Steel Portal Frames For Warehouses

A steel portal frame is the cheapest way to enclose a large clear floor area, which is why it sits behind almost every warehouse, factory and industrial unit in Sydney's west and south.

It looks simple. A row of identical frames, a roof of purlins and sheeting, walls of girts and cladding. The simplicity hides the fact that a portal frame is a light structure covering a very large area, and light structures over large areas are governed by wind far more than by gravity.

How a portal frame works

Each frame is two columns and two rafters, joined rigidly at the knees and the apex so the whole frame resists load by bending.

The rafters are usually hot-rolled universal beams, deepened locally at the knee and apex with a haunch, because that is where the bending moment peaks. Columns are the same family of sections. The frames sit at regular spacing along the building, and cold-formed purlins and girts span between them to carry the roof and wall sheeting, designed to AS/NZS 4600.

Across the building, the frames resist load by bending. Along the building they cannot, so longitudinal stability comes from bracing: rod or strap cross bracing in the roof plane and in selected wall bays, carrying wind and any crane surge back to the footings.

Wind decides the design

Gravity on a portal frame is modest. Steel sheeting, purlins and a bit of services weigh very little.

Wind is a different story. Under AS/NZS 1170.2, a large building with big roller doors has to be checked for a dominant opening, the case where a door is open or blown in during a storm and the inside of the building pressurises. That internal pressure pushes up on the roof at the same time as the external suction lifts it. On a large shed, the net uplift can be several times the weight of the roof.

So the critical load case is usually the roof trying to lift off, not sag. That changes everything that follows. The general principles of wind on buildings are covered in [wind loading on buildings explained](/blog/wind-loading-on-buildings-explained).

What uplift does to the details

Rafter restraint flips. Under gravity, the top flange of the rafter is in compression and the purlins restrain it. Under uplift, the bottom flange goes into compression, and nothing restrains it unless you add fly braces from the purlins down to the inside flange. A frame without enough fly bracing can buckle sideways under a wind load it was never short of strength for.

Purlin connections carry tension. The cleats and bolts that connect purlins to rafters are pulled apart, not pressed together.

Holding-down bolts carry tension. The column base is trying to lift, and the bolts and footing have to hold it down. A footing sized only for bearing pressure can be far too light for uplift. The detail is covered in [steel baseplates and holding-down bolts](/blog/steel-baseplates-and-holding-down-bolts).

Pinned or fixed bases

A pinned base transfers vertical and horizontal load but no moment. It needs a smaller footing and a simpler baseplate, and it makes the frame more flexible.

A fixed base takes moment into the footing. It stiffens the frame and reduces rafter and column sizes, at the cost of a much larger footing and a stiffer baseplate.

On reactive clay or poor ground, pinned bases are often the economical answer. On good ground with tight deflection limits, fixed bases can pay for themselves. The choice belongs to the structural and geotechnical engineers together, not to a standard detail.

Cranes, mezzanines and future use

An overhead crane changes the design completely. Crane runway beams carry repeated moving loads, so fatigue and tight deflection limits govern, and the columns pick up eccentric loads and horizontal surge from the crane braking. A frame designed without a crane does not get one added later without real strengthening.

The same is true of a mezzanine, which is one of the most common changes to an existing warehouse. Whether it ties into the portal frame or stands on its own, the frame and the slab both have to be checked, as set out in [mezzanine floors in existing warehouses](/blog/mezzanine-floors-in-existing-warehouses).

Rooftop solar is the other modern addition. Panels, rails and ballast add dead load, and the arrays change the wind pressures on the roof.

Durability

A portal frame is exposed steel. Inside a dry warehouse it lasts indefinitely with a basic paint system. Near the coast, in a wash bay, or in a building that stores fertiliser or chemicals, it needs a coating system chosen for that environment, which is the subject of [corrosion protection for structural steel](/blog/corrosion-protection-for-structural-steel).

How we approach it

We design portal frames to AS 4100 with the wind case worked first, because that is the case that sizes the restraints, the connections and the footings. The frame is then checked for whatever the owner wants the building to do in ten years, as well as what it does on handover day.

A shed is easy to draw. A shed that is still standing after a southerly with every roller door open is engineering.

ACSES provides structural engineering for commercial and industrial buildings across Sydney. Talk to us about a project.

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