Back to Back Angle Bending 1

[SteelPE]

I have been contacted by a steel fabricator to assist them in the design of supports for some sprinkler pipes in an old gymnasium. By some chance, they have sent me the old structural drawings from 1949 which give us the beam sizes and spacings for our supports. The existing roof decking is wood 2x framing which is supported by steel beams spaced at 18'-0" o.c. We are not being asked to verify the existing structure as the loading is intended to be light as the sprinklers will be quite small (thing 1 1/2" dia water filled pipes.... on the order of 1 psf).

The fabricator is proposing using double angles bolted back to back for support of these pipes. These members would span the 18' between beams and support a pipe with a weight of approximately 4plf (not a lot of load). The client is trying to minimize the weight of the members used as access is limited and everything needs to be carried and placed by hand (which is what they are telling me). In doing some research into double angle bending in accordance with AISC 360-16 I came upon section F9.2 equation F9-12 which requires torsional property J for this double angle section. Is there any reference that provides this torsional property for double angles?

 

[EngDM]

CISC published this a while ago. In the angles section on page 8 they say for double angles the values for torsion and warping are double the single angle values.

You can of course bring this into an FEA section properties calculator to find it as well, but taking double is conservative unless you do everything in your power to make sure they act compositely (bolting the legs together alone likely won't do this from a warping standpoint).

 

[rb1957]

"back-to-back" angles ... to make a Tee or a Zed ?

 

[SteelPE]

rb1957,

back to back is in a Tee (I believe). I am not exactly sure what the fabricator is proposing.

EngDM

Your paper is certainly good enough for me. I wasn't sure if you would use J or 2J for this. Funny how the AISC doesn't really address this (or maybe they do and I just couldn't find it).

 

[KootK]

For for something of this scale, I would be inclined to just support and fasten the angles together often enough that individual angle torsion was functionally a non issue. Angle torsion is annoying for production work.

Something like this with the hangers at 18' oc?

 

[SteelPE]

For for something of this scale, I would be inclined to just support and fasten the angles together often enough that individual angle torsion was functionally a non issue. Angle torsion is annoying for production work.

Something like this with the hangers at 18' oc?

View attachment 7731

I am not sure if the plan on doing hangers or not. Not exactly sure of the detail they plan on using. I envisioned the section being rotated 180 degrees with the web in compression, but then again, that doesn't make sense as that is the section that you want to brace the most.

At this point, I am just trying to get out in front of this so when it does come in I have a good plan of attach on how to handle the issues.

 

[KootK]

You didn't ask but it's common knowledge that I can't keep my nose out of what feels like it might be a steel detailing challenge. I vote for getting out in front of this by convincing them to do it better. You'll have to sell it. And that's where you exert your real value.

 

[SteelPE]

The reason whey they are looking to bolt two angles together is for weight purposes. They have to had carry everything into the building and then do all of the work off of staging (no lifts allowed). A tube would be my first choice of section, but not sure if I can get one under the weights they have proposed for single angles. I would be looking at a HSS5x3x1/8" LSV (not sure if you can even buy that).

I have contemplated using a cold formed girt. Based upon what I can tell, I can get a 8Cx3.5x12ga girt to easily span the 18' unbraced with the moment capacity required.... and that weighs less than what they are proposing (probably costs less too).

 

[KootK]

but not sure if I can get one under the weights they have proposed for single angles.

Right, but that assumes that their single angles work. The cold formed girt sounds slick though.

 

[SteelPE]

I am anticipating the pipes to weigh 4plf..... so not overly a lot of load we are dealing with. At 18' we are talking about 162 ft-lbs.

 

[ANE91]

Wait, so the loads are tiny, the client is weight-conscious, and you're supporting pipes? How is this application not perfect for Unistrut? P1001 can span 20ft.

 

[Eng16080]

Does this have to be steel? With such a small load, you could use a (wood) 2x6. It's readily available and relatively light. Maybe not a better solution than Unistrut, but perhaps a reasonable option.