Hanger Rod Length 2

[PEDARRIN2]

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?

 

[dbill74]

WOW!! That is a long rod!

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.

 

[PEDARRIN2]

We are planning to provide horizontal bracing attached to some lower structure (about every 30') which will be between the clevis hangers.

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.

 

[dik]

Can you attach it with two rods at an angle from vertical. I realise that one acts as a compression member and is really slender. Or, can you add some mass to it?

Dik

 

[PEDARRIN2]

Planning on adding the horizontal (diagonal) at the lower structure and alternate the side of the pipe it is located. That should provide some resistance to the pendulum effect.

 

[dbill74]

That's a nice sway you got going Pedarrin. Is that an elevation change behind the electrical conduit?

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.

 

[mk3223]

It looks like a piece of the water hose except hanging in the air overhead. While the hanging rod to carry the vertical load of the pipe, it's a good practice to have lateral guides every so often to keep the pipe fixed in place. The swinging pipe may hit other objects and cause damage as well as water leak in the screwed joints.

 

[BigInch]

What would we do without those codes and specs.
Use 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.

 

[PEDARRIN2]

BigInch

Where should it be?

 

[BigInch]

Your common sense should also tell you that. Think about it for awhile and go tell them where to move it.

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[PEDARRIN2]

BigInch,

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.

 

[BigInch]

OK. I am trying to say that I would try to find out why a 3" pipe is up there hanging around on 10ft rods and try to move it in hopes that it would result in a better, more cost efficient and less embarrassing design.

Technology is stealing American jobs. Stop H1-Bs for robots.

 

[PEDARRIN2]

OK.

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.

 

[BigInch]

That's exactly what happens when you leave stuff up to "the contractor" to design.
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.

 

[LittleInch]

The original question was "is there guidance / codes / standards to deal with this situation.

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.

 

[PEDARRIN2]

BigInch: That is the nature of the beast of building piping design. We do not get the fee to design to the gnats rear end - so we produce drawings and specs that are tight enough to mitigate most issues, but sometimes things like this fall through the cracks. We have field support that prevent the piping from "going through houses", but again, this was outside the box.

LittleInch: Thanks. This type of scenario will be on the "lessons learned" table and will get more thought/analysis in the future.

 

[racookpe1978]

Use multiple v-shaped rods: each side of the V no more than 15 degrees from vertical.

Little expense, installation cost is the extra rod and the man-lift to get up to the ceiling.

 

[racookpe1978]

For guidelines in the future, the "length" of rod will not affect strength (load capacity) until the rod becomes so long that the weight of the rod is as significant as the weight of the pipe and fluid and insulation held up by the long rod. Unlikely to ever be exceeded.

Maximum pipe movement? Probably best to write it as "Suspended pipes shall not move excessively under normal load at operating conditions without engineering analysis."

 

[MotorCity]

There is a "chandelier" clause (I believe its in ASCE 7) that indicates pipes in high seismic areas are not required to have lateral bracing if suspended from a hanger rod of 12" or less in length (I paraphrased). So that would be your upper bound solution.

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)