Tangent Modulus and secant formula?

[40818]

Is it feasable to use the tangent modulus of elasticity rather than simple E in the secant formula for eccentric loaded struts? I believe that the secant formula is only valid up to the prop limit hence it should only be used by limiting the edge fibre stresses at most to Fty.
Cheers.

 

[desertfox]

Hi 40818

I believe you can use the tangent modulus of elasticity for stresses beyond the yield however below yield it is equivalent to the Youngs Modulus see this link:-

http://en.wikipedia.org/wiki/Tangent_modulus

desertfox

 

[stresscalcs]

The use of the Secant formula is limited to the cross section stress that you can tolerate (allowable stress) which may be very much less than yield in an unstable cross section.

The Secant formula delivers an end load which is related to a simultaneous section bending moment and the two loads together produce the cross section stresses. The limiting load of the Secant formula is the same as the Euler load (long columns). See Perry, Aircraft structures.

Should you have a section where stability is not a problem, opposite sides of your cross section might then be in the plastic and elastic regions. Theoretically this suggests a changing neutral axis position from the pure elastic situation and stresses are not so easy to predict.

The consequences of using Etan in column calculations should be fully understood.
I am not at all sure it is a good idea to use Etan in the Secant formula.