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Unless the struts are extremely lightly loaded I tend to use the limits laid down pre 5950:2000 values, although now I feel have the facility not to use these limlts so rigidly.

Seems extraordinary that 45% don't understand the question, perhaps they understand now!

I have not come across this as a problem in a real project as I have not had to deal with any particularly slender struts. My background is both 449 and 5950 and the slenderness ratios of 180, 250 and 350 are somewhat engrained in me. I think that the limits should be reinstated or some specific guidance given. I have some concerns about use of computer programs when I don't know exactly how they work and your comments tended to reinforce that.

By the way I used to work at Atkins structural division when they were in Ashtead and I knew a Martin Double there who knew a lot about steel design and even more about roses.

Dear Martin

I completely agree with the notion that some form of recommendation needs to be included in the code, and therefore a limiting set of values. I would however include some remark about the limits being exceeded provided a detailed analysis is carried out.

Put me down for answer 'B'.

Dear Martin,
I concur with your opinion that slenderness limits should be reinstated.
Kind regards
D McC (Fellow)

Having been brought up on the 1990 version of 5950, it does strike me as odd that there isn't a limit on strut slenderness in the 2000 version. Put me down as a b).

As a footnote, I think the steel code has become over-complicated (like the wind code). I am a mid nineties graduate, and the amount of radical changes to the structural codes I have had to experience has been a nightmare. I have had instances where I have had to learn BS449 & CP3 in order to check sub contractors calcs, so you end up reading all the blooming things! Talk about inefficient working!

I would have no problem if limits are revised based on sensible logic & research but to provide no giudance at all is folly.

This particularly for younger Engineers who are building their design skills.
I have found that the tables are reliable & trustwothy even though their technology/theory base is now quite old.
Quite frankly I will continue to use them anyway because I know that their use produces safe & economic structures.

I received your e-mail message about the lack of limits to L/r rations for struts in BS5950.

I have no inside knowledge about the recent amendments to BS 5950 but I suspect what happened was that in reviewing the code provisions they investigated what would happen if you applied non-linear modelling to struts with high L/r ratios. The result was that struts that would be acceptable under the old provisions would not be acceptable under the more rigorous analysis. They therefore had a dilemma. If they recommended a reduction in the minimum L/r ratio some existing structures would not satisfy the new code. This would cause problems since it is unlikely that there would be performance evidence to suggest that they are unsafe. Only a catastrophe would justify retrofitting. So the code writers fudged it and took out the upper limit leaving it to the designer to decide. This is rather unsatisfactory but maybe it was right in the circumstances. I am not saying that this is what happened with the code, I am just guessing.

The main question is not what should be the maximum L/r ratio but - ‘At what stage should you go to a non-linear analysis in preference to using elastic analysis plus the code provisions?’ I expect that if those who wrote the code had a good answer to this question they would have included it in the code. If we want to improve the code in this context, it will be necessary to propose a provision which will answer the main question.

1. In addition to non-linear geometry, non-linear material behaviour may also need to be included to get accurate predictions of behaviour. Very few structural engineers have experience of doing this

2. Even the best analysis models available may not be adequate. For example, as far as I know, the safe load tables for Z section purlins are based on tests with little support from theory.

3. I understand the European codes are moving towards suggesting that people work more from first principles using advanced analysis. If this is so, then structural engineers need to be trained in this area. But there is no point in saying to universities ‘Please train graduates to do non-linear analysis’. Structural engineers in the UK normally take a degree course in civil engineering which covers a wide range of subjects. There is very little opportunity for graduates from such courses to have a deep grounding in any of the subjects which they take. This is not the fault of the teachers. There is not enough time in the curriculum.

4. Another factor is that there are no good textbooks to help engineers to be good at analysis. I am presently writing a textbook to be titled Modern Structural Analysis. It will be focused on how to do modelling but will cover mainly elastic analysis and will not good deeply into non-linear modelling. (I am struggling to keep the size down).

In the absence of slenderness limits in BS5950-1:2000 for struts, I apply the limits given in Clause 4.7.3.2 of BS5950:Part 1:1985.

With regard to BS5950-1 as it stands, I feel the argument to be somewhat academic as we will be adopting the Eurocodes in the not too distant future.

In the meantime, however, I do believe the deletion of slenderness limits to be imprudent and that they should be reinstated.

Should you wish to discuss further, either please e-mail me or telephone me on 01905 759 734.

To be honest I haven't had to used the latest amendment, however I would certainly expect some guidance on slenderness limits. Indeed I would even like to see some guidance on slenderness limits for tension members (to limit sag deflection in the case of ties). Some steel codes (e.g. South African) also cover this but alas not ours.

I am in total agreement with you. We tend to fabricate many shop assembled frames & failure to observe these limits would definitely be a problem when transporting & installing. Also, I think that the limits have aesthetic appeal.

I know of others within the company I currently work that are amazed these factors have been dropped.

