Strapping To Secure Ductwork To Supports

[spats]

I have a problem I'm having a hard time visualizing. I'm designing supports for mechanical ductwork above an existing roof. The design of the stands is easy, but I also need to show a method of securing the ducts to the stands. I'm told the method they would like to use is steel strapping around the ductwork. This strapping would be potentially totally vertical, or at a slight angle to the vertical, so the connection can clear the duct. See attached detail under development.

I did a web search and found information for what is called "top-over lashing" used in the transportation industry. The principle appears to be tensioning of the strap and having good frictional resistance between the load and the surface it sits on, thus creating sliding resistance as long as the straps have the proper strength. Obviously in my situation the straps can't reasonably be tensioned, plus it would likely crush the IMP duct. However, even if not tensioned, it seems to me that the duct could only shift under wind load to the point where the leeward strap becomes vertical, and then the strap would go into tension, and everything would be fine as long as the strap and it's connection was strong enough, even if the actual connection was not physically in the way to stop it. I then began wondering how it could move at all if the straps were completely vertical. Assuming the duct doesn't deform, I don't see how it could move. Am I missing something? Does someone have a better way of doing this?

 

[BAretired]

The duct can shift under wind load. It needs a stop at each edge of the duct to prevent the duct from shifting. If you can't tension the strap without crushing the IMP duct, the duct will crush under tension from wind. The duct needs to be strong enough to resist a reasonable amount of tension.

I feel a bit sensitive about this issue since one of my downspouts blew off because the strapping gave way and squashed the pipe. Total pain in the butt.

BA

 

[dhengr]

Spats:
I don’t have any perfect answers, just a bunch of questions.
1. How far will the duck work span btwn. supports, under gravity and lateral loads? There’s some potential for uplift too. How does this compare with the struct. framing spacing?
2. Do you need corner protection under the straps, at the upper corners? Maybe a couple 3x3 angles, 1' long, horiz. legs in and vert. legs down, with something to capture the strap? For strap tension and potential uplift.
3. 3x3 angles, 1' long, vert. leg up and horiz. legs out on top of the L4x4 would hold the duct laterally, without the strap having to move. And, if the duct can move laterally, won’t your strap bolts wear a hole in the insul. or duct. Can you get enough vert. adjustment/tightening in the strap, the way you have this arranged.
4. Why not another L4x4x1/4 over the top, punch them both at the ends for a round rod, threaded both ends which has some vert. adjustment. Fab. twice as many of the same angles and do away with the strap, another different part.
5. How well do lapping x-bracing angles weld to a small round pipe? You’ll get four tack welds, likely to crack. You can at least get a good 3" long weld from the end of the angle to the center of the pipe on your bot. horiz. angle.
6. It is really difficult to find std. pipe sizes with i.d’s. and o.d’s. which match well enough to allow telescoping inside one another. You have to go to mech. tubing, and often still do some machining to boot. Do the telescoping the other way around, shingle fashion, so it tends to drain, not take on water. These types of things tend to take on water, however hard we try to detail otherwise. Put some drain holes in the legs, but where? At the top of the struct. beam where it is already in the bldg?
7. I think I would use to short pipe legs, with base and cap pls. on them; or a light channel, legs down on top and replacing your horiz. angle. Then build you legs on top of this. Place the short legs in two pitch pockets. There are also some good flashing boots like those used on plumbing vent stacks and electrical masts.

 

[TomBrooklyn]

1. By IMP, do you mean Insulated Metal Panel?

2. What is the crush resistance strength of the duct?

3. As an aside, if known, what is the gauge of the metal and the thickness of the insulation?

4. How about placing a piece of sheet metal wider than the hold down strap under the strap to more widely distribute the force from the tensioned straps? This piece could wrap around the top corner to distribute the force along that line too.

5. How much force do you anticipate the strap to create?

6. How is the strap likely to be tensioned, anyway?

 

[TomBrooklyn]

By the way, Re: Flashing Dimension: Min. As Reqd.

Different contractors may interpret minimum dimensions differently. A roofing contractor often prefers to have more room to flash than some other trade contractor who's interest is minimizing his labor and material thinks is necessary.

I'd go with 6" as the minimum, but a little more would be nice.

 

[spats]

Thank you all for your responses. I have been out-of-pocket for a few days, and didn't have a chance to respond back. I decided to take dhengrs' suggestion of an angle at the top with threaded rods, and an angle stop each side. See attached. Tom... yes, I'm talking about insulated metal panel ducts. they are 3" thick, with 26 gauge skins. I instructed them to tighten the rods snug, making sure not to deform or damage the duct.

 

[BAretired]

Much better idea Spats. It looks good.

BA