UC, UB, PFC, SHS, CHS... Understanding Steel Beam References
Updated: Dec 6, 2022
Having trouble understanding steel beam references on construction drawings? We explain what all those numbers and acronyms mean.
1. 'Open' Steel Sections
Common Open Sections:
(Universal Beam [UB] Universal Column [UC] and Parallel Flanged Channel [PFC])
Nominal Size vs. Actual size of open sections
For each nominal serial size there are a range of sections with different thickness of flanges and web. This allows engineers to specify the most efficient beam possible. These variants of the same serial size are distinguished by their weight per meter. The table below shows 3 variants of 152 x 152 UC and the slight differences between them:
The different weight beams have slightly different overall dimensions. The manufacturing rolling process means the internal distance between top and bottom flange remains constant, so the thicker flanges of the heavier versions result in a slightly taller (and usually wider) section. The web of heavier sections is also thicker.
When a project has sections of the same serial size but different weights these small differences need to be considered to ensure consistent levels and fit are achieved.
PFC's only have 1 weight per serial size, and as such they match their listed dimensions exactly.
2. 'Closed' Hollow Sections
Common Closed Sections:
(Circular Hollow Section [CHS], Square Hollow Section [SHS], Recangular Hollow Section [RHS])
Similar to UB's and UC's, hollow sections are available in a range of sizes for each serial size, however they are differentiated by wall thickness, rather than weight. Regardless of wall thickness, the outside dimensions remain constant and so always match the listed serial size. The corner radii of SHS and RHS sections vary depending on the wall thickness and manufacturing process (hot formed or cold formed). See our post on corner radius dimensions for more information. Typically hot formed sections should be specified for structural applications.
3. Steel Grade and Hot / Cold Forming
What is Steel Grade?
The steel grade relates to the strength of the steel. In UK buildings, the grade of structural steel most often used is S275 and S355. The higher the grade number, the stronger the steel: S355 is stronger than S275, for example. The different grades of structural steel typically have the same stiffness, so using a higher grade doesn't reduce deflections.
Steel Grade Availability
In the UK, until relatively recent (say early 2000's) open sections were most commonly grade S275 and closed (hollow) sections grade S355. Nowadays, open sections are typically manufactured in S355, with S275 availability limited, if at all. S355 is still the standard for closed sections, although S420 is also becoming more readily available for demanding applications.
Steel plate, such as that used for fabricating connections is typically grade S275, but S355 is also readily available.
How do I know which steel grade has been specified?
The steel grade for a project will usually be shown within the beam references themselves (i.e. 203 x 203 x46 UC S355) or in the Notes or Specifications section of the Engineering drawings.
What are Hot and Cold Formed sections?
Closed (hollow) sections are available in two different manufacturing methods, 'hot formed' and 'cold formed'.
Cold formed sections are weaker as the cold bending process of converting a flat plate into a hollow section shape puts permanent stresses into the material. Cold sections also have restrictions associated with welding. The hot forming process relieves the forming stress, leaving a section with more capacity to resist external load.
How do I know if hot or cold formed members have been specified?
Structural drawings should be checked to confirm what type of section the Engineer has specified - often this is in the notes or specifications section.
Using cold formed members in place of hot formed ones will invalidate the design.
Structural open sections are always hot formed.
Substituting steel grade and forming method
Steel members should not be substituted with a lower steel grade or different forming method (hot, cold) without verification from the beam designer. In particular, cold rolled closed sections are often available in grade S235 at a lower cost than S355, however such a substitution will result in a grossly reduced capacity and may invalidate the structural design. Typically S235 is not used for structural applications.
Smartbuild Engineering are Structural Engineers specialising in steel connection design. See more of what we do here.