Analysis of Existing Rail Bridge - Built-up Channel Sections

[Pseudo12]

Hello,

I'm working on the analysis of am old rail bridge constructed from built up sections - the standard two channels laced together with straps of metal riveted to the toes of the channels, similar to the image I've attached (this isn't the bridge I'm looking at, but its close enough to get an idea). Working on this, I realized that I can no longer remember what reference I based my understanding of the system on. Intuitively (or maybe just based on my aging memory), I've taken the channels to be braced against buckling individually, with no contribution to cross sectional strength made by the strapping. That is, I consider the channels to act as a unit, but I don't allow the strapping to otherwise contribute to the system.

I was wondering how others around the forum think of these built up sections, and if anyone has a reference that discusses the historical and current method of analysis of one of these sections. I'm reasonably confident in my approach, but its really bothering me that I can't seem to locate any supporting documentation either in my old notes or around on the internet.

 

[spsalso]

Those are called "laced struts" or "laced ties". See mention in the below article.

https://en.wikipedia.org/wiki/Lattice_girder

I just did a search for "loading of laced strut" and got lots of reading material.



spsalso

 

[BridgeSmith]

I think you're on the right track with assuming the lacing for stability/bracing only. It would likely take a fairly detailed FEM to get a reasonable approximation of their contribution to the axial capacity, and it likely wouldn't be much. If you were to use the lacing for axial strength, you'd also need to inspect and evaluate the condition of all the rivets connecting the lacing to the channels, before you could say with any confidence that they will contribute in that way.

What we learned from the collapse of the I-35W bridge in Minneapolis, is that we shouldn't assume the connections of the truss members (gusset plates and rivets) are stronger than the member. The capacity of those will need to be checked, as well.

It looks like you have a big job ahead of you. Enjoy!

Rod Smith, P.E., The artist formerly known as HotRod10

 

[bridgebuster]

Typically, the lacing bars are ignored in the column analysis. AASHTO provides guidance for lacing bar spacing etc.

 

[Pseudo12]

Ah, thanks everybody. I was struggling to find much information in searches, but I was being really stubborn about insisting that the columns were "strapped" rather than "laced" when I was looking for documentation which seems to be the major disconnect. Cheers!

 

[Lomarandil]

There is some research from Caltrans for the bay bridge that better quantifies the limits of when lacing is effective for bracing the member axially, and what other contributions (torsional capacity, etc.) it can make:

https://www.researchgate.net/public...d_Members_of_San_Francisco-Oakland_Bay_Bridge

But in most cases, considering the channels as a fully or mostly composite section and neglecting the other contribution of the lacing is about right. After all, that was the original design intent.

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just call me Lo.