Hanger Rod Length
Hanger Rod Length
(OP)
Is there some type of guidance for the maximum length of all thread in a non seismic project without lateral support?
We have a project where a 3" copper domestic water pipe is suspended about 30' aff using clevis hangers spaced about 8 feet apart. The pipe is swaying about 2"-3" side to side. The water is pumped (~50 gpm) but the pump package is connected to the piping system with flexible connections so no vibration from the pump should be transmitted to the pipe. The all thread rod is about 8'-10' long from structure to hanger.
I am thinking this is too long of a length for the all thread rod, but cannot find any guidance for what length should start having lateral support.
Any thoughts?
We have a project where a 3" copper domestic water pipe is suspended about 30' aff using clevis hangers spaced about 8 feet apart. The pipe is swaying about 2"-3" side to side. The water is pumped (~50 gpm) but the pump package is connected to the piping system with flexible connections so no vibration from the pump should be transmitted to the pipe. The all thread rod is about 8'-10' long from structure to hanger.
I am thinking this is too long of a length for the all thread rod, but cannot find any guidance for what length should start having lateral support.
Any thoughts?





RE: Hanger Rod Length
Unfortunately I'm not aware of guidance either.
You've got an interesting case study on your hands, an opportunity to write a paper and create some guidance for others.
With such a long length of rod, sounds like it's acting like a pedulum and any sort of force sets it in motion. Vibration could be coming from piece of machinery on the roof/deck, wind load on the building, water hammer just to name a few.
As far as solutions, I can think of 3 options;
1. Raise the pipe to reduce the length of all thread.
2. Provide horizontal bracing.
3. Increase all thread size to increase rod stiffness.
RE: Hanger Rod Length
Most of the time our pipe is constrained by above ceiling space and/or is attached fairly close to the roof (slab above) structure so there would be some rigidity to the rod.
We don't have anything in our specs to address the rod length (do have minimum diameter and spacing). I have seen some guidelines/requirements when in a seismic scope, but that would be prohibitive in standard scopes.
RE: Hanger Rod Length
Dik
RE: Hanger Rod Length
RE: Hanger Rod Length
Like you, specs and codes we use provide minimum sizing and spacing, doesn't address extra long hanger rods.
I like your idea of adding the horizontal (diagonal) bracing, especially since you've got the space for it.
RE: Hanger Rod Length
RE: Hanger Rod Length
Use (y)our experience and common sense!
The real question is what is any domestic water pipe doing there.
Now you're going to spend all kinds of time and money supporting and antisway-bracing the stupid thing. It doesn't belong there.
Technology is stealing American jobs. Stop H1-Bs for robots.
RE: Hanger Rod Length
Where should it be?
RE: Hanger Rod Length
Technology is stealing American jobs. Stop H1-Bs for robots.
RE: Hanger Rod Length
I honestly do not know what you mean by "common sense".
Since I have not mentioned where this pipe is, other than what is shown in the video and my original description, I am not sure how you can say it should not be there.
If you mean the pipe should be supported higher (so the rods are shorter), I have found nothing in the applicable codes (Plumbing or B31.9) or other documents to indicate this. That is sort of the reason I asked the question.
This is not a seismic scope, so additional supports were not called for.
In hindsight, the issue is likely due to the rod length, but how long is too long?
Suggestions, guidance, direction is desired, not derision.
RE: Hanger Rod Length
Technology is stealing American jobs. Stop H1-Bs for robots.
RE: Hanger Rod Length
It was there because that was the shortest route from point A to B across an exhibition space in a convention center to get from a mechanical room to other areas of the building.
Moving the pipe would require draining it (it feeds a lot of the building and is currently in use), cutting the pipe and insulation, moving it, reconnecting the pipe and insulation, refilling, bleeding out the air, pressure testing, etc. That would cost a lot more than attaching some transverse supports off of the other structure at about every 30'. So moving the pipe is not going to happen.
Since the final elevation of the pipe was left to the contractor, that is why it is at that elevation. Why it was not higher is unknown. The specifications did not indicate a height requirement and it was installed in the space above other utilities so it was not a concern when installed. Maybe that was wrong, but that is hindsight. Standards that I have indicate rod diameter, support spacing for pipe material, but not rod length.
If this situation comes up again, the pipe would be required to be supported higher. Hind sight will tell me so, but I have no "engineering" to tell me, is a 3' rod too long, is a 6' rod too long, etc.? What do I specify so I can hold the contractor to that?
This is why I was looking for advice, guidance to determine how to engineer the situation in the future.
RE: Hanger Rod Length
If I made a pipeline like that, it might go right through the middle of 1000 people's houses.
Technology is stealing American jobs. Stop H1-Bs for robots.
RE: Hanger Rod Length
I don't believe there is because the number of options are too large.
I think in retrospect, to have any pipe hanging over any distance that is not inherently fixed laterally by either some clip or fixing to a solid object will mean that it has the possibility to move.
Clearly the longer and more slender the hanger is the lower the lateral force resistance is.
Now unless you go and analyse each pipe, these sorts of tings are just left to site run decisions based on practicality. For something like a seismic case then you probably do need to look at each pipe to make sure it doesn't move, bend, clash etc, but in most situations there is no real force to start pipes moving.
I suspect with that pipe you've just managed to hit the perfect storm whereby there is clearly very low resistance to lateral movement from the pipe itself, whatever excitation force / frequency has just happened to be close to the natural frequency of the pipe and its visible.
Maybe the way in the future to address it is simply to add in a minimum lateral force vs movement requirement ( X N force < Ymm lateral movement), but I've no idea how you would get those numbers, or just add a performance requirement -
"All pipes hanging from single vertical rods / hangers shall not move or sway during operation. Suitable lateral supports or bracing to be used to prevent sway or movement of such pipes."
Job done.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Hanger Rod Length
LittleInch: Thanks. This type of scenario will be on the "lessons learned" table and will get more thought/analysis in the future.
RE: Hanger Rod Length
Little expense, installation cost is the extra rod and the man-lift to get up to the ceiling.
RE: Hanger Rod Length
Maximum pipe movement? Probably best to write it as "Suspended pipes shall not move excessively under normal load at operating conditions without engineering analysis."
RE: Hanger Rod Length
As a rough estimate, I would guess that the lateral movement would be reduced linearly in direction proportion to the length of the rod (cut the rod in half, reduce the lateral sway by half)