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Dear Martin,
I personally have not suffered from the problem to date regarding slender struts as you have, although I dare say will do at some stage.
However, concerning adequate guidance on this matter in BS5950:2000:Pt1, I am appalled that such limits have been removed from the code without an apparently adequate explanation.
This and most other structural codes are weighty, expensive, and are supposed to provide important design information to what is, to all intents and purposes, a captive market. They have been riddled with typo-graphical errors on a regular basis. The steel code in particular can be very misleading and difficult to interpret at times, and I have found myself acquiring expensive BCSI books that provide some clarity for what should be a clear code of practice. Therefore, all relevant guidance should be provided.
On a personal note, I am maintaining the use of the original limits until further notice. I was under the impression that HQ were looking into this, but I may be wrong on this point.
I hope this may be of use and you may reproduce my comments with my full permission.
Alan.
Alan Hannaford
Principal
Alan Hannaford Consultants
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Yes I agree we need guidance on suitable slenderness limits.
But why not just work to the old limits. Surely we have enough of rules on just about everything now. Is it not perhaps better to have a choice without have to conform to a rigid rule.
A S
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Martin,
For what it is worth, my opinion is that do not believe that there should be a PRESCRIPTIVE upper limit on slenderness, as it gives too much grounds for rejection by code-slaves when engineering judgment has been used. However, I strongly believe that the upper limits should be SUGGESTED in the code as a memory jogger for the more experienced and guidance for those with less experience.
Regards
A W
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Dear Mr. Double,
I fully support the reinstatement of the guidance on slenderness. However I am greatly concerned by the 45% of respondents who did not understand the question.
regards
B S
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Martin.
I believe some guidance should be included within the new version of BS5950,
therefore I would recommend your option B.
B N
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Martin,
I have read with interest the discussion in Verulam over the past months and agree with your point of view. I therefore would vote for your option b). Good luck with your quest to get things changed.
Regards,
B R
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Martin,
b). I'm away this weekend so Friday's off; will see you next at Corus meeting on 18th.
All the best, Bob
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Dear Martin,
As a practicing engineer in Australia I have not used the particular British Standard in question. BS 449 was still in use when I left the UK! However I do believe the Standards should in principal retain simple useful effective information and the table you refer to appears to be consistent with such a view. On this basis I would support item "b" in your list.
B C.
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I would welcome b) as recommendations for guidance. Especially for those less experienced than ourselves.
All too often and more frequently it seems that the less experienced do not realise how much a horizontal brace will sag under it's own weight far less appreciate slenderness ratios. I had a recent such occurrence that was easily overcome by ties up to the purlins.
Good Luck.
Regards
B S
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Dear Mr. Double,
I am writing as the Chairman of BSI Committee B525/32, responsible for BS8100: Towers and Masts, and as Convenor of the Eurocode Project Team responsible for prEN1993-3-1: Towers and Masts. In drafting both Codes we considered carefully the retention of upper slenderness limits for these light wind sensitive structures.
Both the BSI Committee and the Eurocode project team considered it helpful to the designer to provide some guidance on upper limits, without giving categorical cut-offs. Our concern was related primarily to vibrations, second order deformations as well as the possibility of damage due to bending from local loads (for secondary members).
We thus have recommended, as good practice, upper limits (of 120 for legs, 180 for primary bracing, 240/250 for secondary bracing and 350 for multiple lattice bracing).
We believe these recommended values should be retained for the specialist structures with which we are involved.
Yours sincerely,
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d)
Sorry for the short reply.
I'll try and look up BS5950 when I can spare some time.
It'll probably take me much more time to understand it though!
Regards
Mustafa
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I support the reinstatement of the slenderness limits, therefore please record me as supporting option b)
Regards
C H
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I would fall behind (b)
regards
M C
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Dear Mr Double
Thank you for requesting an opinion from me:
I assume the slendernes
limits to which you refer are 1/180 for struts under permanent loads and 1/360
for struts supporting wind load only.
1) The figures do seem arbitary and are probably based on what 'feels' right . As an exprienced designer I am sure that most of the columns/struts I design will fall within these limits but I never specifically check.
