Dear fellow engineer

As a structural engineer, I would like to ask for your opinion on the question of slender struts.

As you may be aware the slenderness limits for compression members has been dropped from BS5950, Pt1:2000, with no guidance being given to replace these limits.

As you may be aware, a discussion on the subject of slender struts has been taking place through the pages of The Structural Engineer over the past year. The main articles appear in the Verulam column of 4 May 2004 and 21 September 2004. For further background and for copy of the main responses so far then please visit my web site :- http://home.btconnect.com/cadoss

I had hoped for a better response than that received, in order to lobby the BSI over this point. However, there has been a poor response number-wise, certainly not enough to be meaningful.

As you can see, in addition to the poor response, the range of answers does nothing to forward my case!

If you have an opinion, I would be grateful to hear of it, even if it is simply responding with a),b),c) or d) as listed above. If you have been bitten by the problem, as a few have, I would be especially grateful for your feedback.

If this email has been sent to you in error or you do not wish to be included in any further correspondence of a similar nature the please respond with UNSUBSCRIBE.

Hi Martin, as the author of the second verulam letter dated 21 september 2004 (enclosed in word format) I couldn't agree more !

Lets hope we can get the BSI to see sense over this one. We are the checking Engineers for Salisbury District Council and there have been many instances of inexperienced engineers using very slender struts because the 2000 version of BS5950 allows them to do so !

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As an Engineer working in a School of Architecture you will appreciate that I face challenging questions from students - what is the absolute minimum thickness of the strut in my structure for example. I suppose the argument for removing design constraints which may or may not be justified is that it provides an innovative environment. The Building Regulations may be a case in point - as you will know these are performance specification based with the approved documents being the guidance aspect but not necessarily to be adhered to if there are other solutions. Is it mandatory to design to a British Standard? Can you not use guidance from former Standards to justify your design and keep at bay the lawyers!

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We at Fothergill think they should be reinstated i.e. answer b). The limits are necessary for many of the reasons already given on your website. Also from the practical angle, the limits are very helpful when one is being pressured by an Architect to provide ever more slender struts!

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Martin, I would support BS5950 as it stands. I never understood why a strut subject to wind should have a different slenderness limit than one subject to imposed. The calculated compression capacity sets its own limit anyway.

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My suggestion is to carry on using BS449, which I believe is still current although not listed as an approved document in the new (or soon to be) Building Regulations Part A.

I have been using BS449 since I started working even though I had only ever been taught on BS5950 at Uni.

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Dear Martin

I agree totally with you. There should be some upper limit on slenderness added to BS 5950 for simple designs, especially with the trend to use high strength steels. This limit could be ignored if all second order effects were taken into account.

Also, it is good to know that you are alive and kicking. I assume that you have now left Atkins. Please keep in touch

Best Wishes

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In fact I still use them because I do not feel comfortable with some of the member sizes that we seem to be able to arrive at now without using them.

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I did respond following your article and agreed totally with what you said. There is a real danger especially with less experienced engineers that critical element will fail as Pdelta effects are not considered just straight into the tables and off we go - Dangerous to say the least. With the increased use of fancy analysis programs designs are getting worse not better its often near impossible to connect things together and most consultants don't view this as their problem. Slender struts need control of the end connections and care in detail.

As I explained last time, I have insisted that our company maintain the original slenderness limits of 180 for imposed, 250 for wind & 350 for wind reversal elements (Btm boom ties).

Classic example is simple roof bracing where self weight bending and additional bending from Pdelta are often ignored.

The limits must be reinstated as not every engineer takes these into account and low fees mean less time spent and more chance of major cock ups! Time will tell. Its funny that's how I ended my last e-mail and since we've had Charles DeGaul Airport Failure - What's Next?

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Dear Martin,

I would feel more comfortable to set up the slenderness limit in my design
practice,
no matter what kind of steel design code I use.

If there is really a need to choose a VERY slender compression member, it
will obviously need more time to do detailed analysis. Someone will have to
pay for this.

Regards,

Chongsong

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Martin,

I support your concern over the removal of slenderness limits. The fact
that they may have been arbitary does not make them invalid. My logic is
nothing to do with theory; I believe that all structures should be
robust. An over-slender strut is too sensitive to accidental loads
(including impact), accidental eccentricities, etc.

