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Built-up up shape in compression

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I am evaluating an existing steel truss with a built-up top chord composed of a two angles and a plate between. The plate is sandwiched between the two angles to form a tee-shape. The plate and angles are fastened together with a single line of rivets. The stem of the tee (the plate) points downward and I have concluded that it is slender.

I have axial compression and bending (due to the direct application of roof loads). I am evaluating the combined stresses according to the 3rd edition LRFD, and have calculated a Qs reduction factor per Appendix B5.3a to account for the slenderness of the stem plate. The Qs factor is dependent on the b/t value of the plate projecting from the built-up compression member.

B5.1(c) of the Spec says that the "b" of the plate is the distance from the free edge to the first row of fasteners or line of welds. For me, that would mean b = 10.5".

I am guessing that this provision was written so that it would be valid regardless of whether the unstiffened element was connected to the built-up member on one side only (single shear configuration) or on both sides (double shear configuration).

In my case, the downward legs of the angle confine the stem plate, such that one might convince himself that "b" could be taken as the depth of the stem plate beyond the toes of the angles. For me, that would mean b = 9".

Does anyone have any opinions?

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