Martin

Martin, good question - I really miss the guidance on struts, and still use the values from the previous version.

Of the four engineer in our York office, we all think that the information should be reinstated, but as guidance rather than a prescriptive measure.

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I used 449 for all preliminary scheming and when I am happy the structure will work it goes on the computer using 5950. I still use permissible stress design, the race to proliferate complicated standards for use on computers is producing GIGO. Some of the newest Engineers have no idea of how a structure works ask them to draw a deflected form and sketch the bending moment envelope they rush to the computer.

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Martin,
Good to see you looking so well on Thursday evening.
My statement for your website:

I support reinstatement of the original rule because in addition to the fact that most designers forget to check for self-weight stressing, I believe that very slender members are liable to be damaged and hence have excessive eccentricity and become unable to take even the small load intended.
My experience is that young designers do not see a problem - if the code says it's OK then it is!
Alan.

Perhaps now is the time to summarise the points succinctly?
Regards, Alan.

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Martin
Haven't dealt with this problem for some time but would consider clear
guidance from BSS to be essential to safe design. I would therefore agree
with your option (b).
A T C

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Dear Martin,

Thank you for this. Whilst not being responsible for design directly any more, it is certainly important to us as a major D&B Contractor.

I would be happy to vote for your option b) rather than leave it. However, I think that there may be a case for d) - increasing the slenderness limit for, eg posts which are essentially for aesthetic purposes (eg Architectural 'colonnade' type posts), which are vertical and carry very little vertical load. I sympathise with Architects when structural engineers come out with enormous chunky sections for such applications.

A fantastic example of how impressive slender columns can look are the concrete columns in 'new' Coventry Cathedral (opened in 1962). Have you ever visited? I recommend it, if not. I believe that these columns are precast-prestressed, with a pinned bottom connection. They certainly appear to carry a reasonable amount of roof load. They have a cruciform section, so are very elegant I've attached a photo which gives a low res view.

I also think the Code drafters should check for consistency with the other UK Codes (particularly concrete & timber, although aluminium has been referred to). The whole of Europe should also be in step, with the advent of Euro Codes. It may be that the UK Code Drafters need to lobby their Euro counterparts.

I hope this is of interest.

Regards,

A E

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Have you contacted Dr Arun Kamtekar at Birmingham University? I recently went through and commented on the draft of his paper proposing an original approach (for structural steel) for the design of column base plates and I am sure that he could give you a well-informed view on the design of slender struts as well.

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have not been affected by this issue as yet, however there should be guidance in the code. Therefore I would support the following.

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Thanks for your e mail with which I entirely agree. I only recently realised that the SR limits had been dropped from BS 5950 : 2000 but still apply them personally in the interests of good practice.

I feel that the revision to the BS is a bad move and a bad example for inexperienced Engineers and agree with many of the objections quoted in the responses you have already received.

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I wish I could help but as its has been years since I looked at design I fear that I cannot comment with any confidence on the subject.

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Thank you for your Email. The response does not surprise me. I would vote 100%
for option b. I would also vote for the abolition of BS5950 and the full
reintroduction of an updated BS449 and CP3 and I am sure I am not a voice in
the wilderness. I do a lot of checking work and on the smaller schemes say
up to £500k build cost I would estimate that 80% are to BS 449 and where
wind loads are calculated over 50% use CP3. I have absolutely no faith in
the profession to give appropriate direction. Our learned Journal is full of
adverts for beams full of bloody holes and inane articles on what the bloody
president had for breakfast and with whom. Who gives a s*?!, At the
sharp end I check designs to ultimate loads in TIMBER, unfactored load
designs to BS5950 and piles and piles of illogical drivel from large
prestigious firms that have been prepared by children ( unsupervised). I
better not go on I sound like a grumpy old man (48 one careful owner). And
as for f*?!*.....Architects...............

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We were surprised by the omission, but still continue with the old limits which are good practice. As I understand it slenderness is now included under section classification and class 4 slender sections are dealt with by a modified effective area and reduced stress pcs up to a maximum slenderness of 350. For normal lengths of members this is fine however it may be possible to use sections which could be impractical. The blue book gives length limits for members in the tables.

