Why Has a Single Heavy Beam Been Specified Instead of Two Lighter Ones?

On long-span or heavily loaded openings, you might receive a specification for a single large UC (universal column section) and wonder why PorthouseDean hasn’t specified two smaller, lighter beams that would be easier to handle on site.

The Engineering Reason

The governing issue on longer spans is not just the vertical bending strength of the beam — it is lateral torsional buckling (LTB).

When a deep, narrow beam carries a large load over a long span, the top flange is in compression. That compressed flange tends to buckle sideways, which causes the whole beam to twist and collapse laterally. To resist this, you need:

– Greater width (a UC section is wider than a UB of the same weight)
– Or more restraint (provided by the floor or roof above, if properly connected)

On spans where LTB governs, a single UC is typically the most efficient solution. Two lighter UBs side by side may seem appealing, but when you add both sections together, their combined weight is often equal to or heavier than the single UC — and they are still narrower and less resistant to LTB individually.

Why Two Lighter Beams Can Be Worse

Two paired UBs each have to carry half the load in bending — but each one is still vulnerable to LTB on its own. The pairing only helps if the two beams are properly bolted through their webs so they act together as a single composite section.

Even when bolted together, the combined section is typically wider and heavier than a single UC, using more material for the same structural outcome.

Can You Request Paired Beams?

Yes, if you have a site-access or handling reason to prefer two lighter beams, raise a Change Request. PorthouseDean can investigate whether a paired beam arrangement is structurally viable for your span and loading. Where paired beams are specified, they are connected through the webs with M12 bolts at 500 mm centres with spacers to ensure composite action.

Note that the cost of the additional bolts, spacers, and fabrication time often offsets any saving on the individual beam weights.

Contact us to discuss the options for your specific project.