Lattice Spacing and Angle for Built-Up Sections in CSA S16

[pbc825]

I've taken on a design review for an old (pre 1970's) lattice bridge in an industrial complex with angle chords (2 top and 2 bottom) and lattice members. The top two chords are in compression and act as a built-up member. The scope includes reviewing the bridge in the context of the current CSA S16 standard. When comparing the bridge to clause 19 (Built-up Members), there are a few things that the bridge clearly violates. They are as follows:

1) 19.1.4 - Separated chord members are to be interconnected such that the slenderness of any component, based on it's least radius of gyration, does not exceed that of the built-up member. In this case, the spacing is set and the slenderness of the individual chords between interconnectors is close to double the slenderness of the combined section of the top chords. The questions here are as follows:
a) Is 19.1.4 a good practice clause for new construction that could be violated without concern in a re-rate condition, or is there a clear failure mode or reason to space the interconnectors closer together? We could potentially add a brace for the chords, but I think this is unnecessary as the overall slenderness of the built-up section considered in 19.1.4 is the RMS of the slenderness of the built-up section and the slenderness of the individual members between interconnectors.
b) When did the principles of 19.1.4 come into effect, and do you think there's a reason the original designer spaced the inteconnectors so far apart?

2) 19.1.11 - If I'm interpreting the clause correctly, the angle between the longitudinal axis of a chord member and axis of the lattice shall be greater than or equal to 45 degrees. The lattice members are in the 22 degree range. I have the same questions. Can I violate this, and why would someone design in clear violation of this?

I'll look forward to some enlightening responses.

 

[Shu Jiang PEng SE]

1) Setting the maximum spacing of the interconnectors to avoid the buckling (local) of the chord members before the failure of built-up member as a whole.
2) The angle determines the stiffness of the brace can provide (assuming braces have same cross section). Smaller angler mean weak support for the built-up chord member.

I think the requirements should be met in new design. if you are checking existing structure, you may need spend more time to check chord local buckling per actual interconnector spacing and brace angle.

 

[Lomarandil]

Existing latticed members often do not meet the requirements of modern specifications.

My belief is that this is primarily because modern specifications desire to be all-encompassing, and therefore tend to limit latticed members (among others) to the simplest rational forms -- while the original designers years ago were working from engineering principles instead, and fit their designs more closely to the project requirements.

(Also partially because of the labor cost vs material cost shift)

So, it doesn't surprise me that several prescriptive provisions are violated, and I wouldn't let that concern me -- I'd just parallel the original designer and work through it rationally.

Hopefully your client and contract will allow you to be reasonable about working within the intent of CSA if not the letter of it.

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The name is a long story -- just call me Lo.