Determining Slenderness Ratio for Built-Up Section

[BridgEI]

I am currently performing the structural analysis for a historic Warren truss bridge. The top & bottom chords are straightforward being effectively a built-up channel and two angles connected by lattice plates (in tension) respectively. The interior diagonal members, like the lower chord, are built-up sections tying two angles together using zig-zagging lattice plates. The Manual for Bridge Evaluation, 3rd Edition (2017) has a section in Table 6A.6.7 that discusses adjustment factors for computing an adjusted value for L/r to evaluate the column's slenderness. This section only discusses members that have batten plates and only batten plates on both sides or on one side with the opposite side having a solid plate. I have looked in the compression section of the AISC SCM, and I have read Chapter E & the commentary for the section on compressive strength of built-up sections. Chapter E & the commentary both discuss very specific criteria for proportioning in design that I have no control over in this evaluation. Furthermore, the requirements of those sections discuss the use of slip-critical bolts and welds, but nothing about rivets.

I would think there would be some adjustment for members of this type, where lattice plates connect parallel angles to act as a unit, but I don't think I can use the provisions in the MBE, and I also don't think I can use the specific design equations in the AISC. Can anyone help with this? Is there another guide I should reference? Does anyone have experience with this? I can keep going and reading literature on rivets and reading the references mentioned in the Chapter E commentary, but I would prefer to have someone tell me if that's a worthwhile venture before diving into that.

 

[MotorCity]

I would calculate "r" for the section by ignoring any contribution by the lattice plates.

 

[BridgEI]

I don't know how I would include the lattice plates/lacing bars in the calculation of the radius of gyration since the cross-sectional area of those piece is changing in position along the length. My intention was to neglect those members in that calculation. I would prefer not to have to treat the compression members as two unconnected single angles though since they are intermittently connected.

 

[JStephen]

See if the attached is of any help. This is from Structural Engineering Handbook by Gaylord and Gaylord, 2nd Ed, copyright 1968 and 1979.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1679331559/tips/Gaylord_Gaylord_i9nnzz.pdf[/url]

 

[BridgEI]

That seems to make the language in the AISC SCM a little bit more clear, but I may just have to go through and check the individual criteria and go from there with reference to the Chapter E commentary.