2) I suppose that by having no limits an inexperienced designer could design an extremely slender column which works mathematically under a specific design case but has very little margin of redundany should the member be installed out of plumb or should the stucture be subject to a high degree of sway.
On balance, I am of the veiw that some form of
guidance should be reinstated.
regards
C H
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Martin,
I still tend to use the same limits, irrespective of their removal from the BS, call me old fashioned but I think a critical element needs a certain amount of "robustness" and the practicalities of connection need to be considered.
That's a b) then
CM
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(b) please
C Von G
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Dear Martin,
Thank you for the opportunity to contribute to this debate.
I am concerned that the slenderness limits have been removed – and therefore vote for (b) reinstatement of limits.
I do not consider this is over-prescriptive rules (as some have suggested) – because surely as trained engineers we treat these rules as guidance, and if we design outside any limit we are able to justify our decision in some other way. The Building Regulations allow such a procedure.
Has any of the drafting committee responded?
Best of luck with your campaign.
D M
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Hi,
I think Slenderness should be reinstated (b)
D R, Member and Examiner for the Structures.
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Dear Martin,
I continue to be guided by BS5950 : Part 1 : 1990, and would support
reinstatement of the limits. The guidance is useful, so why take it out?
There's nothing to stop a designer exceeding the limits if this can be
justified by other means.
Regards,
DB
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Martin
I haven't come across the problem of late but from reading the various opinions I would certainly think that removing the limits was a retrograde step!
D L
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Martin
I am currently acting as Structural Reviewer on a major project, which includes the design and construction of circa 400 buildings in the UK for the MoD. Whilst the design is supervised by experienced engineers, the majority of the actual design is being undertaken by graduate/junior engineers, who follow code requirements and do not have the experience to know when second order effects need to be considered.
The limits for compression were provided to ensure that second order effects did not need to be specifically considered and calculated. The withdrawal of the limits is leading to the design of very slender elements, some of which would fail when second order effects are considered. As a result, I am insisting that slenderness limits be observed.
In my opinion, whilst the current code permits specialist design there should be defined limits beyond which second order effects need to be specifically considered and calculated. This would enable graduate and junior engineers to produce safe designs and reduce the risk of unsafe design not being corrected through check and review processes. In this respect, I note that feedback from our staff indicates that we operate a far more rigorous check and review than most professional practices. Accordingly, there must be a real risk that structural failure will eventually occur through defective design, as a direct result of commercial pressure to minimise sizing, lack of understanding, inadequate guidance and incomplete/inadequate checking.
I fully support the re-introduction of appropriate guidance within British Standards.
Regards
D P
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Hello Martin
By coincidence I have been designing a canopy for a hospital entrance. I will not try to describe the canopy except it can be likened to a kite with significant wind reversals. There are a pair of tall circular columns that will support a series of struts that connect to the outer edges of the canopy and carry wind reversal. The lengths of the struts are a maximum of 7000mm, the axial loads are small therefore I have simply used Clause 33 of BS449 i.e. limited the radius of gyration to span/250 (perhaps I am showing my age)
Guess what!
The Architect showed the 'struts' as 'wire ropes'.
best wishes
David
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I support b)
D N-S __________________________________________________________________
Martin
I would certainly advocate the retention of a limit on slenderness for struts. If they are designed too slender then it can be a bit unnerving when you actually see them in reality. Also, if one designs too slender then 10/1 the fabrication tolerances actually achieved may require some exotic calculations to prove that they are acceptable.
I have experienced `too slender' on cranes.bridges and offshore structures and usually the extra cost of being `not so slender' is small.
Best regards
D F
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Dear Martin,
I have actually written an article which hopefully will appear soon in
the ISE Verulam section. I am enclosing the text which you may be
interested to read but please don't publish it until the article appears
in the ISE.
As you can see I am very much in support of BS5950-2000.
Kind regards
Esam Al Kelaby.
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Martin Double
I was passed your email on the above subject and would note the
following
1. In BS 5400 Part 3 the maximum slenderness ratio for compression
members is 300, this gives a capacity ratio of 6% of the yield stress.
2. The majority of users would never adopt anything approaching
this value, but I would agree that as the previous version gave value
above 300 then a note should be included to highlight that the user
should refer to specialist literature for values of slenderness above
this value.
