Pipe restraint at Fire Water Lines

[CPENG78]

I have two existing buildings with dedicated fire services. For joints I have called out either flanged joints, mechanical joints or a combination of the two for restraint. I have also given the contractor the option to use thrust blocks with the appropriate sizes called out per plan.

Typically, I understand mechanical joints such as the EBBA Iron Megalug to allow you the transfer of thrust forces from the water to the megalug to the pipe where it gets transfer to the soil via friction between the pipe and the soil with no thurst block required. This would assume that you have enough length of pipe to develop the capacity due to friction to take on that transferred thrust force. Otherwise the force would get transferred to any thrust blocks. Since many of the pipe runs are relatively short in length, I have called out the use of mechanical joints in addition to the thrust blocks.

Am I been over conservative? The contractor would like to know as to why he can't just use push on joints with thrust blocks. By the way, fire lines are tested and are operated at higher pressures than domestic lines, therefore the extra factor of safety, in my opinion, can only be a good practice. Am I missing something? Your thoughts! Thank you.

 

[rconner]

You could find much information using the advanced search feature for these and other forums (you may be interested e.g. in some history I previously provided in the thread

http://eng-tips.com/viewthread.cfm?qid=164171).

Basically, unrestrained push-on systems with even unrestrained mechanical or push-on fittings and thrust blocks, push-on restrained pipes and push-on or various mechanical retainer gland/gasket (restrained) fittings without thrust blocks, or all restrained systems plus thrust blocks (called "belt and suspenders") all work, provided the pressure ratings of all components are adequate for the service and the assumed degree of soil resistance etc. is provided in the field.

 

[Maury]

Flanged joints aren't appropriate for buried service. They lack the flexibility need to accomodate the laying conditions and future soil movement. I would use the Megalug restraints and thrust blocking. The extra cost for doing both is small compared to the cost of a system failure.

 

[CPENG78]

Maury,
Many of the agencies I deal with here in Northern CA require DIP with flanged fittings for certain components of the undergorund system. Like the post from rconner, I tend to call out flange joints for very short runs of assembly, say 2 to 5 feet on the average. Once it gets longer than that and if project specs and the agency allow it, I will use mechanical joints. I agree, flanged joints lack the flexibility but I wouldn't see that as issue on relatively short runs.

I also agree that the use and cost of megalug and thrust blocking greatly outweighs possible system failure.


Does anyone outer know of any data or studies related to thrust restraint by push-on joints alone?

 

[rconner]

The wording of this latest question could perhaps beget some confusion. Actually, the predominant successful usage/experience of "restrained joint" pipes for the last few decades has been with "push-on joints", but said joints also provided with optional special structural adaptations such that the pipe joints cannot be pulled apart by even very large forces (as by unbalanced pressure thrust at unblocked bends, tees, valves, reducers etc.)

On the other hand standard push-on or mechanical joint pipes that have only elastomeric gaskets but with no special adaptations can only be used in the midst of long, reasonably straight runs of pipes that are not subject to such large axial tensile forces. Also, fittings with similar push-on or mechanical joining ends and just elastomeric gaskets with no special adaptations or retainer glands etc. must also be thrust/concrete reaction-blocked in pressure piping applications. Push-on joint pipes or fittings with just rubber gaskets (no gripping metal teeth, or any other structural adaptations for restraint etc.) are thus generally considered to provide no dependable (at least tensile) axial thrust restraint.

To perhaps get a better feel for the subject, you might want to download e.g. the manual "Thrust Restraint Design for Ductile Iron Pipe" from dipra.org (that I think has been available for the last quarter century), and also perhaps visit vendors' sites that provide partial cross-sectional views of the many available joining configurations (e.g.

http://www.acipco.com/adip/products/Sect2.pdf

and portal

http://www.acipco.com/adip/pipe/restrained/).

I guess I should probably clarify in light of a couple posts by others that I am in general also not promoting the contemporary use of flanged joints underground (I prefer instead some flexibility, or at least a little available "wiggle", for buried applications!)

