On plans, in site meetings, and during refurbishments, two terms appear again and again: RSJ and Universal Steel Beam. People often treat them as the same thing – but they’re not. And when you’re dealing with load-bearing walls or structural changes, the differences really matter.

Whether you’re a site manager, facility lead or commercial property owner, knowing the difference between RSJ and universal steel beams can prevent you ordering the wrong item, help keep projects on track, and supports the safety systems that sit around the main structure.

This blog explains what each beam does and how to choose the most suitable option for your next project.

What is an RSJ?

RSJ stands for Rolled Steel Joist. It’s a traditional steel beam that’s been used for many decades in the UK. The shape is similar to a capital ‘I’. Many older buildings, especially those built before metric standards became the norm, use RSJs as their main beams.

True RSJs were produced to older British Standards using imperial dimensions. Although people might still use the word ‘RSJ’ on site, most new steel ordered today is supplied in metric profiles.

Key characteristics of an RSJ

Traditional ‘I’-shaped posts
Flanges are often tapered rather than parallel
Historic sizes, usually linked to older standards
Good vertical load capacity but less resistance to twisting
Commonly encountered in refurbishments and extensions

Where RSJs make sense

You are most likely to come across genuine RSJs when you are working on:

Older buildings that already rely on RSJs
Small-span openings and lintels
Internal alterations where existing steel needs to be supplemented
Projects where the original drawings clearly specify RSJ sections

In many modern plans, the word ‘RSJ’ on a drawing might just be shorthand for ‘steel beam’. This habit is part of the confusion and one of the reasons it pays to double-check what the designer actually has in mind.

What Is a Universal Steel Beam?

A Universal Beam, often abbreviated to UB, is the current standard beam profile used across UK construction. The cross-section looks closer to an ‘H’ than an ‘I’, with a deeper central web and wider, parallel flanges.

Universal Beams are manufactured to modern BS EN standards. This means the dimensions, weight, and performance are highly consistent, no matter who supplies them. Consistency is one of the main reasons engineers prefer them for new projects.

Key characteristics of a Universal Beam

Parallel flanges that simplify connections and fixings
Efficient strength to weight performance
High bending resistance, especially over longer spans
Produced to current BS EN specifications
Well suited to commercial and industrial structures

Typical uses for Universal Beams

You will find Universal Beams specified in most:

New build commercial or industrial units
Mezzanine floors and plant decks
Warehouse extensions and large span openings
Modern refurbishments that have updated structural calculations

In short, a Universal Beam is usually the default choice whenever a building is being designed or upgraded under current standards.

Key Technical Differences

Understanding the key technical differences is what matters for your site.

Flange geometry

RSJ flanges are often tapered. This taper can make bolted connections or welded brackets slightly more awkward.
Universal Beam flanges are parallel. This makes it much easier to attach cleats, plates, and other components with reliable contact and bolt seating.

Strength-to-weight efficiency

Universal Beams are usually more efficient. For a given load requirement, a UB can often achieve the necessary performance with less steel. That can reduce lifting requirements and sometimes material costs, especially across multiple spans.

Bending and torsional behaviour

RSJs cope well with vertical loads but offer less resistance to lateral movement and twisting. Universal Beams, thanks to their wider flanges and modern design, handle bending and torsional effects more effectively. This difference becomes more noticeable with longer spans, mezzanines, and areas where loads are not perfectly centred.

Standards and consistency

RSJs relate back to older standards and may not match neatly across different suppliers.
Universal Beams are supplied under modern BS EN frameworks, which give standardised sizing and performance. That makes life easier for engineers, fabricators, and installers.

Availability

Genuinely new RSJs are less common on the market than they once were. Universal Beams, on the other hand, are widely stocked across the UK in many sizes and weights, which usually means smoother ordering and shorter lead times.

Choosing the Right Beam for Your Project

When the time comes to approve steelwork for a project, a few practical steps make a big difference.

Check the engineer’s specification

Look for the exact section type and size. You should see UB, UC, or another defined profile, along with dimensions and weight. If ‘RSJ’ appears on its own, treat that as a prompt to ask for more precise information.

Consider the type of project

Commercial refurbishments: Older drawings may mention RSJs. Updated calculations might still lead to a UB being selected, even if people on site still use the old term.
New industrial units or warehouses: Universal Beams are usually the starting point for main beams and secondary steel.
Mezzanines and raised platforms: UBs handle bending and deflection well and are easier to integrate with support posts and edge protection.
Large openings in busy environments: Universal Beams often achieve the required spans with a more efficient section.

Think about the environment around the beam

Structural steel rarely sits in isolation. In many facilities it is close to vehicle routes, racking, or machinery. That means the beam may need protection as well as strength.

Beam choice can influence, or be influenced by, safety equipment such as:

Armco crash barriers in loading bays
Warehouse protection barriers near supports
Bollards around exposed columns
Low level guards at vehicle pinch points

Selecting suitable structural steel gives a stronger base for these safety systems to fix to and protect.

Factor in protective finishes

The environment will dictate the level of corrosion protection. External or damp conditions often call for galvanised steel, whereas indoor environments can benefit from powder-coated finishes for both appearance and durability.

The same thinking applies to safety barriers, bollards, and column guards. Matching coatings can improve long-term performance and keep maintenance straightforward.

Learn more about Galvanised vs Powder-Coated Steel Barriers.

Summary: RSJs vs Universal Steel Beams

From warehouse operators to commercial landlords, getting the beam type right goes hand in hand with selecting the right safety barriers, bollards, and protective systems around it.

For most modern commercial and industrial work, Universal Beams are usually the more appropriate choice. RSJs still have a place, mainly where you are tying into existing RSJs, or where older drawings need to be followed closely.

To keep it simple:

Use a Universal Beam for new structures, warehouse projects, and long-span work
Retain or match RSJs where you are modifying an older building that already relies on them

The decision should always come from a qualified structural engineer. The terminology might feel casual on site, but the calculations behind the scenes are not.

Contact Us

CT Safety Barriers manufactures UK-made steel safety products from our facility in Yorkshire, supplying projects across the country. We work with real sites every day so we understand how structural steel and protective systems fit together in practice.

With CT Safety Barriers, you’re in safe, local hands.

Call us on 01977 801279 or send an email to sales@ctsafetybarriers.co.uk.