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Resisting Torsion Due to Guy Wire Anchor Attached To Roof Framing

Eng_Struct

Structural
Sep 23, 2022
61
Hi Group,

I need to provide guy wire anchors to support the mechanical duct for wind uplift. The wires will be installed at roughly 45-60 degree angle and the guy wire anchor will consist of a round HSS tube with eye-bolt and base plate that will be welded to the roof beam top flange.

There is no access from below to install any additional members to take the torsion out. I wanted to confirm if the anchor is placed close to the joist shoe, is there a way for me to resolve the moment into axial force into the joist?

I have tried resolving the moment into horizontal tensions and compression force at the top and bottom flange - the top flange will be OK as it is restrained by the joists however there is nothing restraining the bottom chord for the entire span length and the horizontal force from the anchor moment will reuslt in a huge moment in the bottom flange bending in laterally.

Any suggestions or ideas will be helpfull. I have attached a sketch to explan the condition.

Thanks in Advance!
 

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  • Roof Anchor Sketch.JPG
    Roof Anchor Sketch.JPG
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My gut feel is that you'll want to find some way to add kickers as @HTURKAK suggested, even with the access problems.

Even if you can moment connect the post to the joist seat, I don't imagine that the top chord of a 12" / 17' joist has much moment capacity / bending stiffness to work with.

Conceptually, I buy @Tomfh's proposal. That said, this feel like a situation where precise control over the cable angle would be a challenge. So, if you go that route, I would at least estimate a reasonable tolerance on the cable and investigate the beam for the amount of torsion that would imply. Perhaps that is workable.

You could engage the joists by adding HSS frames under the roof deck for that purpose and welding the posts to those frames. Similar to RTU support frames. That said, you'd have to pull off a lot of deck to manage such a solution and I would have to think that kickers on the bottom would wind up less costly than that.

Are you sure that the beams don't already have kickers? Given the proportions of things, I would have expected at least mid-span kickers for uplift etc.
 

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