Space Frame vs Portal Frame: Where the Time and Money Actually Go

Every production hall goes through the same meeting. Somebody stands at the drawing, points at the row of columns down the middle of the floor, and asks whether they really have to be there.

The honest answer depends almost entirely on one number, and it is not the budget. It is the span.

Below about 25 metres a portal frame is very hard to beat, and if that is what your building needs we will tell you so and lose the enquiry. Above 30 metres the arithmetic turns, and it keeps turning in the same direction for every metre you add. This is a piece about where that line sits, what moves when you cross it, and how much.

The span where the answer changes

A portal frame carries load in two dimensions. Each frame is an independent structure spanning from one side of the building to the other, and the frames beside it help only through the purlins and the bracing. Make the span longer and every frame has to grow — deeper rafters, heavier haunches, bigger baseplates — and it has to grow on its own.

A space frame carries load in three. Force entering at any node has many routes to a support instead of one, so the roof behaves as a single shell rather than as a row of parallel frames sharing very little. That is the whole difference, and everything below follows from it.

There is nothing clever about the left-hand side of that chart. At 15 metres a portal frame is a simple, well-understood building that any competent fabricator can produce, and a space frame at that span is an expensive way to solve a problem you do not have. We quote those enquiries honestly and we usually do not win them.

The building this article is about

To compare anything you have to fix the building, so: a production hall of 60 metres clear span by 90 metres long, 12 metres to the underside of the roof, 5,400 m² of floor with nothing standing in it. A moderate seismic zone. Sandwich panel roof in both cases.

Built traditionally that is a trussed portal frame — at 60 metres a plain hot-rolled portal is no longer sensible — with site-welded splices and a lattice rafter. Built as a space frame it is a double-layer grid on a 3 metre module, sitting on perimeter columns, delivered as tubes and machined nodes and bolted together on the ground.

Where the weight goes

At the spans where a space frame is the right answer, it comes in 25 to 40 per cent lighter than the equivalent portal-frame hall. For this building, working from the middle of that range:

A hundred and thirteen tonnes is a real number in a market where steel is priced by the kilo. But the tonnage is not the most useful part of it. What matters more is what those tonnes were doing to the rest of the building.

The foundations were paying for it twice

A portal frame at 60 metres does not just push down. It pushes out. The frame wants to spread under load, and that horizontal thrust has to be resolved somewhere — either in moment-resisting bases, which means large pads and a lot of reinforcement, or in tie rods running across the floor, which means a floor with tie rods running across it.

A double-layer grid on perimeter columns delivers a load path that is close to vertical. Smaller pads, less reinforcement, less excavation, and no ties in the slab. The saving in the ground is not a rounding error against the saving in the steel; it is roughly a third of it again.

Where the weeks go

Steel is a purchase. Programme is a cost you keep paying, because the building is not earning while it is being built, and on an industrial project the rent, the finance and the delayed production line usually add up to more than the frame did.

Erection of a bolted space frame typically runs 25 to 40 per cent faster than site-welded portal frames, with a smaller crew. The reason is unglamorous: almost nothing is decided on site. Members and nodes are CNC-fabricated to millimetre accuracy, they arrive finished and galvanised, and the crew bolts them together.

Seven weeks. And they are seven weeks bought at the least forgiving point in the job, when the building is open, the crane is on hire and everything downstream is waiting on the roof going on.

Weather stops being a variable

Site welding at 12 metres is weather work. Wind stops it, rain stops it, cold stops it, and every splice has to be inspected before the next thing can happen. A bolted connection is torqued and checked, and a rainy morning costs you the morning rather than the joint. On a large space frame the grid is often assembled at ground level in bays and lifted into place complete, which takes most of the work off the wind entirely.

Four things neither chart shows

• Redundancy. Lose a member in a triangulated network and the load redistributes through alternative paths. Lose a rafter in a portal frame and you have lost the frame. This is a safety property, not a saving, but it is the reason space frames are specified for hangars and stadium roofs.
• Seismic behaviour. Inertial force scales with mass. A roof that is a third lighter attracts roughly a third less horizontal load, which propagates all the way down through the columns to the footings. In a seismic zone this is not a marginal benefit.
• Extension. The geometry is modular. Adding three more bays in five years is a continuation of a pattern rather than a structural intervention, and industrial clients extend far more often than they expect to at design stage.
• Quality control. Shop-fabricated components are measured against the model on a coordinate measuring machine before they leave the workshop. Site-welded splices are inspected after the fact, at height, by someone standing on a platform.

When we would tell you to build a portal frame

There is a version of this article that ends here with a straight recommendation. This is not it, because the recommendation is conditional and pretending otherwise would waste your time and ours.

• Spans under about 25 metres. Build the portal frame. The chart at the top is not decoration.
• A building that genuinely tolerates internal columns. If the process does not care where the columns are, you are paying for clear span you will never use.
• Very heavy point loads hung from the roof — large travelling cranes in particular. These are solvable in a space frame and we do solve them, but a portal frame with a dedicated crane gantry is often the more direct answer, and the honest one.
• A programme where the steel is already ordered. Nothing in the comparison above beats a decision that has already been made and paid for.

The case for a space frame is strong in the conditions where it is strong. It does not need to be oversold in the ones where it is not.

What this looked like on a real floor

The Arian Motor Factory in Arak is the version of this we built. A clear-span space frame over the production hall, erected from prefabricated members and nodes, with the floor below it free of columns because a production line is a poor thing to design around a structural grid.

CORTEX designed, engineered, fabricated and installed it. That matters more than it sounds: the analysis, the shop drawings, the node production and the crew on site were all the same company, so nothing was lost in a handover and nobody spent a week arguing about whose drawing was wrong.

Where to go from here

If your building is over 30 metres clear and the columns are in the way, the comparison above is worth running properly against your own brief. We will do it before anyone commits to a system: send the geometry, the loading and the site, and we will come back with how it gets built, what it weighs and what it costs.