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Dear Martin,
Thanks for your message. I have been following the discussion in Verulam but have not had time to get involved due to other commitments. (See forthcoming article on BS6399-2 for one.)
However I support your view that the slenderness limits which are in BS449 are sensible for a design code and should be reinstated in BS5950. I have done a lot of research into slender column behaviour (including developing a new method for the accurate analysis of slender columns made from non-linear materials such as reinforced concrete) and it is certainly true that from a theoretical point of view a weightless strut could be made in any proportions and it would also be possible for a strut with a finite weight to exceed the BS449 limits provided a careful analysis was made.
However this is pure academic hypothesising: the fact that such struts can exist in the laboratory or a computer model does not mean that they have any value or relevance to real engineering design. On one hand they are too fragile for use in real structures and on the other hand working with steel at such low permissible stresses is rarely economic, so they do not serve any real useful purpose. An experienced engineer would never use them and an inexperienced engineer should never use them.
The real mistake has been to remove the limits but not to insert a cautionary note drawing attention to the problems which can occur with excessively slender struts: the need to consider dead load stresses in great detail, lateral wind loads, vulnerability to accidental damage. Uut in the real world the role of BS5950 is to provide technical regulation in a fiercely competitive market where designers of limited ability working on limited fees with only limited time vie to produce the lightest, cheapest design which can be claimed to 'comply with BS5950'. In this situation, removing sensible slenderness limits will simply encourage people who don't know what they are doing to do silly things.
My own view is that the real need in the modern commercial world is for a much simpler, clearer, more defined code than BS5950 which will be clear enough to ensure fair competition and with sensible limits so that people are not encouraged to do silly things. (BS449 is actually very suitable for the job.) In this spirit, I would favour simply reintroducing the BS449 limits and telling the researchers who object to the theoretical impurity of this to grow up, remember that BS5950 is supposed to be a code of practice for design and go out and do some proper useful research, (They could start by working out a proper definition of column length for slenderness calculations: the definition in BS5950 Cl. 4.7.1.1 is completely wrong and implies that a cantilever column has zero strength.) If for some reason this is regarded as totally unacceptable, as a compromise I would accept leaving the formal position as it is but asking for a cautionary note to be added to the text as follows:
'Although in theory if compressive stress is very low it may be possible to justify an extremely slender columns or strut, the strength of such members can be may be very vulnerable to accidental damage and the effects of even small lateral loads (such as lateral wind pressure) can have significant effects on their strength. Therefore in normal structures it is advisable to restrict the slenderness ratio to one of the following figures (** insert BS449 figures) unless it can be shown that by careful detailed assessment of all relevant factors that a more slender member will perform fully satisfactorily.'
To answer your questions:
a) Support for BS5950 as it stands - no
b) Support for reinstatement of slenderness limits or similar - yes
c) Don't understand the question - no, I understand the question perfectly well
d) Other - no.
Best wishes,
A B
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Dear Martin,
As a South African Engineer I don’t use BS5950 but I would agree that it doesn’t seem a good idea to drop slenderness restrictions and would support the view that these be reintroduced
Regards
A N
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My reply may
muddy the water but I would suggest that the restriction is
removed as such, as in BS5950, but the values are re-instated as trigger
points that require more thought/calculations to justify going beyond the
guidelines. Useful for younger Engineers to warn them of possible dangers.
Regards
B A
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am comfortable with the original slenderness limits so I'll go for (b).
Regards
S B
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Dear Sir,
I am an Engineer with over 25 years experience of designing steel structures mainly for steelwork fabricators but for the last 10 years in my own practise.
The question of empirical rules for the slenderness of struts was, in my opinion, grounded in past experience for structures when design by computer was not possible and manual design was the only real way forward. The limits may have been conservative but they did minimise the chances of failure.
I have experienced problems with the design of slender struts due to wind excitation especially on exposed structures. The problems arose due to vibrations resonating through the building at relatively low wind speeds and occurred during both the temporary case for buildings under construction, which were then scheduled for enclosure, and for open structures such as multi-storey car parks. Fatigue effects then can become a problem.
Exposed feature (slender) struts have also suffered the same problem.
It is my opinion that designers would benefit from a guide which stated that:-
a) if you comply with the guide limits then no secondary checks need applying, and
b) beyond these limits you must undertake these additional checks such as self weight deflection induced bending moments, P delta effects etc.
In this way we could attain a reasonable solution for those inexperienced in the precise design of steelwork being able to achieve sensible results without resorting to Computer Aided Design and allow experienced practitioners the flexibility to optimise building designs should they wish to do so.
The overall savings for these changes must by virtue of the low frequency of such components in a typical building be small and yet the matter is being debated at some length.