 

[CPENG78]

rconner,
Thank you for the valuable information. The intent of my previous question was to obtain possible data on any potential load capacity (if any) of the push on joint and gasket alone. More on the side of independent studies on the product itself, not intending to show confusion on my part or cause confusion for others.

On a different note, I have come across some manufacturers that claim that the use of their mechanical joints with their pantented products can eliminate the use of thrust blocks. I agree with you, thrust blocking should also be used to provide more dependable restraint.

Much of the work that our office has been producing with regards to these applications has been in deveoplemnt of Shop Drawings for contractors seeking to obtain approval and permits for the installation of fire lines. Believe or not I have had to battle agency offcials on requiring the use of thrust blocks in combination with flanged fittings. I supppose that is the other end of the spectrum in asking for way too much and not understanding how forces are distributed through different pipe applications.

 

[cvg]

I would not assume any practical thrust restraint for bell and spigot pipe. With proper design and installation, mechanical joints can handle thrust without concrete blocks. Belt and suspenders is fine, however most of the agencies I work with prefer to not use concrete blocks. I rarely see thrust blocks constructed properly with the appropriate amount of bearing area. They are also hated by the maintenance crews and other contractors. When they have to remove old thrust blocks, it is a lot of work to remove the concrete just to replace a valve or install another pipe nearby.

As far as city officials goes, it seems that review of permits is always done by the youngest staff in the department. Rarely ever done by an engineer. All the older, experienced engineers are now being given retirement packages so that cities can balance their dwindling budgets. It's a huge brain drain which is leaving the cities (and the states) being run by a bunch of lower paid, inexperienced newbies.

 

[rconner]

I must again attempt to clarify what I have said. It would appear that there has been a little, in the words from "Cool Hand Luke", "failure to communicate", and I apologize for any contribution I have made to that! What I am simply saying however is that I have seen pressure thrust restraint (e.g. at bends) successfully handled in more than three ways, including:

1. Unrestrained joint pipes and/or also fittings (bends etc.) but with concrete reaction blocking (i.e. thrust blocks) buttressing any significant thrust foci (large angle bends).
2. Restrained joint pipes and fittings with no reaction blocking.
3. Systems employing all restrained joint pipes and fittings AND reaction blocking at the same time ("belt-and-suspenders.")

I am NOT advocating any one of these over the other for any specific local application; I feel to do so would imply I know more of local conditions (a great many variables could come into play) than I do!

Now, I will again say that IMHO you should not however expect significant axial tensile strength for pressure thrust restraint purposes at least for any high pressures and/or larger angle bends from only simple, unrestrained push-on joints like Carnegie joints for steel and pccp, stab joints for steel, standardized and unrestrained mechanical or push-on joints (like Tyton or Fastite) for ductile iron piping (but without retainer glands, or without embedded gripping teeth in gaskets etc.), nor say unmodified Reiber joints for pvc piping.

You should use instead special restrained joint adaptations (such as provided at link) when you are trying to restrain without thrust blocking any substantial bend angles in pressure piping.

 

[cj5]

Have you checked with the water utility that is providing the water service? They should have a standard specification on what they will accept. Of course if it's a private fire line, they may not be too involved with what you're doing onsite... just a thought.

 

[s0eebuch]

You should ask the contractor why he's questioning the design and what background support he's using to propose his alternate design. I would venture to guess he's trying to save a buck and hoping you'll agree.

I also design waterline thrust restraint "overly conservative" because it's my PE stamp on the plans and I feel more comfortable with a little more thrust restraint than a little less. For higher pressure and larger diameter lines, it becomes a public safety issue. I have never seen nor experienced a catastrophic watermain break but I don't want to either.

I have learned that, once in the field, the inspector and the contractor have scaled back some of my design based on their experience - not saying this is right or wrong, just saying how it goes around here. But I've never given them the go-ahead to do so and probably wouldn't.

If it were me, I'd make the hard stance and tell the contractor that the design you put on the plans is (a) the design you, as the engineer on record, have proposed for the given application and (b) is the design the contractor choose to accept at the time of bid. To deviate from the design could cause issues with local laws regarding government purchasing/procurement.

Good luck with your situation.