Long-span solutions

Why long spans are hard

Because the structural depth and the quantity of material needed grow fast with the span. Doubling the span does not take twice the material but substantially more.

So a long span is always about the choice of structural system, not about sizing members.

The direction of the solution: trussing

The essence of a truss is that it puts the material at the edges of the cross-section: the chords sit far apart and the web members tie them together.

That achieves large bending stiffness from a small quantity of material.

It suits thin-walled steel particularly: the members work in tension and compression, and the machine does the connection preparation. Trusses.

Where the limits are

Manufacture

On the usual manufacturing plant the clear span of a truss goes to a defined size. Beyond that it takes split fabrication, a site splice or another system.

Transport

Transporting long members is subject to size limits. Panel size and transport.

Erection

Lifting large members requires a crane and temporary bracing.

Structural depth

The depth of a truss takes up space — a constraint where the ceiling height is fixed.

When a hot-rolled structure is the answer

Where the span or the load is so large that the truss solution is no longer economic, and where the structural depth is limited and a more compact cross-section is needed. When HRS is needed alongside LGS.

Serviceability as the constraint

At long spans it is often not the capacity but the deflection and vibration that limit. A floor that passes the strength check can be unpleasantly springy. Serviceability criteria.

So at long spans the serviceability requirements have to be settled in advance.

The point

A long span is a question of system. Trussing keeps the thin-walled solution competitive for a long way — but serviceability often limits before capacity does.