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b) Support for reinstatement of slenderness limits or similar, with option to use judgement

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Just thought I would give you a taste of my limited experience with BS5950. When it first came out, the lack of the slenderness limits was one of the first things I noticed As an engineer who knows about the reasons for their existence I just continue to use them anyway. My personal copy of the code has the table of limits sellotaped into it at the location where they used to be. I have had my design queried by one client, but I explained the reasoning behind my section choices and about the slenderness limits and the client accepted this. No sure if all clients would be this accommodating though!

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Further to your E-mail, I confirm that I still employ the slenderness limits from the former version of BS 5950 whilst designing, in order to provide a more robust structure. (ie L/250 for members carrying wind loads only, L/180 for members carrying other loads.)

I would support the re-introduction of the slenderness limits from BS 5950:1990, as I haven't really stopped using them since the introduction of BS 5950:2000.

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Whilst I have not been affected by this in my current role, I remember replying on this guidance in the past. I therefore believe that the limits should be reinstated and would therefore fall into your category (b).

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At the moment there is still a generation around that understands the dangers of using very slender sections for struts, who at least know where to draw the line. Whilst I don’t think absolute, must have, limits need to be imposed, I certainly do consider some guidance should be given for those less experienced designers who may not necessarily have the safety net of someone more experienced around to guide them.

We are starting to see enough poor design as it is and this kind of omission in the code will only serve to encourage even more.

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I have just received your e-mail and I too am surprised that the slenderness values were removed from the latest version of the code.

The slenderness values were in the BS449 code (which is the one I grew up with) and then followed on into the first version of the BS5950 code.

I have always used the values for slenderness and I consider it unwise to not to use them. Robustness is one of the criteria used in the design of steel buildings and a major source of this is the stiffness of a structure. This stiffness is starting to be eroded when the slenderness values are not taken into consideration especially when lateral forces are taken into consideration (possibly due to inaccuracies in erection, wind etc).

Sometimes lateral forces cannot be quantified during the design period and some spare capacity should always be maintained in a member to cater for unforeseen loadings etc. Very slender struts are inclined to be "whippy"

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Martin,

My choice of response is b) Support for reinstatement of slenderness limits
or similar.

Reasons: helps guard against over reliance on computer design and none
consideration of p delta effects, design requirements of projects often
require strut and tie bracing systems for seismic performance, excessive
sagging of bracing systems would be less reliable for dynamic loading
creating additional impact loading on connections.

DN

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Dear Martin,

I have to admit to ignorance in this issue. I always apply slenderness
limits (as per last revision of 5950 and possibly 449?) Anyway as I did
not study structural engineering in Britain, but in Czech Republic (which
had and still have excellent structural degree courses) existence of
slenderness limits was always with me, as introduced during my masters
degree. As I said, I have to admit I did not notice this missing from
BS5950, 2000, because I was not looking for it, foolishly assuming that it
is there and therefore applying it anyway.

I fully support your curse and it is absolutely imperative that this is
included in B5950.

E S

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Although I have not carried out any steelwork design for a while now I may have to manage others doing so, & I believe that clear rules on slenderness will avoid confusion & trouble.

I will not be able to help you with your query at this stage in time. I am working in Australia at the moment and have not received any magazine from the Institution since August last year.

I am trying to work with the Australian Codes and I am amazed at how low they take their slenderness ratios. The “in” figure in Western Australia stands at 0.6 L in the x-x and y-y directions irrespective of bracing / portal effects.

The lowest ratio I work to is 0.85L considering braced effects. Anything lower than that and I feel that I will then need a PhD when standing in court!!! These lads use Microstran with adapted Ozz regulations attached. The Ratios are set in the program and are applied to all columns and struts. I feel that this design practice is a bit dangerous. I like to consider each member separately when sizing members as I get to “feel” the structure!. To-date, I have had to check two major structures 20m high and designed by “experts”. In both cases, the site lads have experienced 90mm horizontal deflections at mid height to vibrating screen columns!. Crazy but true and what is even more funny is the fact that on bringing the problem to the attention of the controlling powers, I have lost my job on the grounds of causing “mistrust”. The “Structural Engineer” has lost more than it’s glamour here in the “Outback”. Furthermore, anyone who is “deemed” to be suitable, can do structural design work and you do not have to know where WL/8 comes from!.

