In product catalogues, beams like UB 152x89x16 have dimensions written in mm to the nearest one decimal place. Are these beams cut and measured using an SI ruler or an imperial ruler. I am asking this question because I have trouble matching some of the section properties given in the table for some of the beams. So I presume that the dimensions given are rounded off dimensions and the actual dimension could be given in exact fractions of an inch.

Am I correct?


2 Answers 2


There is a misnomer in the question in that the beams are not cut to section at all, and not measured with any sort of ruler during manufacture. The sections are rolled on a hot rolling mill where billets of hot (hot enough to have softened) steel is driven between a series of rollers that force it to the required shape. See for example https://www.youtube.com/watch?v=6xnKmt_gsLs (particularly around 2:09)

(Other sections may be formed by cold rolling, but those to which the question relates - BS UB and UC sections - are hot rolled. Cold rolling is used for relatively thin sections, such as purlins or cladding rails supporting the cladding common on industrial sheds.)

The figures which designate a section are the 'serial size' (which is nominally in millimetres) and weight of the section. The dimensions of the sections are now explicitly defined by reference to SI dimensions. The current relevant standard is BS 4-1:2005 'Structural steel sections. Specification for hot-rolled sections'. http://shop.bsigroup.com/ProductDetail/?pid=000000000030128029

Section 3 of that standard states:

3 Designation

A section shall be designated by the serial size in millimetres (mm) and the mass per unit length in kilograms per metre (kg/m).

This form of designation shall be used in any enquiry and order.


A beam of 920.5 mm × 420.5 mm at 388 kg/m is designated a 914 × 419 × 388 beam.

However, the values of the units are not round numbers partly because they do originate as imperial values. BS 4 is (not surprisingly, given its very low number) one of the earliest British standards. The first version of BS 4 was BS 4:1903 http://shop.bsigroup.com/ProductDetail?pid=000000000030304821 In 1903, Britain was imperial (digression - BSI originates when Sir John Wolfe-Barry instigated the Council of the Institution of Civil Engineers to form a committee to consider standardising iron and steel sections on 22 January 1901, the day Queen Victoria died - about when Britain's empire was near its peak).

Sections very similar to BS 4:1903 sections are still in current use. Some of the nominal or serial size sections match exactly. For example, the BSB20 from BS 4:1903 was a 12" x 5" x 32 lbs/ft beam which had 0.55" thick flanges and 0.35" thick webs. In precise conversion, 12"=304.8mm, 5"=127.0mm, 32lbs/ft=47.631...kg/m, 0.55"=13.97mm, 0.35"=8.89mm. Now, 305 x 127 x 48 is a current section designation in BS 4-1:2005 (within table 2 in the standard) and has 14.0mm thick flanges and 9.0mm thick web.

(Although even where the nominal figures match exactly, the sections are not a precise match - BSB beams had tapered flanges, early UB sections had somewhat less tapered flanges, and modern UB beams have parallel flanges.)

However, if you actually put a ruler on this beam, it won't read as a round number in either imperial or SI units, because it isn't actually that size at all. A 305 x 127 x 48 UB is actually 310.4mm deep and 125.2mm wide. The designation is just a label, which reflects a nominal size termed the 'serial size'. The various beams designated as 305 deep vary between 303.8mm and 312.7mm deep.

This arises because you can roll all the beams from the same serial size from one set of rollers. For example, 305 x 127 x 48 UB, 305 x 127 x 42 UB, and 305 x 127 x 37 UB can all be rolled from the same rollers, you 'simply' set the rollers slightly further apart. You can see that from the section tables - those three sections are different depth and width, but the internal distance between flanges is the same (264.6mm) and the root radii are the same (8.9mm). The rollers can physically be the same set for all those sizes, the rollers that form the outer faces are set further away from the inner ones and the inner ones are themselves spaced further apart, making the flanges, web and also the overall dimensions thicker and greater.

That is, in summary:

The 'actual' dimension is not either of the designation dimension in millimetres or the conversion of that dimension into inches.

The actual dimensions are, however, defined in millimetres.

Some of those metric dimensions are the same dimension as a round number in inches, rounded to the nearest 0.1mm (but not all of them).

If you want to explore historical steel sizes, in addition to the BSs I've linked, there's a useful British Constructional Steelwork Association (BCSA) book - the Historical Structural Steelwork Handbook - http://www.steelconstruction.info/index.php?title=Special:ImagePage&t=Historical+Steelwork+Handbook.pdf

  • $\begingroup$ Is the 305x127x48 really the same as BSB20? I don't have copy of BS4:1903 to check, but I understood that old RSJs had tapered flanges, whereas universal beams have parallel flanges. $\endgroup$
    – AndyT
    Dec 7, 2016 at 17:20
  • 1
    $\begingroup$ OK, that's true. The section is very similar, but not precisely the same, so I'll update the answer when I have time to do it properly. In summary - BSB flanges have 98 degree internal angle between web and flanges. The first UB beams (in the 1959 version of the standard) mostly had a 92 degree 52 minute angle, UBs now have a 90 degree angle. UCs have been parallel flange from the beginning. The Historical Structural Steelwork Handbook has details of each section. $\endgroup$
    – achrn
    Dec 8, 2016 at 11:11

Universal Beams (UBs) are defined in millimetres, and are not based on the imperial rolled steel joists (RSJs). So no, the dimensions are not rounded off exact fractions of inches.

However, the designation of a section does not match its dimensions exactly. The online blue book (navigate to "Advance UKB" for UBs) shows that for UB 914x419x388, the depth is actually 921mm, whereas for UB 914x419x343 the depth is 911.8mm - neither is actually 914mm deep!

Additionally, small errors in matching section properties may be caused by failing to take account of radii.

  • $\begingroup$ If the original dimensions are measured based on SI, why aren't the beams defined and cut to nearest 10mm or even 1mm? 914x419mm corresponds to 36x16.5inch approximately and 152x89mm corresponds to 6x3.5inch. I would tend to think that these dimensions originate from imperial measurements. $\endgroup$
    – Jamme
    Nov 11, 2016 at 2:14
  • 1
    $\begingroup$ @Jamme It doesn't matter what units the standard sizes were originally derived from. Since 2000, it has been illegal to use "Imperial units" for any form of trading in the UK, with a very few exceptions like measuring beer in pints and precious metals in troy ounces. $\endgroup$
    – alephzero
    Nov 12, 2016 at 2:37

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