Chilled Water Line Hanger Rods

[Lutfi]

I am dealing with the following situation on one of the projects we designed. During punch list, we noticed that contractor supported chilled water pipes to the bottom of a sloping roof beam (the beam span is perpendicular to the roof slope; the beams are 16 inches on center). The contractor bent the all-threaded rod and bolted it to the bottom flange.

The detail we supplied utilizes an angle that is bolted to three roof beam bottom flanges and the all-thread rods were to bolt to the angle and we called for beveled washer under the nuts.

I do not think what the contactor did is a good practice since it will introduce bending to the rod. Our company is being challenged by the contractor and owner to leave it as is.

As I look, all I can find is the 2004 RCSC specification section 6.1.1 which reads as follows:

“Sloping Surfaces: When the outer face of the joint has a slope that is greater than 1:20 with respect to a plane that is normal to the bolt axis, an ASTM F436 beveled washer shall be used to compensate for the lack of parallelism.”

I would like to get my hands on a documented reference (established one such as AISC manual or publication) so I can show them written documentation why they should change it. Does any one have more details or references?

Please feel free to comment on this situation as well.


Regards,
Lutfi

 

[gwynn]

Can the rod take the bending forces introduced by the bend?

 

[Lutfi]

To do so we need to verify the size of the rod, grade, and the actual bend degree. The contractor did not submit calculations. We can do calculations. However, for liability reasons we prefer not to accept the installation since it is does not conform to our details.




Regards,
Lutfi

 

[Lutfi]

I would like to add that in my entire career of over 27 years, I never used a threaded rod to carry nothing but tensile load! I think this is what they are best suited for.

Regards,
Lutfi

 

[gwynn]

The liability issues I understand. I would consider giving the contractor the options of either fixing it to conform to the design or having an engineer qualify the change at his expense.

 

[JLNJ]

Failure of the rod seems unlikely if the bend is right at the support. Maybe this is a change to bank up some goodwill with the contractor which you will need to spend elsewhere.

 

[smb4050]

Don't allow this contractor to do this! I've previously looked at this situation in some depth and threaded rod doesn't have much capacity when combined with axial tension AND bending. I doubt the pipe in you're referring to will always have a constant temperature chilled water running through it 24/7, so there will be bending in the threaded rod as the pipe moves back and forth. Now add the cyclic effects of movement to the kinked threaded rod and there will be a fatigue failure at the kink a.k.a stress concentration location. See Grinnel publication Pipe Hanger and Engineering, ASME 31.1, Power Piping, section 121.7. There is so much out there that I think you could put the contractor in their place fairly easily.

 

[miecz]

Lutfi-

smb4050 is right. Hanger rods subject to axial tension and bending do not follow classical mechanics. If you have any expansion/contraction of your piping system, the bending stresses can skyrocket under tension.

Bending a rod at the thread is such a bad idea, you may have trouble finding anything written on it. First of all, the machining of the thread has created a stress concentration around the circumference of the bar. OK, so there are zillions of threaded parts doing just fine. But bending the threaded bar into a permanent set means the material has been strained beyond yield across the stress concentration. Unless the material was properly heated before bending, I would expect there to be cracks at the root of the threads.