker flange plate can be used in the middle and thinner at the ends, joined by a butt weld. Depth can also be tapered or haunched. The result uses less steel than a constant section sized for the worst point.

Because an efficient girder has a deep, relatively thin web, and a thin web is vulnerable to buckling under shear long before it would fail in direct stress. Transverse stiffeners divide the web into panels and raise the shear buckling resistance, and longitudinal stiffeners are added on very deep webs. They also carry concentrated loads and reactions into the web at bearing points.

It depends on the loading. On statically loaded members, fillet welds between flange and web are usually sufficient. On fatigue-loaded members such as crane runway girders and bridges, the weld detail determines the fatigue category of the whole member, so full-penetration welds, controlled profiles and specified non-destructive testing are normal. Weld detailing is a structural decision on these members rather than a workshop matter.

Considerably more expensive per tonne, because the cost is in cutting, fitting, welding, straightening and inspecting rather than in the steel. That is why plate girders are used where a rolled section cannot do the job rather than as a general alternative. The material saving from optimising the section rarely covers the fabrication cost unless the member is large or the quantity is significant.

It is a real issue and is managed rather than avoided. Welding a long flange to a web puts heat into one side and the girder tends to bow and twist. Fabricators control it with welding sequence, balanced welding on both sides, restraint during welding and heat straightening afterwards. Camber is often built in deliberately at the same time so the girder is straight under its permanent load.

Bridge main girders, crane runway beams in heavy industrial buildings, transfer structures carrying columns or walls across an open space below, long-span roof girders, and heavy plant support structures. The pattern is a long span or a heavy concentrated load, usually combined with a requirement that no rolled section can satisfy directly.