The development of design rules
for restrained columns
Following on from the previous two articles, David Brown of the SCI looks back at the
development of design rules for restrained columns. Looking at the work in the 1950s and
1970s reveals the background for many of the features found in today’s design standards
The June and July/August articles on restraints around portal frames
encouraged a closer look at the rules defining the resistance of restrained
columns, making the link between the current rules in BS 5950
and BS EN 1993-1-1 and the salient technical papers published in the 1970s.
The work on columns with restraints refers back to earlier rules covering
unrestrained columns and unadopted recommendations to modify BS 449,
the design standard of the time.
The most significant papers covering restrained columns are Design of columns
restrained by side-rails1 and Failure of columns laterally supported on one flange2,
both by Horne and Ajmani, published in 1971 and 1972 respectively, and the
record of the associated discussion published in 19733. The authors and
contributors include a number of very well-known names in the steelwork world.
Professor Horne OBE is a co-author of Plastic design of Low-Rise Frames4 which
used to be the definitive work in the UK on portal frame design and detailing.
A contributor to the discussion was Dr Morris, co-author of the aforementioned
publication, and forever known by the shear stiffener which takes his name.
Other contributors to the discussion include Dr Wood, known for the effective
length curves found in Appendix E of BS 5950, Professor Nethercot, widely
known for most things in steelwork and Mr Needham, who is known for his work
with CONSTRADO, the forerunner to SCI.
Dr Ajmani was the Chief Design Engineer for the Tata Iron and Steel Company
of Jamshedpur, India. Clearly Dr Ajmani would not know that decades later, his
company would buy Corus, previously known as British Steel, Jaguar Land
Rover and Tetley Tea - and be so significant in the UK steel industry.
The 1971 and 1972 papers present the rules for members restrained on one
side only – the tension flange, as typically found with a portal frame column.
The work undertaken by Horne and Ajmani leads directly to the stable length
rules found in Section 5 and Annex G of BS 5950. In turn, this leads directly to
the rules found in BS EN 1993-1-1 section BB.3. Some 50 years later, current
design rules depend on this research from the 1970s.
The discussion of the paper is perhaps most interesting. At the time, the UK
design standard was BS 449. This standard offered guidance on the effective
length of “stanchions” in Appendix D, and proposed that if a stanchion was
restrained by side rails, the effective length factor in the minor axis was 0.75L.
No limitation was placed on the maximum spacing of side rails – the effective
length was always 0.75L. Figure 1 (Figure 15 from BS 449) is interesting in that
the side rails are angles, and are drawn as a considerable proportion of the
stanchion depth – around 50%. This is quite different to details found today.
A Professor Bryan was moved to comment that “as Sir John Baker once said,
the effective length concept is most unsatisfactory in that one takes the
length of a column and then multiplies it by a factor which very much
depends on what you had for breakfast”. Professor Bryan was complimenting
Horne and Ajmani for their contribution in advancing the guidance. The work
of Baker et al is discussed later in this article, though Baker himself correctly
credits a Mr John Mason with the original quote.
Another contributor, Mr Dwight, noted that “it will at last be possible to
take account of the restraint afforded by sheeting rails connected to the
tension flange”. Mr Dwight also commented on “the practice sometimes
adopted of bracing the sheeting rail back to the inner flange of the stanchion,
thereby supposedly providing restraint to the compression flange”. Mr Dwight
appears to be sceptical about the effectiveness of the system commonly
employed nowadays. Mr Dwight assumed that there was “negligible
advantage in doing this because of the great flexibility of the sheeting rail”.
Professor Horne proposed verifying the relative stiffness of sheeting rail and
restrained member – the checks appear in the SCI publications on portal
frames with the recommendation that the verification is important when the
member size starts to be disproportionate compared to the side rail.
Dr Morris recalled previous practice (he referred to the mid-1950s) and the
“relatively simple calculations one used”. He noted that “it would seem that as
our knowledge of structural behaviour is extended, the design process is
refined and becomes complex, and it may be the case in the near future of
reverting back to simple elastic design”. Although Dr Morris was apparently
enthusiastic about elastic design, some ten years later he collaborated with
Professor Horne to publish the definitive guide on plastic design of portal
frames4. Since 1970, the design process has become ever more complex,
frequently reliant on computer aided analysis and member verification by
software, rather than the simplicity Dr Morris suggested.
The 1970s were clearly a significant time for the development of the design
rules for portal frames. In 1979, Professor Horne collaborated on a further
paper considering the stability of haunched members5. The design rules for
haunched and tapered members in Section G of BS 5950 follow from this
paper, and from then were “translated” into Eurocode nomenclature in
Figure 1: Effective lengths of stanchions according to BS 449 (Figure 15) 26