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have tended to ignore this change for critical members and have stuck with the limits of 180, 250 and 350 as previously defined

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Dear Mr. Double,

I am a structural engineering student due to obtain my masters degree in less than 2 years. The problems concerning the Eurocodes has been brought to my fellow-students and me numerous times on many different occasions and lectures. Once our lecturer pointed out that the Eurocode is the only standard that lacks the restrictions of slenderness, unlike the codes of Germany, Sweden, Russia, Great Britain, and my country Bulgaria.

I'm surprised to learn from you that the BS has adopted (or is being prepared to adopt) that change. The curious thing that we learned from that lecture is the reason why there's no mentioning of restrictions of slenderness in the Eurocode -- the members of the countries preparing the new Eurocode have different limitations of slenderness as far as concrete values are concerned. Therefore, we were told, the question of slenderness limitation has been skipped for as long as the members reach an agreement.

My country is due to enter the European Union in 2007, which is not 100 per cent sure. The Eurocodes are being translated at the moment and we, as students, are learning to design structures by our current code and the Eurocode at the same time. Therefore, I'm concerned with what is going on in Europe and hope for as quick as possible a solution to the major disagreements between the states members.

In my opinion a limitation of slenderness is necessary, as most structural engineers will agree. I have no personal or professional relation to the British Standards, therefore my opinion might be of minor significance, but what I strongly hope is that all differences between the Codes of the members are settled and a good compromise solution is found as quickly as possible, so that the engineers will have to work with two different codes (their current and the Eurocode) as short a period as possible. I think we, as professionals and future professionals, will all benefit from such development.

Best regards,

F S

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As far as I am aware, my local Council and local structural engineers have even got as far as the old BS5950 yet let alone Pt1:2000 and still use BS 449 (I think).

They don't even use computers either as everything comes back to me hand written with calculations done on an abbacus.

Martin
I have not experienced this but am concerned at the simple dropping of guidance from BS5950 and would therefore vote (b) for reinstatement.

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I give below my comments on your query. My children always used to say: "Don't ask Dad because you will get more answer than you want". I fear this may be the case here.

Regards,

J R

First I should give you my antecedents. I have exceptionally long ex[erience, having designed my first structure at the beginning of 1948. When I last saw it, 50 years later, it was still in service and looking good. I have worked on the shop floor fabricating steelwork and on site erecting it. I have been in the competitive design and supply field and in the professional office and I have done time in the ivory tower. I have a number of publications and awards to my credit. Unlike most of my generation I am computer literate and an "expert analyst" in the Finite Element Method.

1.                      I am strongly opposed to prescriptive codes of practice. I am inclined to support the one-line code: All structures should be designed by a competent engineer. Sadly I have to concede that guidance is necessary for the inexperienced engineer and the regrettable number of incompetents among us. This said, a code should be a recommendation, not a rule-book, allowing the designer to use his initiative. Unfortunately the deficiencies of the legal system tend to force it to be the latter. If your structure falls down and you can convince the judge that it complied in every respect with the relevant code the chances are that God will get the blame; if you cannot, you will. But more emphasis needs to be placed by the drafters of codes that they are not prescriptive so as to encourage judges to view them in the light of mere recommendations.

2.                      Since I am now retired I have not seen the new BS5950 but in principle I am strongly in favour of dropping the illogical limitations on slenderness ratios from the codes. How can we justify the assertion that a member that is normally in tension is more capable of resisting compression than one which is always in compression?

3.                      Nevertheless, I am extremely wary of slender struts, not because of the risk of their failure in normal service but because of the risk of accidental damage during handling in transport, erection or otherwise going unnoticed by the designer and increasing their susceptibility to buckling. I would not object to the inclusion of sensible guidelines for the benefit of the aforementioned inexperienced and incompetent, drawing attention to the dangers of slender struts and recommending especial attention when a certain value of slenderness ratio, say 200, is exceeded. I have a garden shelter which clearly required horizontal bracing to resist wind forces. The struts inserted have a slenderness ratio of about 400 and have proved perfectly satisfactory. It has just successfully resisted a severe hail-storm.