By leaving out the limiting ratios, you open yourself to very light and poorly designed “not for purpose” structures. Someone has then got to go back and fix the poor situation with huge site expenses and possible litigation. The killing of someone on site is something I do not want to experience! I would rather go and sell second hand cars instead!

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Thanks for your message. I have to say that I have no affection at all for the latest BS5950 & I can only see things getting worse with the Eurocodes.

Notwithstanding, I support the re-introduction of slenderness limits (option b on your list)

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The 180 requirement led to some of my previous design being very chuncky, not aesthetic and frankly being down right silly.

The removal of this archaic restriction gives me freedom to use engineering jugement to acieve the right solution to the problem at hand.

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In my opinion the removal of slenderness limits only serves to remove robustness and future use flexibility from a structure, whilst at the same time reducing redundancy.

I have in the past attempted to 'prove' old Dorman Long buildings, to both old and new codes, elastic and plastic, without success. Now consider the future if the slenderness limits are removed.

The preponderance of analysis programs and individuals prepared to believe them implicitly is concerning. Now a very useful and quick sanity check tool has been removed. An intuitive engineer, in my opinion, carries out many rule of thumb designs in his head, one of them being the slenderness check on a compression member. This he can do whilst very quickly looking at a drawing or prelim calcs. He must now go through a full process.

There is now an argument that graduates will rely on computer analysis further. The codes are solely written for computer analysis, not for hand analysis. As packages become more powerful so the engineering input becomes less. How long before we draw a structure in Autocad and the analysis suite identifies key frames and members and 'designs' them? I bet it is not that far away. I fear for some of the outputs.

Graduate courses are becoming more diverse, with shorter modules for each subject. When I was at college, I spent two years studying analysis and design. These days it is more likely to be two terms. There must be something lost in the process. The onus will fall on employers to educate trainees. This in an environment of competition, framework contracts and falling numbers of engineers.

A bad idea that will mean further reliance upon analysis programs to be accurate, and the user to be fully aware. The essential check will now take longer for the over-pressured engineer struggling to make a living in a highly competitive market. ODPM initiatives in respect of Gershon and European Procurement rules will only make this more difficult, with more framework contracts being generated, with more pressure on fees. The alternative is to hope that the graduate, who is receiving far less practical training will pick up on any problems, or even know what to look for. The result can only be at best slender structures with no feasible possibilities for alterations, or at worst failure/s - God forbid. Still at least the insurance companies and Solicitors will be earning money - as usual!

I would however suggest, that regardless of what is in the code, a sensible engineer will always adopt a practical rule of thumb. If there is such opposition to this move, I feel sure that most engineers will adopt this stance. There is no rule that says you cannot 'over design' after all. I for one, will always adopt the 'sensible' slenderness limits, unless the code requires me to adopt a more onerous design. Unless the structure has a vast amount of steelwork, I feel sure that this approach will not generate significant cost. After all, we either have a bigger column or we introduce a multiplicity of pressed steel bolt on bits, that bolt to other bits, to provide restraint. Alternatively, we spend a vast amount of the client's money trying to check Pdelta effects. It all costs money, so why not provide the steel, and robustness, at the same time.

As a final word, if we make struts more slender, their susceptibility to fire becomes far greater, except for the saving in self weight which would become an issue in certain circumstances. The minimum requirement for fire protection is bound to change - more cost?

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In absence of any other guidance I have always followed the limiting criteria of BS449!

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Being a busy, but hopefully reasonably competent Engineer and not regretfully having sufficient time to keep fully up with my reading of The Structural Engineer I was not aware of this controversy. I have read some of the response from your web site and I have to say that I am appalled that BSI have taken the step of removing the limit. I can't say I was aware of it because I still use BS449 whenever possible, but I am very aware that many of the Engineers produced by our current education system are very much less capable than those trained by years gone by. I cannot agree with any relaxation in a standard which places responsibility on the Engineer to make his own assessment when the quality of the Engineers taking that responsibility seems set in decline. This is madness and will surely lead to disaster. So my vote must be with (b).

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I support the code as it stands. The old slenderness limits had no theoretical justification - struts do not suddenly fail to resist compression at an arbitrary slenderness. There is no cliff edge situation; rather, a gentle decline in efficiency and an increasing proneness to side effects, but grown up code users are capable of working this out for themselves.