Regards
Dr J F
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Dear Martin Double
The question of struts etc. has been around since the ancient Greeks. I am 71 and still working and whatever BS says I will continue to base my designs on some limitation of slenderness. Ultimately we need to remind ourselves that before BS there was Euler! I suspect that on occasion the decisions made by BS committees or more often these days paid members are based on commercial rather than theoretical considerations. Just make sure your PI is fully paid up.
Regards
Geoff Watts Chairman EKG ( part of the SEC Branch)
PS If you live and/or work in East Kent join us at our monthly meetings in Canterbury-none of us feel like travelling to Croydon in the evening!
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Martin
Thanks for your e-mail
I believe a lot of the new codes are becoming both too academic and woolly
I support the reistatement of slenderness limits
Regards
G S
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Dear Martin
Do I recall a colleague from WSA back in the early 1970's?
I'm pretty much out of steel design these days so can't really help you. Slenderness ratios seemed to be a straightforward if imprecise solution to a difficult problem. Does BS 5950 retain the concept but leave it to the designer's discretion or is there a new theory? Is Euler dead?
Best regards
G M
(I think Euler was assassinated by Perry & Robinson who stole his theory then went on to mess it up -Ed.)
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Greetings!
I opt for (b) in your choices.
Thanks
F G C
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Martin,
I would support some form of guidance on slenderness limits.
gerry
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Hello Martin
(Martin who?), the main issues governing strut slenderness are (obviously) Euler
buckling and bending under self-weight...both of which could lead on to
second-order (P-delta) effects in tall structures. I find it hard to believe
that BS5950: 2000 has ignored years of such theory and research. Sorry, but I
work mainly in small-scale structures where slenderness is seldom taken to
extremes - unless I specify too light an "ACROW" prop, say. I haven't followed
the discussion in VERULAM in detail, but would vote for the old ratios (180 and
250). What does EC3 require?
Yours, Graeme O
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Martin
Sorry, I have no recent experience to be able to comment sensibly.
Regards
C H
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Martin,
Like some of your other contributors I no longer perform such calculations, however I do believe that guidance is required in such codes. So put me down for a b).
In todays supercharged world, designers do need all the guidance they can get.
regards
Iain
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Dear Martin,
Working to the NZ Standards - with the majority of design governed by Earthquake criteria (where we commonly require columns to be stronger than beams - to control the collapse mechanism) slender struts are seldom a problem. I'm sorry that I can't be more definitive - but have not had to face the problem (or the NZ controls) for some time.
Regards
Ian
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Dear Martin,
As a structural engineer practising for the past 40 years I never cease to be bemused by the structural updates from the powers that be. I have a tendency to update rather then reinstall my knowledge and would be of the opinion that the slenderness ratio guidelines of yesteryear still hold good notwithstanding that there is no current guidance on the subject.
I would be inclined to support (b) as an incorporation into 5950: Pt 1 - 2000 as I believe that from broad experience strut failure has been known as a cause of major member failure particularly in trusses and the limits we are used to working to are appropriate. I am also of the opinion that without some control or limit on the slenderness ratio less experienced engineers may be inclined to adopt an overly slender section to their cost or perhaps that of their employers. PI is expensive enough already!
I look forward to following your crusade with interest and trust my small contribution may assist.
Kind regards
I I
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In answer to your email:-
Having designed steelwork for over 40 years and classifying myself as “practical engineer” working in the “commercial sector”, I still retain in my mind the slenderness guidelines set pre BS5950 : Part 1 : 2000. The magnitude of the changes, requiring often complex and time consuming calculations, in both the latest steel and wind codes is a direct result of too much academic input and interference. Unfortunately the academics have the time, and whilst research and sophistication can be advantageous this can often be mis-guided in an industry where on site quality control still allows the holding down bolts to be cast incorrectly – even before the steelwork is delivered.
Category (a)
Although I consider I have been ‘rail roaded’ into using the latest codes I have little support for either BS5950 or BS6399 : Part 2 on the basis that the amendments of previous codes with detailed guidelines on their use would have proved more cost effective and beneficial to our industry.
