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FTB of double angles 1

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NS4U

NS4U

Structural
Apr 2, 2007
320
I am checking flexural-torsional buckling strength of a double angle in a truss.

According to the spec, for double angles, I should following equation E4-2. However, there is also an equation for FTB of singly symmetric members (Eq. E4-5). According to the commentary, E4-2 is just a simplification of E4-5.

Therefore, I would expect that I would get essentially the same critical buckling stress from using either equation… but I don't. They are 10-15% different. Does anyone know why?
 
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I remember running into the same issue before. I don't remember the exact details. And, I don't think there was a difference that was quite that high.

But, I definitely remember getting a difference between the general formula and the singly symmetric formula.
 
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Double angles are singly symmetric whereas single angles are asymmetric about either the X or Y axes. A single angle can buckle about its minor principal axis whereas a double angle buckles about its X axis. The value of rx is always larger than ry'.

BA
 
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Actually, it has to with the fact that FTB in double angles is an inelastic buckling mode. The AISC critical stress curve is based on the tangent-modulus method for computing the inelastic buckling stress. This method uses a modified modulus of elasticity, which in turn modifies the shear modulus (which is used in calcing the torsional buckling stress).

This is not correct and overly conservative; research has shown that the shear modulus does not change when a member has yielded.

Therefore, for the special case of double angles (because warping is neglected), method E4.a is used to reflect that the shear modulus should not be modified by the tangent modulus method when determining the critical torsional buckling stress.
 
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NS4U -

Excellent explanation.... where did you find that information? The SSRC guide?

Josh
 
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Read the paper by Galambos, "Design of Axially loaded Compressed Angles" which is references in the commentary.
 
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