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Definition of secondary members

Definition of secondary members

Definition of secondary members

I am working on a steel box truss for an overhead sign structure. I am using the AASHTO "LTS-3" design code along with the AASHTO Standard Specs for Highway Bridges, since this is an ASD design (lucky me). I am also referring to the AISC code.

My question is about the connection for web angles bolted to gusset plates, which are welded to a tubular chord member. The AASHTO Std code § states "Bolted bearing-type connections ... shall be limited to members in compression and secondary members."

Can webs be considered as secondary members in a truss? I cannot find any solid definitions regarding "secondary members" in the AISC or AASHTO Std codes. The AASHTO LTS-3 code mentions it once in §1.9.2 .

The peak loads are about 50 kips tensile force, and the 'standard' design calls for only two bolts per web. As a result, I will require 1-3/8" A325 bolts if limited to friction-type connections. As the design requires 5x5x½ angles, this violates AASHTO Std code §, requiring the fastener diametre to be 25% or less of the leg width.

Thanks for you input!

RE: Definition of secondary members

Right off the top of my head I would suggest the following rough definitions:

Primary member:  Takes calculated stresses - on the load path of your structure.

Secondary member:  Does not take calculated stresses on the load paths, but simply provides bracing to the primary members to keep the primary members from buckling.

RE: Definition of secondary members

I agree with JAE.

Secondary members as you are probably aware can be connected with bearing type connections.  This should ease your 1-3/8" bolt issue.

RE: Definition of secondary members

Thanks for the feedback, JAE & Qshake! Your advice is appreciated.

This confirms my thoughts that the webs are essentially holding the chords apart, and are indeed secondary members.

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