Category (b)
Regarding slenderness I consider that some guidance should be provided for the design of struts – particularly as computer generated calculations can produce some very strange and slender sections in the hands of inexperienced non practical users
I trust this assists your survey.
Kind Regards
C W
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My Opinion is b)
J P
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b please!!!!!
Regards
J K
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Martin
I continue to use the original limits as a personal guide.
I would think the limits would be useful for inexperienced engineers.
I am in favour of the reintroduction of slenderness limits to BS5950.
J B
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b)
J D B
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martin
support option b
regards
john
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Martin,
I strongly believe that there is a need for guidance, particularly for the
routine structural design which makes up 90% of the building structures
market. There simply is not enough time to undertake complex analysis.
Therefore reintroduce the slenderness limits! This need not preclude an
option to undertake a more rigorous approach when justified.
John
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Hi Martin
Well done on taking some action.
I realised the relevant clause had been taken out when I spent an hour looking for it!!!
I still use the old limits but of course younger engineers would not know what the limits are.
Anyone who has used say angles for vertical bracing and then seen how the angles sag, would certainly use the old limits.
My vote is b, Support for reinstatement of slenderness limits or similar.
Hope you have a good response.
Regards
K D
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I would like to see the values reinstated. At the very least a clause that says no further checks are required if the strut is designed for a slenderness value less than X
K C
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b)
R J L
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Martin
I do not have much first hand experience of slender struts, but would
support your option b) for the limits to be reinstated, if only for general
guidance, rather than being mandatory.
Regards
martin
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Martin,
I was amazed when I first looked for the strut limitations in BS 5950 and realised that they weren't there !?
My own view and approach is as follows...
I think that BS 5950 should include limitations on slenderness ratios for struts.
In the absence of guidance I usually stick to the limits as shown in BS 449.
Occasionally when circumstances require it I work from first principles and make an informed judgement.
Best regards and good luck
I T M
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Martin,
With the codes becoming so much more complex to use these days some kind of sanity check for the inexperienced user is definitely required.
Put me down for option (b).
Regards,
MAK.
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Dear Martin: I have been retired for some time but the USA fairly-recent Steel Code (AISC 2002) does contain the following
1. for members primarily loaded in tension but that might be subjected to occasional compression max length/r (l/r) is 300
2. for members loaded primarily in compression KL/r (length is now L) max is "preferably" 200 (I assume that this gives the designer some leeway for personal judgement)
3. For more complicated situations (e.g. combined stresses) there is some guidance in the Appendices and some references for more detailed study. If you cannot get access to the AISC Code I'd be happy to copy some pages for you (if time is no problem - which is unlikely).
4. It seems to me (I was born, raised, and worked for several years in Manchester U.K.) that the old Reynolds Designer handbooks also had suggestions.
5. My recollection ii that the limits in 1 and 2 have been around for a long time and have served fairly well. To hear that the new BS code no longer addresses slenderness is a surprise - maybe just an oversight???
6. USA also has a light Gage Steel Code (written by my old professor George Winter) if you can get access to that. (Same as 3)
7. Possibly the best (also the most thorough and hence the most detailed) book, even though now a little old, is McGuire. (same as 3) Salmon and Johnson is pretty good also.
Hope this helps - keep me informed if you can. Though I am very busy this seems important
M T Prof Emeritus Univ Calif Davis USA.
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Martin,
Thanks for your emailed message and would say that I don't really get that involved in major structural projects, only small schemes, however, common sense should prevail and some form of guidance should be available in the standards, therefore pencil me in for support of ''item b''.
Regards
Michael
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Hi
My understanding (which may be incorrect) is that the slenderness ratio limits ( which were very simple to comply with) were replaced by second order analysis (which is not so simple to deal with without using computer software) the output from which cannot be checked longhand.
As BS5950 is to be replaced by eurocodes in the very near future I think you should campaign for the eurocode to include slenderness ratio limits as an alternative method of compliance without having to undertake secong order analysis.
In other words why bother about changing a code which is on its way out
regards
M S
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Dear Martin,
I support re-instatement of the slenderness limit. Last time I used it I think it was 250?
Regards,
M Y
Sharon (near Boston), Massachusetts,USA.
