FTB of double angles
FTB of double angles
(OP)
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?
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?






RE: FTB of double angles
But, I definitely remember getting a difference between the general formula and the singly symmetric formula.
RE: FTB of double angles
BA
RE: FTB of double angles
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.
RE: FTB of double angles
Excellent explanation.... where did you find that information? The SSRC guide?
Josh
RE: FTB of double angles