Truss or rolled beam: which one pays off, and from what span

October 26, 2026  ·  Indumor

1 Rolled beam · 2 Top chord · 3 Bottom chord · 4 Diagonal · 5 Vertical. L span, h₁ beam depth, h₂ truss depth.

Buying a rolled beam is a lot like buying a shirt off the rack. It comes off the hanger and you wear it. A truss is the tailored shirt, which uses less cloth and fits exactly, but someone has to take measurements and sew, and that gets paid for too.

To compare with numbers we set up a worked case. A 20 m roof beam, simply supported, with a 4 kN/m service load and deflection limited to L/200, which is what Argentina's CIRSOC 301 code asks for rigid roofing in industrial buildings. F-24 steel (similar to A36) in both options.

The rolled beam

For strength an IPE 450 would do, but it deflects 123 mm against a 100 mm limit. Deflection governs, as it tends to on long spans, so the beam goes up to an IPE 500 weighing 1,814 kg, in a single piece half a meter deep. It is cut to length, the end plates are welded on and it is ready for paint.

If it is going to be a rolled section, IPE beats IPN. An IPE 300 weighs 22% less than an IPN 300 and gives more section modulus per kilogram, and its parallel flanges bolt up without the tapered washers that the sloped IPN flanges require.

The truss

The same beam as a 1.6 m deep Pratt truss in angles and gusset plates weighs about 810 kg and deflects about 24 mm, against 86 for the IPE. The UK Steel Construction Institute's design guidance puts it this way: for the same weight a truss is stronger and stiffer, and the advantage grows with span and load. Ducts and pipes can also run through it.

What the weight hides shows up in the shop. It is about fifty pieces between chords, diagonals, verticals and gussets, against a single one. And the paint area is almost the same, 33.6 m² against 34.9 for the IPE, because a small angle has nearly twice the surface per tonne of a large section. The truss also needs 1.6 m of depth where the beam took half a meter. If it is built in bolted segments, ordinary bolts in oversized holes slip and add deflection, so they have to be preloaded or the splice welded.

So does the lighter one win? It depends on what a kilogram of steel costs against an hour of shop time. By the same institute's figures, material takes 30 to 40% of the cost of a steel frame, fabrication about the same and erection 10 to 15%. Saving a tonne of steel while adding fifty pieces can come out even, ahead or behind, depending on the shop and on that month's steel price.

Verdict

For a one-off beam, with a short span or little depth to spare, the rolled section is hard to beat, because it arrives cut to length and goes straight up. The truss starts to win at around 20 m of span, with heavy loads or when there are many identical beams, since the assembly table and the cutting list pay off in series. In between, each job's numbers decide, at that moment's prices.

Further reading

• The Suite's truss beam and column calculator (in Spanish) gives the force in every bar, the lightest sections that pass and the cutting list.
• When no rolled section is big enough, the next step is a welded plate girder.
• For the whole building, welded or bolted steel buildings.
• To keep truss bars from ending up as offcuts, how to get the most out of each bar.
• Where the I-section came from, in the history of the I-beam.
• Industrial steel structures and steel buildings and warehouses at Indumor.

In this guide's case, the truss saved a tonne of steel on a single beam. With your span and loads we can price both options, so you choose with numbers.

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