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Dear Martin
I had noted the discussion, without necessarily seeing all the correspondence or getting particularly wound up about it. I may have missed some elements of he arguments.
The point I have not seen is any mention by the compilers of BS 5950 of why they have omitted the guidance. It crossed my mind that the move towards the different style of codes demonstrated by the new Eurocodes may have something to do with it. The new codes stick to the requirements, and avoid guidance. The ISE (or someone) will be needing to produce guidance for the Eurocodes, and matters such as slenderness limits, or what to allow for when designing struts, will probably end up there.
How have people been 'bitten' and how? Difficult checkers? We get those anyway.
I'm pretty sure that the rule of thumb limits will not reappear in the formal BS (EN)'s
I don't 'support' 5950 as it stands, but I recognise that the world is moving on whether we like it or not.
Regards
N E
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Dear Martin Double,
As a retired engineer, I am no longer concerned with design from Codes of Practice or British Standards.
Anyhow, when I worked in Dam Engineering we deliberately never worked from CPs but from guidelines. I am talking now of the design of dam walls and not appurtenant works. However, the point is that you did start off with a set of principles or criteria, based on theory, experimentation and experience. Once a design profile had been established, the wall was analysed and reshaped - a convergent iterative process - until the design satisfied bracketing permissible stresses and displacements for all known load conditions.
In the case of concrete dams, one of the first considerations would be on the initial profiles. For example, the well-known middle-third rule would dominate the design of the cross section of a gravity dam. It would be unthinkable, in fact, not to start from this simple stability criterion. The same sort of thing applied also, on the other end of the scale, to arch dams. In other words, a pretty good idea of the shape and profiles could be obtained from design guidelines before the "design proper" was undertaken. Furthermore, not to obtain an initial shape in this way would result in an unacceptable amount of effort afterwards to get to a suitable shape.
Obviously, in the case of column design, the Slenderness Limit is a leading criterion. A designer can quickly get to the right order of proportions by using a Slenderness Limit to start off with and saving time and effort in what is again a convergent iterative process. Why on Earth this should be omitted from the initial design process I can't imagine, as it is clearly extremely useful.
I would have thought, though, that 45% in support for reinstatement against 0% for leaving BS5950 as it stands is an enormous majority.
I think that the 45% who don't "understand" the question reflect experience of a life spent in design by code-bashing on standard structural elements, rather than through being involved with special structures that demand an overall approach to design.
P O'C
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I Work in the offshore sector of the construction industry and we do not use BS5950 for routine design. However I would be nervous about removing the kind of limits of which you speak. If you only design for the very specific load cases you have considered and do not provide additional redundancy or robustness there is no spare capacity for unexpected events e.g. accidental damage, fabrication errors, new load cases or load cases missed in the original design. In offshore, we have to consider vibrations, fatigue and vortex shedding (for tubes) and these are often critical in setting the member sizes (slenderness ratio), but do not always come into the load carrying capacity calculation.
Hope that’s useful.
P S
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Martin
I would favour a return to some form of guidance in the code.
More often than not when pushed to the limit the result just feels wrong.
We often come against this situation in Building Reg checking work.
Of course the fabricator based designers have to squeeze the last % out of their designs to remain competitive and so won't give anything back unless every one is forced to do the same.
Regards
PJT
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b)
P T
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I have read with interest the responses to your query. I have just finished an expertise project where very slender architectural structural masts (though not struts the problem is related) had to be taken down at great expense due to two failings, :
The first is that the Pdelta calculations had not been carried out on the slender sections L/,
secondly the welds that were carried out were of a poor nature, and insufficiently checked at erection.
The consequences of the above items lead to some fissuring in the steel and some potential corrosion stress disorders. The masts were considered unfit as the cracks over time evolved and it was decided to remove the masts before they caused harm to persons and material damage. Furthermore the calculations on full checks including pdelta did not meet the requirements (at the time of the project ie 9 years ago) in terms of maximum deflection under wind load on the assumption that all the welds were fit for purpose (which they were not). The calculation under the current wind loadings demonstrates a shortfall in the material resistance by around 25%.
The value of guidance notes is essential particularly on very slender sections and engineers straying into the field of very slender sections beyond those recommended in the previous code need to consider fully the consequences of doing so.
