Portal frames and pre-engineered buildings

A distribution warehouse, a factory, an assembly shop and an aircraft hangar are all the same structure: a row of portal frames at 6–8 m centres, tied together along the building, wrapped in sheeting, standing on a concrete floor.

It is the most-built structural form in the world, and almost none of it is complicated. What makes it interesting is where the money and the risk actually sit — which is rarely the frame.

The interior of a large new warehouse, a long run of steel portal frames with tapered rafters and deep eaves haunches, purlins and rooflights overhead, cross bracing in the wall of the near bay, and a power-floated concrete slab with sawcut joints running away into the distance. — Four things worth naming in one view: the haunch at the eaves, the purlins restraining the rafter, the cross bracing in the end bay, and the slab — which cost more than the steel above it.

The frame

A pitched portal frame in elevation with its parts labelled: columns, rafters, the haunch at each eaves formed from a cutting of the same rolled section, the apex splice, nominally pinned bases, and purlins on the rafters. Beside it the bending moment diagram for gravity load is drawn, peaking at the eaves and reducing toward the apex, showing the haunch placed exactly where the moment is greatest. Below, a second diagram shows the moment reversed under wind uplift with the bottom flange in compression and fly braces triangulating from a purlin down to that flange. — The haunch is not decoration. It puts depth exactly where the bending moment diagram says depth is needed.

The frame is a rigid jointed structure. Horizontal load applied at one eaves travels round through the rafters to the other column, which is why the frame needs no bracing in its own plane — and why the eaves haunch and the apex splice are the two details that carry the whole design.

Bases are almost always nominally pinned. Fixity would attract moment into the foundation, and a mass of concrete big enough to resist it costs more than making the frame a little heavier. The base plate gets four bolts for erection stability, and the analysis assumes a pin.

Restraint, and why uplift is the awkward case

Under gravity load the top flange of the rafter is in compression, and the purlins bolted to it provide the lateral restraint it needs.

Under wind uplift on a light roof, the moments reverse. The bottom flange goes into compression — and there is nothing attached to it.

PEB against hot-rolled

	Hot-rolled portal	Pre-engineered building
Members	Standard rolled sections	Built-up plate girders, tapered
Section depth	Constant, sized for the worst point	Follows the bending moment diagram
Steel weight	Higher	Typically 15–30% lower
Design	By the project engineer	By the supplier, to their system
Fabrication	Any fabricator	The supplier’s works
Future modification	Any competent engineer	Effectively back to the original supplier
Suits	One-offs, complex geometry, heavy cranes	Repetitive spans, speed, cost

Both are portal frames. The PEB simply removes the constraint of standard section depths by welding its members from plate, and it is that freedom — depth varying continuously along the member — that produces the saving.

Along the building

An isometric view of a single-storey shed showing portal frames repeated along the length, with a horizontal bracing truss in the roof plane at one end bay collecting wind load from the gable, cross bracing in the side walls of the same bay taking that load to the foundations, eaves ties running along the building, and the gable frame with its wind posts. Arrows trace the load path from wind on the gable, through the roof plane bracing, into the wall bracing, and down to the ground. — Across the building the frames do the work. Along the building there is no frame action at all — the bracing is the only load path there is.

The frames handle load across the building. Along it, nothing does, so the shed needs:

Roof plane bracing — a horizontal truss, usually in one or both end bays, collecting wind load from the gable.
Wall bracing — cross bracing in the side walls of the same bays, taking that load to the foundations.
Eaves ties — running the full length, tying the frames together and delivering the restraint.
Wind posts in the gable, spanning vertically to share the gable wind between ground and roof bracing.

Bracing bays have to line up in the roof and the walls, or the load path has a gap in it.

The floor slab

Here is where the cost and the risk actually sit.

The frame is a commodity. The ground-bearing slab is not:

It is loaded by racking leg point loads, not a uniform pressure. A rack upright can put 60–80 kN onto a 100 mm square baseplate.
Narrow-aisle and automated equipment need flatness tolerances measured in a few millimetres over defined lines — free-movement or defined-movement classifications.
It frequently costs more than the steel frame.
And unlike the frame, it cannot be repaired without emptying the building.

Where this connects

The steelwork detailing is steel connections and erection, the member stability is structural stability and buckling, the slab is placing, curing and testing, and if the shed has an overhead travelling crane the runway is crane gantries and runway beams.

What to take away

A portal frame resists lateral load by frame action through rigid joints, so the eaves haunch and apex splice are the design.
The haunch puts depth where the moment peaks; bases are nominally pinned because fixity costs foundation.
Plastic design needs Class 1 sections and restraint at every hinge.
Wind uplift reverses the moment and puts the unrestrained bottom flange in compression — that is what fly braces are for.
PEB tapers built-up members to follow the moment diagram, saving steel and tying you to the supplier.
Along the building, bracing is the only load path, and the bays must line up.
The floor slab is usually the critical and most expensive element, and it cracks from restrained shrinkage rather than load.