4.                      The whole thing is a pursuit of a will o' the wisp until people learn to estimate effective lengths correctly. That many cannot has become clear in Verulam on occasions. I have known of the chief design engineer in a very large concern who decreed that effective length can always be taken as 0.7 of actual length. The truth is that the ratio cannot often be taken as less than 1.0. One Verulam correspondent came up with something under 1.0 when the correct figure was 2.3! The same strut can carry a larger load in vertical bracing than in rafter bracing because in the latter case sagging under self-weight introduces a Pd effect which is not present in the former case. Have you tried calculating the deflection of an unequal angle diagonal rafter brace in a sloping roof under self-weight and what this does to its capacity as a strut? I have never seen this taken into account. The whole thing is a web of uncertainty which cannot be covered by a single arbitrary rule. Far more to the point would be sensible guidance about effective lengths in place of waffle about fictitious fixed and free ends.

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Dear Sir,

With regard to your query I would support the reinstatement of limiting the
slenderness of struts within the code.

Regards

J R

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Regarding your Email dated 26^th October I have now been retired about 2 years now so my experience of BS 5950 2000 is limited. However I did attend a Seminar when the revision of BS 5950 was issued. At this Seminar the omission of the slenderness limits of struts was not mentioned, but when we came to implement the code we decided to retain the slenderness limits as contained in the previous codes. This was done in the interests of having a robust structure. There was also no indication that I could find that the software used for analysis included for the effect of the significant shortening to be expected if very slender struts were used. Mind you having been an Engineer for 30years or more I did tend to be conservative in my approach.

I would have thought by now that the committee responsible for the drafting of the code would, in the interests of clarity, have explained the reason why these limits were excluded. Even if these limits are seen as arbitrary I consider that retaining them would have been a sensible approach and one which I would support.

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I have previously run into problems with this area of the code. I think the slenderness limits should at the very least be reinstated or else an adequate alternative method should be clearly detailed in the code. A decent explanation as to why it was removed should at the very least be given!!!

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Martin,

I've spent some time recently trying to justify the design (not mine) of
some very slender roof bracings - so have sympathy with your cause.

b) Support for reinstatement of slenderness limits or similar

Best regards,

M H

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In my opinion the slenderness limits set by the old code provided good guidance, but could also be unrealistically obstructive. However, the new code allows more flexibility, for instance when a slenderness ratio for a roof brace is 260 it still has an axial capacity. However, under the previous code you would not have been able to use it, even if it were only taking a very small load.

As such I feel that the removal of the limits is useful. However, engineering judgment also needs to take place; for instance it would be foolish to design the top chord of a lattice to a slenderness limit of 350. In this respect some form of guidance along the lines of the old code would be helpful. I would suggest simply removing the limit of 180 and replacing it with 250 for all members in compression except those members carrying wind reversal which could have the limit increased to the old 350.

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I agree that minimum requirements for slenderness should be reinstated. Beauty of steelwork is its ductility giving plenty of warning of failure. A very slender struct could result is catastrophic failure with no warning if actual eccentricity of load was significantly greater than design assumption made.

I have been in the industry for over thirty years now and have witnessed standards become thicker, more complex and more theoretical whilst at the same time practical guidance has been removed. In some cases this is dangerous, for instance:

The new wind loading code allows the shielding effect of surrounding buildings to be taken into account.

The building regulations for structure has only two requirements ie that the loads shall pass safely through the structure and that no structure shall rely on its neighbour for stability.

Any building designed using the above clause would not comply with Part A and could be subjected to increased loading if the adjacent building were to be demolished leading to possible collapse.

Why is this clause present and why isn't practical advice given with regard to its use.

As a practical engineer I still adhere of course to the old requirements but I am not usually (unlike many others) in a situation where I am designing competitively.

I have checked many other engineers calculations and have become very concerned at the lack of a deep understanding of how structures work in practical rather than theoretical terms so practical guidance is needed in the codes to avoid possible failures.

I passed your e-mail on to J McN who is a Meccano Engineer and works near me, his reply is below.

-----Original Message-----
From: McNay, John
Sent: 28 October 2004 09:10
To: Page, Ron
Subject: RE: Structural Engineers Opinion

Personally I still stick to the Le/180 as a guideline. I think that this should be reinstated for guidance only to aid younger engineers but not made mandatory.

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Re: The dropping of slenderness limits for compression members from BS5950, Pt1:2000

Thank you for your e-mail requesting feedback on this very important topic. I am equally concerned and amazed by this given the new codes emphasis on secondary effects. I have not had cause to refer to that particular part of the code and did not realise thay such a basic omission had been made.