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The slenderness limits should definitely be re-introduced. Taking them away was in my opinion a grave error. An old adage springs to mind..... "if it looks right it usually is right!". The converse is of course true.

With such heavy reliance on computers and in the hands of an inexperienced engineer very slender struts will be used, but without the background knowledge this will not be realised!

I have not worked to EC2 yet, but wonder what the code requirements are there regarding slenderness limits. Presumably the section accompanying the Eurocode and drafted for the individual country would deal with this, but if we have already moved away from slenderness limits this would not be taken into account ..... worrying!

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b)

I still tend to use the previous slenderness limits as a guide for section selection as they 'just feel right'.

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Martin,

As an engineer who has used BS449, BS5950 pre and post changes, (and I'm not that old!) - I still believe that engineering judgement should be used and I still used as a rule 180, 250 & 350 to limit slenderness - good engineering I believe

rgds

J P

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Dear Martin

My response is "B - reinstate the slenderness limits." Codes are becoming increasingly theoretical, unworkable and detached from reality.

I think that codes such as in the old BS5950 or 449 should enable a reasonably competent engineer to design a structure satisfactorily. An engineer may deviate from this code and could, for example, use greater slenderness limits than the rules of thumb previously given, but to do this, he would need to be fully researched, experienced and aware of what he was doing. The deletion of prescriptive data assumes that everyone is at the same high level of ability, which they're not, so my answer is "B".

Well done on chasing this through.

With regards

J O

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In an ideal world, with all inexperienced engineers properly supervised and checked, I would go for an answer of a). The reason for this is that the extreme cases mentioned (slender CHS etc) would not comply with other checks that an engineer should be performing. In addition, there may be cases where such slender members can be safely designed and would be highly desirable for aesthetic reasons.

However, knowing that there are unsupervised, and in some cases incompetent engineers out there, it would probably be wise to reinstate the slenderness limit with a note to the effect that it can be bypassed with more rigorous analysis methods. So reluctantly my answer would be b).

I am alarmed at the number answering c) and suggest that this represents either apathy or a real problem with regards to understanding structural behaviour. I would be very interested to know how you progress with this.

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It has always been a nonsense to me that a slenderness ratio of, say, 185, was not permitted in respect of a normal stanchion.

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QUOTE As a structural engineer, I would like to ask for your opinion on the question of slender struts.

As you may be aware the slenderness limits for compression members has been dropped from BS5950, Pt1:2000, with no guidance being given to replace these limits.UNQUOTE

I am long retired and, though I still read and enjoy the Structural Engineer with undiminished interest, have not followed this case closely. However, you describe a not uncommon predicament that is not entirely unknown to me and with which I can empathise. So, based on what you have said, I willingly give you my support, choosing item b) of your email.

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For myself I would always look to limit the slenderness of compression members to those limits laid down in BS449, so a lack of guidance in BS5950 does not really impact upon my designs. Having said that I would support option (b).

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Actually my focus is currently on professional fee levels and the unsustainable Consulting Engineering World.

I suggest you move toward this before using up your valuable spare time quibbling over decimal place stuff.

I trust you don’t mind my offering a tip to allow some “head-on-the-pillow” factors in your design work:

Try making your magnitude of loading more accurate then you won’t need to worry about the slenderness clauses.

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I have read about this in Verulam - I use the guidance given in BS449 (which I still use anyway) - ie max slenderness of L/250 for any strut (or tension member that mat be subject to load reversal) and L/180 for any critical compression member ie one carrying DL & LL.

This usually gives a minimum member size that looks about right and is capable of carrying a reasonable load without buckling.

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Dear Martin,

We as a design office have indeed come across the lack of guidance in the
latest steel code for slenderness limits. We still retain the old
slenderness limits for strut design of 180/250/350. Interestingly the
IStructE/ICE "Manual for the design of steelwork building structures"
Second edition retains the limits on page 43 / section 6.1.

I would support the reinstatement of the slenderness limits to the code.
This would also give the BSI a good opportunity to sell us all a new
version of the same code. Joking aside I am worried that younger Engineers
may find themselves in trouble without the safety net of the previous
guidance.

Thank you for the opportunity to comment on this.

Regards

N M

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I agree slenderness limits need to be checked, especially in temporary condition as this is probably the most critical loading condition in many cases.

