Efficiency in one plane. Because the section is deeper in one direction, its capacity and stiffness about that axis are considerably greater than a square section of the same weight. For members that bend predominantly in one plane, such as beams, rails and mullions, that means less steel for the same performance, or greater span for the same section weight.

Critically. The greater depth must lie in the plane of bending, so a beam is installed with its long dimension vertical. Fitted the other way up, the same section delivers only a fraction of its intended capacity, and because both orientations look purposeful the error is easy to make and easy to miss on site. It should be shown explicitly on drawings rather than left to assumption.

For columns and for any member loaded significantly in both directions, where the weaker axis governs the design and the additional depth in the strong axis contributes nothing. In those cases a square or circular section uses the material better. Rectangular sections earn their place specifically where the loading is directional and predictable.

The flat faces give the same easy connections and fixings as square sections, and the deeper face is often useful in its own right: it provides room to mount equipment or fixings, a larger surface to attach panels or glass to, and in some designs enough internal depth to conceal cables or small services within the section rather than routing them alongside.

Balustrade top rails and mullions, glazing and curtain wall framing, canopy and walkway beams, spanning members in equipment frames, and supports in process, food and marine environments where the corrosion resistance is needed. The pattern is a member that spans and bends in a known direction while also needing the corrosion or appearance properties of stainless.

Use section properties about the relevant axis rather than overall size or weight, since two sections of similar mass can differ greatly in capacity depending on how the material is distributed. Manufacturers publish section tables giving properties about both axes. For stainless specifically, check the design standard being used, as design rules for stainless differ from those for carbon steel.

Broadly, though the range of rectangular sizes is narrower than for square, and combinations of large size, heavy wall, 316 or duplex grade and a polished finish can carry long lead times. Where an installation mixes rectangular and square sections that will be seen together, order them together so that finish and corner radius match across the whole assembly.