Bracing and Secondary Members

[JLNJ]

Up through the 8th Edition, the AISC spec allowed an increase for "axially loaded bracing and secondary members when l/r exceeds 120". I do not see this provision in the 9th or 13th editions, though I may be missing it.

I have two questions.

Would any of you use this provision when looking at the capactiy of an existing installation of 7th Edition era?

If the answer to the first question is "yes" and if the member in question is a main compression diagonal in a 200' span "joist" (10' deep truss, really), would you consider it a "secondary" member?

I have my own feeling about this but in the office here we are not all of the same opinion.

 

[kslee1000]

Without the diagonal, will the truss function? How could it be secondary?

 

[271828]

I wouldn't use it and I don't know of anything post-89 that allows this.

Like kslee1000 typed, the truss might completely collapse (assuming statically determinate) with the loss of this one member.

 

[warrenw]

JLNJ
I think you answered your own question when you called it a "main compression diagonal"

 

[JLNJ]

I am in the camp to not consider such a provision, but I play devil's advocate.

With chord continuity the truss will not be unstable simply because a diagonal is cut or has has limited capacity. As a roof joist there might be a chance for some load redistribution to adjacent joists. One assumes pinned ends on a diagonal but there is usually some restraint provided by the connection.

 

[csd72]

In my opinion, A secondary member is one in which its failure would only result in its own collapse (or a very local failure) e.g. a floor joist.

Any other member should be considered a primary member.

That said, I would consider a truss diagonal part of a built up member and not a separate member in itself. You would not consider the web of an I beam as a separate member to the rest of the I beam even though it performs the same function as this truss diagonal.

csd

 

[kslee1000]

To me, a secondary member is some thing rudundant in a structural system. The failure of such element shouldn't result in drastic increase/change of stresses of other members, even the system may not fail under such circumstance. Please note lacing is quite different from truss members. The former is acting more on the stability concerns, the later is a true Main Force Carrying element.