Eccentricities in tension only diagonal x-bracing

[racefanf1]

I am an EIT and I am designing some X-bracing to stabilize a relatively small platform. I was using single angles back to back with a spacer plate to match the gusset plate thickness and using them as tension only. Sounds easy enough...

I was looking closer at the gusset plate to angle connection and there are some eccentricities in that connection (from the centroid of the gusset plate to the centroid of the angle) that I think will transfer some bending moment into the angles. I asked my very experienced mentor about it and he said he generally sizes the angles so the one leg welded or bolted to the gusset plate can resist the tensile force and he considers the other angle leg as a stiffner to the strap bracing (as he is effectively treating it as a strap). This design method seems quite reasonable to me and the eccenricities become so small I think they are negligible.

Just to satisfy my curiousity, I looked at combined tension and bending on an angle versus the strap bracing method and I came up with using a slightly heavier angle with the combined loading.

My real question is, what do you look at when designing X-bracing using single angles? Have you ever looked at the eccentricities? I know there are design procedures to take the compressive strength of the other angle into account, does anyone use that procedure?

Thank you.

 

[JAE]

I don't look at the eccentricities as they are fairly small. I do most certainly use Chapter B of the AISC spec with the U value, though. This takes into account the fact that ALL of the axial load is at first taken through the bolts which are only on the one leg (per your mentor) and then through a shear lag drifts out to the full section beyond the connection.

 

[racefanf1]

I too always use the U value when calculating the tensile rupture strength and understand it's purpose was to take into account eccentricities at connections (and I thought at connections only). My thought was that there might be some yeilding near the connections due to combined bending and axial stresses, but not at the connections. However, I didn't read the last paragraph of Chaper D Part 3, subparagraph 3 (in the 13th Ed.) that actually states:

Single angle, double angle, and WT sections shall have connections proportioned such that U is equal or greater than 0.60. Alternatively, a lesser value of U is permitted if these tension members are designed for the effect of eccentricities in accordance with H1.2 or H2.

So I guess if I would have read more carefully I would have answered my own question...sorry about that. I do thank you for your reply.