Slender Columns - Why not allowed

Why is there a slenderness limit for columns in codes of practice?

If the permissible safe load is reduced to allow for slenderness in the
first place why is there a need for any limit?

 

To prevent buckling (lateral failure in bending).
___

 

Codes with which I'm familiar often include "prescriptive" requirements,
which may be used when separate engineering calculations are not supplied.
Normally, such prescriptive rules provide design solutions that are more
conservative than those that would be provided by strict engineering
calculations. I don't know about the UK, but in many US jurisdictions,
certified engineering calculations are accepted in lieu of prescriptive
requirements, and visa versa.

If I understand your comments correctly, British Standard 5950 no longer has
a prescriptive slenderness ratio, which, indeed, is "daft."

Perhaps I'm missing something here, but Martin's original question was "why
is there a need for any limit?" -- to which my answer would be "for
prescriptive applications."
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"...taken into account when calculating the compressive strength...."

Because it's not a compressive problem, it's bending problem. Imagine a
slender column with compression loading axially at the column ends PLUS
bending loading at each end as well.

Try it with a soda straw. You cannot fully compress the straw before it
buckles!

WDA

end

 

Martiefficient

Because slender columns are flat-out nightmarish to construct and the limits
keep you in the bounds of reality while sitting at a desk.

Same goes for flimsy-ass plate girders.

So you design a slender shape that is braced to hell and back in the final
configuration - tell me, how do you pick it up from the truck in full
bending without expensive bracing. Next, supposing that you actually get it
up there, how exactingly plumb must it be? Are the plumbness tolerances
such that I have to spend an extra 1/2 hour with my rasing gang? Lets see:
Foreman, four ironworkers, an operator and oiler, PLUS the crane,
supervision and engineering for all the special picking devices. Way
expensive. You can buy a lot of dumb steel for that.
Second, and more importantly, is safety. The flimsier the section, the more
creative the designer is with bracing, the more likely the failure. You
can't (or I won't) put a cost on that.

You are quite literally tripping over dollars picking up pennies when you
make things unstable in the construction (i.e., unbraced) phase.

Overefficient design regularly leads to inefficient structures.

 

Tony

You have sussed me!

Explaination:-
I was about to post a summary of engineers responses to my original comments
on slender struts but found that I had too few replies (and too many not
understanding the problem) to make a fair analysis. So I thought that I
would provoke a few more replies.

You are also a mind reader it seems, I have often visualised leaning on a
column and considered its consequence. Your example provides yet another way
of looking at the problem of slenderness. Thanks.

I had actually hoped for a wider discussion on the subject which could have
lead on to slenderness of beams, and the problem of limit state design vs
permissible state design with respect to analysis of slender members. But
that will have to wait it seems!


Regards

Martin Double
CADOSS

Email:
Web Site: http://home.btconnect.com/cadoss

 

Death? :-((

 

Sure you can. You just buckled it.

Bucling does not equal crumpling. If a column is unstable beyond it's Euler
limit, the entire member will buckle in simple sinewave-like failure modes.
If the column is within it's Euler limit, the section fails and it crumples.

However, when you start talking about the inevidable bending moments from
real world connections, slender columns perform poorly.

 

Another nice one Tony.

It seems that these slender columns have more to answer for than I first
thouht!


Regards

Martin Double
CADOSS

Email:
Web Site: http://home.btconnect.com/cadoss

 

I have been wondering how all the so-called proto-Aeolic capitals unearthed
recently in Palestine were found in secondary locations. May all the
capitals have been salvaged from the Philistine temple destroyed by Samson.

 

p-delta effect!