Yours faithfully
Eur Ing W F B BSc CEng MICE FIStructE.
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Fortunately being based in RSA and using far simpler codes than imposed by the Eurocrats, we still retain slenderness limits in our standards.
As our engineering professionals can be somewhat cavalier, to say the least, elimination of any limits could prove exceptionally interesting here !
Regards,
B M
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I support the reinstatement of slenderness limits in Codes of Practice. However, I object to imposing such requirement in Regulatory Documents. Engineers should be given the freedom to use their knowledge.
CKL from Calgary, Alberta, Canada
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Dear Martin
I'd opt for (b) as a traditional engineer.
P C
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Martin,
My response is somewhere between b) and d). As technology develops, surely we should not go back to use BS449 type of elastic/working stress design method. I partially support b) for the reinstatement of slenderness limits but I also object to strongly discourage engineers to design above the slenderness limits. For example, large deformation of steel struts could dissipate a lot of energy and may be helpful when the structure is experiencing extreme dynamic loads.
BS5950 may recommend engineers to carry out further checks when designing strut members exceeding certain slenderness limits.
Regards,
cyho
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My only comment on buckling of compression struts was
a collapse I studied with Tony Ganju many years ago which
Tony thought was "snap through" buckling. Tony produced
an interesting text book on the phenomenom, and he may
have views on this if you ask him.
Regards,
D G
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Haven’t been following the discussion.
D C, Professor
Dean
Faculty of Engineering
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Dear Martin,
Our (Alexander Scott Ltd’s) practice for the design of slender struts is as follows:
Main stanchions and major load bearing elements, Slenderness not to exceed 180
Minor stanchions carrying modest loading (where slenderness of 180 results in a disproportionate member size increase), slenderness not to exceed 250
Bracing members carrying wind load, Slenderness not to exceed 250.
Horizontally orientated slender struts are designed for P_Delta effects of axial load with self weight bending deflection – this can reduce capacity by up to 25%
Members subject to wind reversal; slenderness not more than 350.
In other words, we generally follow the old BS slenderness limits but modified in light of our on-going consideration of relevant issues. I personally consider eliminating slenderness limits to be unwise, being likely to give rise to occasional problems with robustness and unlikely to result in any significant savings for a well designed framework.
Since our firm sets internal standards, sometimes more stringent than the BS, we have no intention of abandoning slenderness limits, whatever the BSI may do.
Hope this helps, disappointing that the response has been poor. If our profession can’t get a message across due to apathy we only have ourselves to blame when codes and standards are unworkable, wrong or stupid.
More power to your elbow
David
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Hi Martin,
Good for you. It's too bad more engineers don't take action on issues facing the profession.
Anyway, I would stick to the old limits, or at least ensure that the strut that i designed was 'reasonably' robust, to allow for unforseen & unspecified accidental horizontal loadings, such as a surge of bodies in a Bar Area, that sort of thing. Nothing fancy, just ask myself, does it look right? I haven't actually checked a slenderness limit for a long time, but i've got 40 years design experience. It's a bit scary thinking of recent graduates working without this guidance on stiffness.
In answer to your question, i would support:
b) Support for reinstatement of slenderness limits or similar.
Regards, Richard.
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I have been following the debate and personally continue to work to the limits in the earlier version of BS 5950.
b) please Martin.
G J
Senior Engineer (Structures)
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(b) for now.
regards
G M
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Hi Martin,
Thanks for the effort.
I support option b) stated in your e-mail.
Why is I Struct E not supporting your motion in this matter??
Regards
G C
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Hi Martin
I agree, and pick b).
It was good enough for BS449 and BS5950. Pt1:1990
Thanks
Ian
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Dear Martin
I support your campaign to have slenderness limits set for structural steel
struts. My concern, based in part on structural testing for my MSc, is about
loss of strength associated with out of tolerance straightness - are the
calculation methods that accurate?
The problem you will find is that BSI is now refusing to revise BSs except to
bring them into line with Eurocodes. I would suspect that the reason slenderness
limits have gone from BS 5950 is because they are not in the steel Eurocode. If
I am right getting the Eurocode changed will be orders of magnitude more
difficult - there doesn't appear to be a mechanism in place to do this yet.
Best wishes
Dr J B
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Martin,
a) There is nothing wrong with continuing to use the "old" limits, as
long as they are recognized as "good practice". Just because
something is not in a code it does not mean it cannot be used. A good
engineer is required to use his/her engineering judgement if
information is not codified. Codes do not indemnify the engineer
against negligence if the engineer can argue a code is "deficient".
b) I think you are also caught with respect to BSI over the
introduction of EuroCodes. BSI will now do as little work as possible
on BS's as they will effectively become defunct when the EuroCodes
come into full force. EuroCodes have to be reviewed and, if
necessary, updated every five years. It may well be appropriate to
target this rather than BS5950.