Dear Mr Double

Thank you for your email but I am retired and did little steel work. In general I am against losing any information proved over the years which may aid the designer.

Good luck

P L

But then if I personally design anything, which is few and far between, I would still use BS449. This is the last code I used when actively involved in structural steelwork design. Our company today mainly use outside consultants to carry out our structural design.

I have seen the correspondence you refer to and I think I remember a recent contribution from Keith Jeremiah.

Having retired some 14 years ago, I am completely out of touch with current developments in this field and could not add anything to the present debate. But you are no doubt right and I wish you well with the campaign.

I am currently working in the USA and working mostly to AISC working stress design codes although I have used a variety of different codes in different countries.

The guidance criteria does vary however I do agree that there should be some guidance on slenderness limits for compression members.

Thanks for the e mail – I have to be an (a) – leave 5950 as it is – for the following reasons :-

1) Codes should not be prescriptive – they can give a commentary if they feel that second order needs consideration in certain circumstances. Even the Building Regulations are going away from prescriptive clauses.

2) If you take the famous 26.9dia x 3.2thk in Grade S355 x 10m long! If the member was made 1.0mm thick the ry improves and, therefore the lambda improves – a weaker member with a better lambda – crazy – lambda is not the only measure of robustness. I have been in the situation where I had to make an SHS valley column thinner in 5950:1990 to bring in the lambda.

3) The loads we arrive at for buildings are never achieved - 0.6 kN/m2 roof imposed 5.0 kN/m2 floor imposed. The relationship between loading codes and material codes works and produces an economical ‘British’ answer. The loading codes being too high allow us to get away with a lot of sloppy neglectful design/ draw/build and give ‘serviceable’ buildings.

4) I suspect that you are primarily discussing hollow sections as bracing members in buildings. The skin of a building acts as a diaphragm/ box and the bracing members are never loaded to their theoretical loads. Look at a 3 span x 30m portal, 10m to eaves – a rafter bracing girder spanning 60m has a theoretical deflection of 0.75m at mid point – at this deflection the roof liner would have stripped all its fastenings.

5) I would be more concerned with other areas of the buildings – some of the bracing to beam/ column connections that end up on site these days – if there were real loads in them we would be in serious trouble.

I am concerned with the lack of guidance for slenderness limits in BS 5950 (or any other code if it should come to that). I haven't any direct experience of the problem to give you.

I am always concerned that graduates and people who are gaining experience may overlook such considerations. It is too glib to rely on their being supervised.

Martin,

The removal of the slenderness limits seems a 'step backwards' and, although
not a requirement any more, I still use them when sizing members in my
designs. I originally designed to BS.449 using these 'limits' and currently
to BS.5950 (learnt due to the advent of eurocodes..!) and feel no need to
change the way I work.

A lot of my designs are for simple 'fag packet/bread and butter' projects
(excuse the use of these terms) and, given the usual time constraints, I
cannot spend hours analysing/designing to the Nth degree. A simple approach
is required and the use of these limits should produce safe robust
structures and buildings.

If an-depth analysis/design is warranted then the maybe the limits need not
be considered as all possible scenarios should have been looked at. However
for the 'simple stuff' some guidelines should still be in place.

P M. B

As a "self-employed garrett practitioner" I still calculate without the benefit of sophisticated software and work to BS449 for most purposes. I always refer to BS449 for slenderness ratios for strut design even when working to BS5950. Whilst I wouldn't normally recommend mixing design standards, I see no practical alternative here. For small-scale projects, the time and effort of detailed analysis that would otherwise be required would result in disproportionate costs, with no saving in material and fabrication costs and thus of no advantage (or interest!) to the customer. So, I would suggest incorporating the BS449 slenderness recommendations into BS5950, whilst still recognising that in some circumstances a more detailed analysis could be appropriate and could perhaps permit a more slender or economical section

I think that I would need a little more time to read up about this issue. My gut feeling is :

In principle I think that it is wise for British Standards to give simple clear guidance and that should include maximum slenderness ratio limits for normal use . But there should always be an opportunity to break such limits if adequate alternative justification or testing is done to verify something special. The key thing, to my mind, is to emphasize that British Standards are Guidance Documents for use by most of us most of the time (i.e. when we need reliable answers quickly) which is a very different thing to regarding them as Biblical Tablets of Stone never to be challenged !

Sorry, as I have been retired de jure for fifteen years and de facto for eight and never had to deal with slender struts, I cannot help.

Regards

P B

I work in Paris - the French codes have never imposed "arbritrary" slenderness limits, and I don't know of any examples where this has led to problems.

For architectural reasons, it has often been useful for us to push the slenderness a bit beyond 5950 limits for lightweight structures carrying very low compression loads, provided the compression loads have been determined from a suitably precise analysis, usually second-order. When the structure is particularly slender, we systematically carry out a buckling analysis on the local and global framework, with result assessment to Eurocode recommendations.

