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# Fco expression ?

## Fco expression ?

(OP)
Has anyone seen this expression for plastic compression allowable ...

Fco = Fcy*(1+sqrt(Fcy)/1333) "for Al plate", Fco, Fcy in psi ... dates from mid-60s
used as a column allowable ...

attached the calc

### RE: Fco expression ?

Mil-Hdbk-5"C" should be your first hint that this has gone by the wayside.
It looks more like an empirical formula than some kind of exact one.
Is this for working out the transition from elastic to plastic buckling of long columns?
Does the end of the chapter have references to the source of this calc?

STF

### RE: Fco expression ?

Off top of my head it is probably the short column Johnson-Euler column stability cutoff value.

### RE: Fco expression ?

Fco is a parameter is the empirical column buckling curve used for "short" columns. For long columns, Euler's formula applies. For short columns, plasticity is a factor, and empirical formulas are often used. I think the more "modern" aerospace approach would use Euler's equation with the Tangent Modulus, where long or short is automatically included.

The transitional L'/rho value referred to is the transition between long & short columns. If your L'/rho is greater, you have a long column, which is evidently the case the author had, and you can use Euler's formula. Since he had a "long" column the author does not go on to show the short column formula, which would be a linear function of L'/rho containing Fco. The Johnson-Euler formula is another empirical short column formula. It is parabolic and is often used for steels, whereas a linear short column formula is often used for aluminum alloys.

I do not have a copy of MIL-HDBK-5C to confirm this. Newer versions of MIL-HDBK-5 do not seem to have these formulas. Similar (or identical?) equations appear in Bruhn, chapter C4.5 titled "Column formulas for aluminum alloy tubes". He gives different forms of the equations depending on the exact alloy.

### RE: Fco expression ?

(OP)
thx sdm919, I understand the use of transitional L/rho.

Yes, the Bruhn reference is where it is, LPS for you !

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