J P
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b)
Regards
K C
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Martin
I am in favour of reinstating the guidance. Without it there is a risk of
flawed designs, particularly by inexperienced Engineers.
Regards
Mark
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Having discussed with 2 other engineers we concur with b) one engineer uses BS449 anyway
M E
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Martin
I would support (b), the reinstatment.
M G
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Martin,
I fully support the view for the reinstatement of slenderness limits for
compression members to BS5950, Pt1:2000. If this clause continues to be
omitted then guidance must be given to replace these limits. I believe
that many problems will arise in future because of this omission. It's a
great pity that experienced structural designers are not, also, (that is:
designers with hands on knowledge), on these committees that review/draft
these codes.
Regards
Mike
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Martin,
The slenderness limits were a serious omission from 5950, and I thought that they were to be re-instated in the next amendment, if there is to be one before it is superseded by EC3. The UK NAD of EC3 does require the slenderness of struts to be limited to 180.
Regards,
R M
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Martin,
Thank you for your email. I have recently picked up on this in the ISE magazine and am pleased that a discussion is taking place. I would very much like the guidance as in BS449 to be reinstated. The problem I now encounter is that you could have junior engineers designing long compression members without appreciating the need to think about slenderness. In our projects we are very much aware of these issues and continue to pay particular attention but guidance would be useful.
The other area where I would welcome more guidance is where a column is of varying section properties along the length of the member.
Trust this helps you in a worthy effort.
Regards,
M P
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Well Martin, not sure how this email came to me but here a response.
Personally the main objection I have is the reliance upon second order analysis to ensure elastic instability doesn't become a problem. As Engineers we develop a feel for the solution, this feel is developed through years of sizing elements based on thing like basic slenderness ratios. What are the new graduate engineers learning apart from how to let the computer calculate the 'elatsic critical load factor' by pressing a button - probably how to design an accident waiting to happen. On the reverse side, it does take the handcuffs off the Engineer and allow designs to be developed that theoretically will work but are outside the bounds of the old version of the code.
Personally I'd be in favour of a simple version that adopts slenderness limits and an appendix that permits second order analysis to override these limits in certain situations.
On a sceptical note, think of all the additional software that has been sold since the introduction of this code.
Hope this helps.
P V
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Martin,
Put me down for b).
PD
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Thanks for your email.
I support the reinstatement of detailed guidance on slender struts.
Regards,
Phil
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Dear Mr. Martin Double,
In my opinion there must be an upper limit to the l/r Slenderness ratio, well below the Euler Limit, to take care of built-in eccentricity on account of manufacturing and erection defects and the non-isotropic metallurgical composition of the material and property variations frrom point to point.
In the absence of new experimental data, the old provisions of BSS should be restored.
Regards,
Dr. R V. B, PhD ( Civil), MIStructE, F.ASCE, PE,
SANTA CLARA, CA
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Martin
You can put me down as a b).
No need for me to state the reasons I think this as they are obvious.
It's good you are doing this.
Rob Struthers
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The formulation of the P-Delta and p.delta issues is not the whole issue as the equations used mask the residual stress issues and other criteria. My greatest concern is when very slender struts are not used vertically and thus have additional out-of-straightness from a gravity-induced bow or even when used vertically they are moved laterally by wind effects.
The Eurocode formulation is more general in that it allows/expects the designer to model the P.Delta and p.delta effects explicitly. Good in theory but one is reduced to using something like Perry and Robertson type factors anyhow unless one has a FEM model that formally allows for residual stresses!
RJP
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Martin,
My opinion is (b).
Regards,
R H
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Martin,
I have not practised structural design as a discipline for a number of years and I am afraid I have not followed the debate in the Structural Engineer, so I think that you will have to put me down as
c) "don't understand the question".
Regards,
R R
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Dear Mr Double,
Re your e-mail, I support the reinstatement of slenderness limits.
Although load tables are published for members in compression, I am concerned that without applying slenderness limits limits there may be a risk of under-design, with the potential for possible collapse.
Indeed, how are the load tables arrived at if slenderness limits are not applied?
I trust this is of assistance.
R T
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Martin Double
CADOSS
greetings
probably go for
b) Support for reinstatement of slenderness limits or similar - 45%
but having trouble with your website :- http://home.btconnect.com/cadoss
can you fix it
best regards
G R
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would like to see slenderness limits reinstated.
regards,
R R
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My response
is
b) Support for reinstatement of slenderness limits or similar
It appears a degree of caution on robustness of structures should have been
included (as has been for years) by the Code Committee.
