Re: Concrete Encased Hollow Circular Steel Section


"Tom S." <tdscanuck@xxxxxxxxx> wrote in message
news:JIGtIy.G0t@xxxxxxxxxxxxxxxxxx
<jrm2002@xxxxxxxxxxxxxx> wrote:
For a concrete encased hollow circular steel section (compposite
section), the Eurocode4 says that its section axial resistance may be
enhanced to the effect of confining provided the non-dimensional
slenderness(which is the ratio of plastic resistance to axial
compression to the elastic critical buckling load) is gretaer than
0.5.That is, the resistance need not be enhanced if the non-
dimensional slenderness is less than 0.5.

My questions:

1) But, if the non dimensional slenderness is greater than 0.5 , it
means that column is all the more slender, right?

Yes. The numerator (plastic resistance to axial load) is fixed by the
column cross section. It is length independant. The denominator (elastic
critical buckling load) decreases with increasing length. So as your
slenderness ratio increases, the denominator must be dropping, which is
equivalent to an inceasing length.

In that case confining effect should become less, right?

I'm not sure about that. The confining effect is basically taking credit
for the concrete providing support to the steel.

I think what's going on is this: in order for confining support to happen,
you need to get enough deflection for the steel to "notice" that the
concrete is there and supporting it. For a very short (i.e. very stiff
column) the deflections will be very small, so the steel can't really see
the effect of the concrete before the load gets so high that you fail
plastically. For a slender column, the deflections will be much larger at
lower load and the steel may pick up more stabilization from the concrete.

Tom.

The new AISC Steel Construction Manual, thirteenth edition, has total
composite specifications for design and analysis. It also has composite
properites for circular composite sections and capacities with either
'Allowable Stress Design' or 'LRFD design. It also includes checking for
buckling and slenderness ratio.

CID...


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