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Hi Martin,

I would definitely support proposal B to give clear 'guidelines' on slenderness limits. These may need to be slightly conservative for general use in simple designs. They should not prevent capable engineers justifying more slender struts where they are able to. I think it is important that there should be some conservative guidelines for quick reference and to assist less experienced engineers.

I am a bit worried that 45% of respondents couldn't understand the question!

Regards,

Phil

P J

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seems sensible to reinstate slenderness limits and comply with Bs5950 : 1990 in this respect
thanks

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Dear Martin Double
I am no longer a practising engineer so I am not directly affected by
any changes or omissions to British Standards.
In the (good) old days I always took the provision of slenderness limits
for granted. But my senior engineers would never have allowed me (or
themselves) to approach such limits - except in an emergency to verify
an existing structure. I suspect things are different now with - as it
seems to me - computer programs having captured the structural analysis
market completely.
The provision of slenderness limits would seem to be - at the very least
- a useful guide for junior engineers. And the more initiated could
always exceed these if they so wished. Wasn't that always the case? A
British Standard is, after all, not a statutory instrument.
The lack of any guidance on slenderness would seem to be a sorry
omission, but obviously some more learned engineers now deem guidance to
be unnecessary. I suppose the best guidance is to stick to the limits
given in previous editions of the standard.
I would say that engineers need slenderness limits and guidance should
be reinstated, but apart from that I'm sorry that I can't be of more
positive help to you.

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Dear Martin,

Sorry, I don't think this is an area of speciality for me, but reducing the hurdle for compressive performance for struts can only be a bad thing. I may be a bit old fashioned, but unless a clear case can be made for change, I think the status quo should be maintained.

Did you see the plaudits for Sir Norman Foster seven span cable stayed bridge in France? It cost £400m, isn't actually required and is justified on aesthetic grounds. To me it has just spoilt a beautiful landscape to massage a few over inflated egos!

Perhaps, you should have asked someone else and I should be auditioning for "Angry Old Men".

Regards
D W

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My personal view is that the slenderness limits set out in the old 5950 should be reinstated but with appropriate guidance. The Engineer should have the freedom to exercise his/her judgement to work around the limits where it can be demonstrated that adequate safety is maintained.

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Dear engineers,

I refer to the question raised by Martin below for opinions of structural engineers.

The Department of Civil & Structural Engineering of the Hong Kong Polytechnic University, jointly with Ove Arup and Partners, is involved in the code drafting of the coming steel code in Hong Kong and we considered the issue of limitation of slenderness . In fact, the article mentioned by Martin below and appeared in Verulam of the IStructE journal is written by myself, together with friends in U.K. but Mr. Gary's Wyatt's and my affiliations were incorrectly printed as Leeds University.

With the use of computational method, the limitation is no longer considered as necessary, PROVIDED that the effect due to load (self-weight, wind or snow loads) along the member and its subsequent P-d effect is considered. Otherwise, if this P-d effect is ignored (i.e. when using a linear analysis or P-D-only analysis), we should then include the restriction to limit the ignored effect of P-d moment induced by these loads through maximum slenderness ratio. In summary, we believe that the code should have limitation when one uses a linear or P-D-only analysis, which ignores the effect, but not for a non-linear P-D-d analysis which has already considered the effect of high slenderness.

The mentioned design code will be available early next year and please contact us for a possible copy through the Buildings Department here.

Note : P-D (big D) effect refers to change of joint geometry and P-d (small d) effect refers to change of member bow.

S.L. C

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I support the the reinstatement of guidance on strut slenderness limits. In the absence of guidance in BS5950, Pt1:2000 I would support the tried and tested recommendations given prior to BS5950, Pt1:2000.

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Many thanks for your e-mail. Like many others I suspect, I haven't yet fallen foul of this one.

This is mainly due to the fact that steel frames make up a very small part of our turnover on a design front.

However, I would have to say that my preference would be to have some guidance on slenderness limits, so I'm probably a "(b)", for the record.

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Although long retired, I was once a steelwork design specialist before moving to wider civil engineering responsibility and management. I have always kept in touch with design issues though and was once a member of the ICE Structural Engineering Board.

I was astonished when guidance on the slenderness limits for compressive members was deleted from the code and can see no justification for the present position.

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Personally I still use the rules in BS 449 for limits on slenderness, but without making specific reference to it!