In the absence of a comprehensive review of slenderness (together with the lateral restraint / effective length guidance for beams) I would support the reinstatement of the original guidance.

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I would call for the reinstatement of the slenderness limit option b, and still limit the slenderness ratio of struts to similar guidelines as previously used

I think you have every reason to be concerned about the lack of guidance on slenderness ratios in the latest BS 5950. Although I have now retired, I have been following with great interest the discussions in Verulam. It appears to me that our profession is being taken over more and more by the academics, who appear to have lost sight of the fact that the majority of structures are designed by engineers who have to earn a living preparing designs for the more mundane type of structure. Their calculations are subject to scrutiny by local authorities and in the event of any dispute the engineer needs as much help and reliable sources of reference as he can get. This has always been work of the British Standards.

In my opinion, the importance of the slenderness ratio of any strut cannot be over-emphasised. I have no objection to sensible revisions to the previous recommendations, but there must some form of system to guide the practising engineer. If the powers that be cannot improve on the traditional slenderness ratio guides and tables, they must keep to the previous tables used in the now outdated British Standards. I hope this message helps you in your endeavours.

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I like the idea of direct action as a method of lobbying and getting engineer’s to contribute for a change.

We are currently worrying about the possible effect of the introduction the Eurocodes and the likely chaos that will cause. Generally I consider that the British Standards have steadily improved over the years because they have allowed more freedom of design. The one caveat to this is the loss of ’simple’ design criteria for straightforward structures. The compromise reached over 449 and 5950 was a good thing because it allowed more options for design.

My alarm at the Eurocodes is both about the bureaucratic way that they are appearing to force engineers to design and about the lack of design freedom.

I think slenderness limits should be retained as part of a simple design approach which should be permitted. They can then be exceeded but only with a lot more checks including a risk assessment of issues such as accidental damage and workmanship.

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Martin,

Further to your recent E-mail regarding slenderness limits, I would place my
opinion firmly in category b) i.e. Support for reinstatement of slenderness
limits or similar.

In addition to the above, I feel it may be worthwhile for me to clarify my
opinion.

1) I cannot see how limits specified previously in both BS449 & BS5950
(early editions) can suddenly be declared "arbitrary" as AD 270 states.
These limits were specified for a purpose & have, in my opinion, served the
industry well over the years i.e. minimising the possibilities of structural
failures. I have never found these limits to be particularly onerous.

2) Mr. Wyatt, Dr. Lam & Professor Chan in their letter published in "The
Structural Engineer" of 21st September state, initially, that "columns with
any slenderness limit can be used provided that the second-order effect such
as additional deflection due to member self weight must be considered " etc.
However, later in their letter they refer to lack of "a firm understanding
of geometrically non-linear behaviour in undergraduate education & perhaps
in the industry as a whole". I believe this to be true, which rather negates
their earlier comments. The requirement for slenderness limits would be a
safety check on any design not complying fully with any second-order
effects.

3) With regard to the infamous strut 26.9 dia x 3.2 thk CHS, consider the
case of this strut, in position & loaded, simply supported at each end,
which, I admit, is unlikely in practice. Work is proceeding on site when a
heavy operative props his ladder against the strut, & carries a heavy load
up the ladder. This action imposes say 0.5 kN horizontal force at the mid
point of the strut. The strut would then deflect almost 300 mm. Would the
second-order effect then result in the failure of the strut if it was fully
loaded ?

4) A further point regarding this strut. Even when being carried by one
person at each end, the theoretical deflection, almost 70 mm seems excessive
to me.

5) Without limits, it is inevitable that certain designers will push the
design as far as it can go, until failure will occur.

6) Presumably the idea of no limits etc. is to reduce material weight, &
hence cost. But is this really essential ? What would be saved is a small
amount of material weight, & hence a saving of the basic material cost.
However other factors such as design costs, which could possibly increase,
drawing, fabrication, finishes, transport, site works costs etc. would not
be reduced. Result, minimum actual savings, but any possible safety margins
would be eroded.

I trust the above assists your case.

Finally, I note from your E-mail that category c), Don't understand the
question, gives a reply rating of 45%. I would question the ability as a
structural engineer of anybody who does not at least understand this
question.

Regards,

M T

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Problem with Strut Deformation

Response from News Groups