Can I Use an I-Beam / RSJ Instead of an RHS Box Section?

This is one of the most common substitution requests we receive. UB sections (also called I-beams or RSJs) are more widely available and generally cheaper than RHS (rectangular hollow section) box beams. PorthouseDean specifies RHS sections only when a UB will not work — so if your drawings show an RHS, there is a reason for it.

When RHS Is Required

The critical scenario is when the beam supports a cavity wall where the outer leaf is not centred over the beam — that is, the outer brick skin bears onto a shelf plate welded to the bottom flange of the beam, offset to one side.

In this configuration, the weight of the outer leaf is applied eccentrically to the beam bottom flange. This creates torsion — a twisting force acting along the beam’s length.

An open I-beam has very poor torsional resistance. Under the twisting force from the offset outer leaf, a UB can rotate unacceptably, potentially causing the shelf plate to lift and the outer leaf masonry to become unsupported. The torsional stiffness of an RHS section is many times greater than an equivalent UB, making it the correct choice for this situation.

When a UB Can Be Used

Where the beam is centred under the full width of the wall above it — with the outer leaf sitting directly over the beam centre rather than on a cantilevered shelf plate — torsion is not an issue and a UB section is adequate. PorthouseDean uses UBs in the majority of domestic beam designs.

If you believe the beam on your drawings showing an RHS might actually be in a configuration where a UB would work, raise this as a Change Request. The engineer can review the specific wall arrangement and confirm whether substitution is possible for your project.

The Cost Trade-Off

RHS sections can cost more and may have longer lead times than equivalent UBs. However, using a UB in a situation that demands an RHS is not a cost saving — it is a structural deficiency that will eventually result in visible cracking of the outer leaf and potentially require costly remedial work.

Contact PorthouseDean if you want to discuss the beam sections specified on your project.