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Dear Martin,

I am a bridges engineer and tend to use BS5400 Part3. Have a look at section
10, it may be of help.

Regards
Samer

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I believe that limiting the slenderness ratio may not be the complete answer to the question of slender struts, since the influence of secondary effects may vary significantly for individual structural arrangements with different geometry, loading and load combinations. In the present world of extensive use of computer software, frame analysis should deal with the secondary effects and distinguish between members subjected to tension and compression in combination with shear, flexure and torsion as appropriate. I believe that the modern codes, e.g. BS5950 and Eurocodes, assume that the codes will be used by competent engineers and they tend to do away with generalised limits on application of rules or any such guidance.

Regarding design of common buildings, I take the view that the modern codes are not as good as the older codes, e.g.BS449. Application of the older codes was subject to limits and the rules were given with the understanding that secondary effects would not be fully accounted for in the design of common buildings. I prefer the older codes personally and believe that the complex structures should be left to specialist designers, who would understand the significance of secondary effects, dynamic effects, etc.. As regards design of steelwork in low-rise and medium-rise buildings, I cannot understand why one should use complex design methods. Steel behaves elastically up to 67% of its yield strength and BS449 is the perfect code of practise for the purpose,

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I am not an expert in this field however as a practising structural engineer I consider it is correct that standards give guidance. For instance much design is now carried out using computers and you may wish to contact Cads or Oasys to see how they propose to approach this in their software.

As to slenderness of columns a set limit is good practice and will set the alarm bells ringing, of course engineers can and do go outside limits set by codes of practice by using alternative methods.

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I visited your web site and read with interest the range of comments you have received. The most accurate is one I will commit to memory and came from America - 'Over efficient design regularly leads to inefficient structures'.

My own work comprises predominantly steelwork design , and includes connection design. I am often faced with situations where someone else's 'design', has paid little or no attention to the real world connection difficulties. (I was just as guilty myself, earlier in my working life! ).

The 'Blue book' has of course churned out some 'interesting' figures, but no more so than many analysis/design packages. It is the intervention of the Engineer that should (but sometimes doesn't) turn this information into proper design. Proper consideration of slenderness is just one of many issues which can be overlooked.

Despite these views, I don't necessarily agree that the previous limits should be reintroduced. 'Prescriptive' limits might offer the degree of safeguard many are requesting, and the previous 1990 limits would be a sensible start. There will always be situations where there is good reason to exceed these limits, with appropriate consideration of the consequences. The Engineer should be left to do some Engineering rather than be force fed with a rule book, and I don't think BS5950 should try to make up for any lack of understanding by some.

I generally apply the 1990 slenderness limits, but reserve the right to stretch them where appropriate. I guess this puts me somewhere between a) and b) or possibly d) in your categories !

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My understanding is that there are no lambd<= 180 limits in the Eurocode
and that removing them from BS is in preparation for this.
The issue is whether one is prevented from using lamba>180 by code or just
by common sense.
Generally very slender struts are not cost efficient though they can be
structurally sound.

The EC3 approach is to prevent only those things which affect safety. It
does not pretend to give a best practice guide. The BS approach has
traditionally been to mix practical advice with structural integrity
requirements and codify both.

Tim

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I would support option b and in fact still design struts to the old slenderness limits

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Sorry Martin, I work in the offshore industry where we don’t really use 5950, so I don’t have any opinion on this issue. I hope you have success elsewhere.

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Dear Martin,
I noticed the issue but did not go through the details or the BS.
Despite that I have been away from design for long with current focus
mainly on project management, being a structural engineer, I strongly
support your view on reinstatement of the slenderness limit (i.e. (b) in
your email below). Although we could calculate the buckling strength of a
strut, it'd better to provide some guidelines in Code for peers as if it'd
already allowed for in BS8110 for deemed to be satisfactory span/depth
ratio for crack limits.

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I, too, regret the omission of slenderness limits from BS5950 and agree with you that AD270 gives insufficient emphasis to conduct the further checks necessary. I have semi-retired from my practice and now (for my sins) teach design to undergraduates in Swansea University. Students of steel design have little grasp of what should 'feel' right and set or recommended limits would help. I tell them of the previous limits and advise them to use them having no time in the course to teach them non-linear effects.

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Problem with Strut Deformation

Response from News Groups