I don't know the new clause in 5950 but would personally prefer that arbritrary limits be replaced by pertinent commentary/analysis guidance which takes due account of available software capabilities. The Eurocode commentary on this subject is fairly opaque.

Why was the limit removed in the first place. I don't recall any research to justify

Martin,

I have been a 'non-designing structural engineer' for some years now,
having moved into engineering management. When called upon to do so, I
would limit the slenderness of compression members to those limits laid
down in BS449 as a first check and then seek to satisfy the current BS. A
lack of guidance in BS5950 does not really bode well when we see the % of
replies to (c) !

I would always advocate BS's and CP's giving guidance to engineers, but
with the option to go outside of the guidelines for other solutions when
they can be proven by specific design/empirical methods in special cases by
those 'suitably qualified' to do so.

Maybe the Institution should be challenged to follow up on the alarmingly
high proportion of (c)'s returned given that there will be many more in
this category too embarassed to respond accordingly?!

The bottom line is that I would support option (b).

RG

It is certainly necessary to provide guidance on slenderness limit in the BS or code.

Design of normal size member has always been easy. It is the slender secondary members that cause most of the problem. I have seen tension X bracings in the plane of the roof that dangle in the air like cloth strings. The BS should actually provide guidance slenderness limit for tension ties also!

I tend to spend more time getting these straight than to work out the mail columns and beams because they cannot be wrong by a large margin but the secondary members will. The code or BS are for saving time and for producing good design. I therefore see no reason why such limits are not provided. They will not hider innovative design. All you need is a crazy architect and a rich client, then you will have INNOVATIVE design.

I don't want engineers to fall into the embarrassing trap when they are push too far by the crazy people.

I would agree that the revised code leaves something to be desired. I think that some advise on maximum slenderness should be included. We all know that the deflection induced by self weight of a very slender strut can be enough to make any compressive resistance irrelevant!

I do not agree that the old limits should be re-instated, they were perhaps too conservative, however some general advise, allowing limits greater than previously but with the codicil that self weight deflections, third order effects etc need to be considered could be introduced.

Replies from a quick poll around the office today - we agree that the limits should be reinstated as recommendations at least.

I'm all for any simple guidance in the codes. It is also a great help for younger grads!!

This guy is biased. He implies that no one supports BS5950 but there are people supporting it in his replies pages. For me they are arbitrary limits all be it reasonable general limits. Perhaps it should state the limits as guidance with a recommendation to check P-delta effects above these limits. What does the Euro code adopt?

I agree, they were always a good rule of thumb and it helps built elements to look right

I am one of the 45% who replied supporting the reinstatement of slenderness limits! I'm amazed that there were also 45% who didn't understand the question!

I work in South Africa, and we have our own structural steel code, SABS 0160, based on the Canadian code. In this code the slenderness limits are still featured.

The removal of slenderness limits from the latest version of BS5950 has also caused me some concern. As a result, I still apply the old rules in my design calculations. A quick survey of the structural engineers in our practice show unanimous support for the reinstatement of the slenderness limits.

The IStructE journal I received yesterday had a very interesting letter in Verulam on this topic and I fully agree with the comments made.

I'm away from my office at present so cannot give you full reply. I think the limits should be used as they provide a level of robustness. They were dropped to align with Eurocode developments, the European position has always been to not require slenderness limits.

It is proposed to put them into the UK non conflicting national requirements of EN 1993-1-1. For details contact Charles King at the SCI of professor David Nethercott at Imperial college ( he's our outgoing president and chairman of the BSI committee for 5950 etal.

I would be grateful if you would pass this query on to Mr Charles King for his response:-

During a recent survey, of which you may be aware, I have received back the following information, to quote:-

" It is proposed to put them (unquote, the slenderness limits for struts) into the UK non conflicting national requirements of EN 1993-1-1. For details contact Charles King at the SCI or prof. David Nethercott at Imperial college ( he's our outgoing president and chairman of the BSI committee for 5950 etal."

I would be grateful to know if it is the intention to include guidance on maximum slenderness for the design of simple struts within the national annex for EC3.

Charles knows of no plan at this time to put slenderness limits into the national annex for EC3.

Martin, when we spoke in March concerning this issue and BS 5950 you agreed to write to me in order that I would have your views placed before the code committee. I have seen other things in the ISE mag but have not received anything from you?

I have not come back to you, so far, as I fear that my personal thoughts, on this subject, will only fall on deaf ears. I am therefore taking a poll of the opinions of Structural / Civil Engineers with a view to providing a greater representation than just my lone voice.

There is still some way to go, but in the meantime, you may care to look at some of the feedback that is posted on my web site. The feedback is also a bit of a revelation as to the mood of engineers on subjects other than that presently under discussion!

If you have any further thoughts on this subject, either way, I would be glad to receive them.

Problem with Strut Deformation

Response from News Groups