A beam does not have one weight rating independent of the building. Span, load position, restraints, steel grade and support details change the answer. Earlier unsourced capacity tables on this page have been removed.
Total mass is not a load pattern
As an arithmetic illustration, a total force of 20 kN spread uniformly along a simply supported 4 m beam gives a maximum moment of 10 kNm. The same 20 kN applied at the centre gives 20 kNm. Both cases have 10 kN reactions at each end, so identical support reactions do not imply identical bending effects.
For the uniform case, M = wL²/8 with w = 5 kN/m. For a centre point force, M = PL/4. These elementary relationships illustrate load placement; neither value is a resistance check. Our online tool models only the uniform case.
What a load assessment needs
Record which floors, roofs and walls bear onto the beam. Tributary width converts a uniform area load to a line load, but it does not represent a chimney, column or other local force. Do not add tonnes, kN and kN/m together: mass and force have different units.
The engineer also needs lateral restraint, connections, masonry condition and the actual support-centre span. Capacity may be limited by stability or the supports before the steel reaches its material strength. A larger beam alone does not remedy an inadequate support.
Read a supplier table before using it
Ask for its design basis, steel grade, assumed restraint, load combination, deflection criterion and treatment of self-weight. An old RSJ identified only by approximate depth is not automatically a modern UB.
British Steel section data provide geometry and mass. SCI P183 documents beam-analysis relationships. Neither approves the illustrative loads above.
Structural work requires a chartered structural engineer and the applicable Building Control approval. This educational article has not received an independent engineering review.