In France, where I am practicing, calculations are generally extremely close to the resistance of materials, . The problem has been demonstrated however that if the full calculation is not done, or that the site follow up and construction is not as rigorous as the full calculation then the margin for potential collapse becomes too small to accept as a designer Charles de Gaulle is only but one mediatic example.
As such, I believe that the guidelines in the code should be reinstated with the possibility to take the calculation to the level the engineer is competent to do so (ie calculation of secondary effects, consideration of QA procedures and their implementation, etc). Response b) then.
Regards,
Philip G
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It seems to me that the academics can't find an equation that gives the same answers as the old "rules of thumb" regarding minimum slenderness, so they ignore it. The old BS449 limits should at least get a mention in the margin as a guide.
RAD
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I'll support for (b)
Krishna
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Have you contacted Colin Taylor about the background to why the limits were removed?
There may be practical limits that one would apply but if you have a reason to want to use a very slender one at well below 10% of its squash load then why not?
R P
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b)
R R
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I support
option b
Regards
C S
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c)
W H S
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I consider
that some form of guidance should be introduced into BS5959:2000.
After all we regularly ignore minor effects which can suddenly become not
minor with very slender members. What about robustness?
S M
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Option B preferred
No chance of going back to BS449 I suppose!!!
At least that code hardly ever changed, and still nothing fell down
T T
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Dear Martin
Rather than opinion I would make comment based purely on my experience as a Chartered Engineer who is officially retired but kept active sorting out the result of Graduate Engineers errors. But that is another subject much aired in the Verulam column of the Institutions Journal.
I personally believe that modern day engineering design is too accurate! How many times have I argued with Local Authority Checking Engineers that rounding down a hand calculated stress of 0.401n/mm2 to 0.40 is not a fiddle but takes into account the arbitrary nature of the load assessment. The great belief is that computer generated calculations are more “accurate” than hand driven and are “more correct” and “infallible”. Computers are pushing Engineers into the belief that they can be more accurate in design and therefore economies can be achieved by reducing the material required. Each nibbling away at the code in my view is reducing the commonsense element of Engineering and increasing the opportunity for failure.
On the specific issue that you question, does it matter? At least 95% of design is run of the mill and the excessive lateral buckling that appears to give you concern is not applicable. The other 5% will generally be part of some prestigious development when all aspects of the engineering design will push the boundaries. Then economies and prestige apparently are the guidelines used to control the design! A bold statement I know but when hasn’t the developer always pushed for that little bit more.
I suppose I am saying that the deletion of slenderness limits from the code is not really a bad thing as compliance now devolves to the expertise of the designer and goodie goodie, the Checking Authority will not have a table on which they can hang their hat of rejection. On the other hand in ten or less year’s time, the “new graduate” will not have a yardstick to measure against. He will just prod numbers into a computer and accept the answer that comes out. Will the Programmer then have been a sufficiently competent Engineer to put the correct numbers in to cater for all cases. As an aside, I had a simple beam deflection calculation rejected by a Local Authority as the “engineer” could not follow my method of design. The computer used to check the figure rejected the beam as over deflected by 0.01%. The method I used was basic in the extreme. I took the area of the moment diagram multiplied by the various lever arms, etc to establish the deflection. The reason for the difference was that I used the still permissible BS449 but the computer could only manage BS5950!
No, the table should stay. As one of the authors of the original BS5950 (one of my Partners) was heard to say “these are recommendations that are not set in stone and a competent Engineer should be able to read them in the manner intended. That is as a guide. As for the others, they either cannot read or are in research”
Regards
T G
A Stone Age relic who is still uses a calculator, as programs are too expensive!
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As a consulting engineer and frequent user of design codes, I entirely agree with your view that a limit on the maximum slenderness ratio should be given in the design code, for reasons given by you and other respondents. This is not to say that the limit in the old BS5950 is the most appropriate value.
I agree that re-instatement in some form should be made, by:
Using the old limit or a new limit;
As a second option, if the limit is not to be specified, then a warning clause should be added stating that for slenderness beyond a certain value, the effects of strut shortening due to compression and lateral buckling should be checked. A warning should also be given on reduced robustness.
Regards
Dr W W L C
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