This is to ensure struts are not "too whippy" (new technical term!!)
R T
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Martin
I have followed the discussions in the structural engineer over the past
weeks and must admit to not being too worried one way or another. I am
though selfishly looking at it from the point of view that as experienced
steelwork contractors employing experienced engineers who look after our own
graduates that we often size members by what feels right rather than just
what calcs prove ought to be right.
We usually would not go beyond slenderness ratios in previous codes but in
some instances we might. I see the point that some engineers are winning
work by producing what they believe is minimum weight design hence minimum
cost without thinking about connectability or what looks right, so in this
respect i would be in favour of bringing back reasonable limits which would
go some way towards producing robust designs and put us on a level playing
field when quoting design and build projects.
I hope the above will be of assistance.
Regards
S M
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The slenderness limits in the BS 449 and something similar to the RC codes should stay. Something which can become a "cantilever" is bound for failures and I support the reinstatement.
S C
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Hi Martin
I haven't looked into the details of the question but...
The absence of guidance/rules might be an opportunity to go return to the 'good old days' where we have to use broadly accepted methods of analysis tempered with good engineering judgment.
Euler buckling loads with a suitable factor of safety followed by a step back and a question on the lines of "do I really want a 152 UC 4m long carrying that floor?" would be my suggestion
S W
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Dear Mr
Double
I back the code as stands i.e. option a)
When I was working for a steelwork fabricator in the UK we came across
situations
where a strut/tie was carrying no meaningful load but we had to limit the
slenderness
ratio to conform with the old version of 5950.
Also, on principle, we should leave these decisions to engineers instead of
expecting the code to define the limits. Any reasonable engineer should
know what looks right and adjust the size accordingly!
S H
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Martin,
Unusual to be spammed by a useful e-mail...
I still carry out the checks in accordance with the old revision of the code, after all the codes are guidelines so I believe if one is lacking in information then rooting out alternative codes for guidance is acceptable.
However I do see this as a problem for people who are unaware of the previous codes and what they entail.
To avoid confusion then I guess I'm going to plump for b).
Regards,
S R
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Martin,
I agree that guidance on limits should be included in the BS.
Regards,
Stewart
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I still work to the same slenderness limits of the previous code
and in my opinion these limitations or similar should be reinstated.
S H
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(b) - without
going into full technical research on this subject the
slenderness limits set in the past have always served us well, as far back
as the BS449 days. Perhaps a limited increase in threshold to permit for
plastic design parameters?......!!!
Regards.....Stuart (M.I.Struct.E)
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b)
T S
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Martin,
Thank you for your email regarding BS5950 slenderness limits. I have read with interest responses from other engineers and can see another problem associated with this. I personally have always used BS449 so I am in total agreement with limits for slenderness.
Assuming that BS5950 stays as current and I as an engineer use my judgement and expertise when designing a specific building or element and decide, for example, that I am OK with a slenderness ratio of any number greater than the previous BS5950 values, and then make a submission to the Local Authority for the necessary Building Regulation approval, where do I go when another engineer, who is checking my calculations and also has good judgement and expertise, refuses to grant approvals because he has a different view? My client will possibily see this as bad judgement on my part and probably want some recompense if I need to make changes that may cost him more.
At least with a set of practical and sensible rules everybody is 'singing from the same songsheet'.
Regards - Tony C
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Dear Martin
I do not use BS 5950 very much, as I work in the power transmission industry and we have our own specialist codes.
However, we have limits on members for design of lattice towers, (ie max l/r=120 for legs, 200 for bracing members, etc.)
As if happens I am at this moment undertaking design checks on some ancient towers (pylons) dating from the 1930s, some members of which have slenderness substantially above the above limits. These towers have given over 70 year's successful service and the design checks are to determine what strengthening is necessary to accommodate larger conductors. After all that service it hardly seems justified to increase section sizes merely to achieve these slenderness ratios!
In general I do not believe that it is justifiable to impose what are effectively arbitrary limits on slenderness, although engineers need to be aware of various issues concerning struts of long slenderness, for example:
They tend to be more vulnerable to lateral loading (accidental or otherwise),
Initial out of straightness will be more significant
P delta effects may be significant, and failure can be much more sudden than is usual for steel structures
They may be prone to vibration, wind induced or otherwise.
There is also the point that struts of high slenderness will be structurally inefficient, and thus usually uneconomic.
I suggest there is a case for adding guidance notes/recommendations along the above lines, but would not support absolute limits.
I hope this is useful.
Regards
C T
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B!
T M
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