Why both restraints and thrust blocks?

[BernieH]

Why is it that some municipalities demand both joint restraint and thrust blocking? I guess thrust blocks can be altered or removed during subsequent work on other utilities. So backing up thrust blocks with restraints could make sense. But why should restrained joints be backed up by thrust blocks? Do they think the restraints may be removed or damaged? Or that they may corrode? Maybe they wish to guard against poor workmanship or design...
Thanks

 

[cvg]

I would be happy to specify welded PE pipe, however to this point I have not had municipal water client that was willing to use it. ductile iron pipe and PCCP with bell and spigot joints is what they want and that is what they get.

 

[BigInch]

We're building gas distribution lines with it.

 

[EngWade]

Ok, so why don't the civil guys and petro guys each pull it out and see which is bigger? Seriously. As flawed as it may be, based on whatever opinion you have, local code, politics, and money make the decisions. You're kidding yourself thinking otherwise.

 

[BigInch]

Money is only half the story. You must also meet the engineering criteria. Sometimes the optimal solution falls towards one material, sometimes the other. That's why both materials and a lot of others are still on the market.

 

[rconner]

I sat back and watched this thread appear to get somewhat hijacked or sidetracked from the original question, and in the process it appeared some didn’t give much credit to those who have come before (and before even some of us older folks). In this regard I feel compelled to offer just a few words. The water infrastructure of the USA e.g., now in areas approaching at least two hundred years of age, is an interesting study. It represents the design, manufacturing, maintenance, and many other efforts of thousands of well-meaning USA citizens over two centuries. While we are all now some deluged(so to speak)with reports of breaks and outages(along with many other forms of bad news)from activist and omnipresent multimedia, it is also true that our water systems have supplied(and for a very long time, to generations of our citizens)water that has been among the best, if not the best taste and quality(and at the highest percentage of coverage to our populace)and at lowest supplied cost of any in the world. We are also currently lobbied with reports of very large percentages of lost or “unaccounted for water”, that I’m sure much of the populace (cultivated by this media, and also many vested interests, some of whom know better) identifies mistakenly with leakage out of their main water pipe and joints. When detailed water audits are eventually accomplished, what many have in fact discovered in many areas is that quite small percentages (even when quite old joint designs, but some still rather effective, are employed) are actually main pipe or mainline joint leakage.
The availability of good quality water made possible with USA water treatment and these systems has also very likely been no small factor in a near tripling of mean USA life expectancy since e.g. gray cast iron pipes were first introduced here in the early 1800’s. It is hard to make a convincing practical case against the performance and general cost-effectiveness of even formerly supplied gray cast iron piping and even some rather ancient joining systems(that have reportedly in just North America provided continuous service to Customers, and presumably generation of revenues etc., for more than 100 years in more than 628 utilities, and in more than 23 utilities for more than 150 years). While some pipes and/or joints must indeed be replaced or renewed and leaks fixed with new technologies, in some cases it is obvious systems that were installed for a pittance a very long time ago are now still effectively delivering even better water to Customers, and still generating dollars per month revenue to the utilities.
Try as you might you will thus have a hard time convincing me that most of our forebears did not or do not now know what in the hell they were are doing at the time (or necessarily had other motives than their utility’s interests, their piece of mind, and service to their tax and rate-payers)! In my mere 36 years in the industry I have been made aware of a good many other alleged “newer” ideas and types of pipe(line)s etc. that have really not made it beyond infancy.
While some welded or fused water and other pipelines, that have really come on board only for the last few decades, have shown some effectiveness in areas, they have also exhibited some unexpected problems, and it remains to be seen if they will survive as long underground(as have some other systems)with any widespread installation for water service.

 

[waterpipe]

While enjoying reading the above comments, just wanted to mention a point with regard to the main question of the post:
Even a restrained joint MIGHT need a thrust block, depending on the minimum required restrained length of pipe on each side of the bend. For a buried pipeline with restraint joints (for example a welded steel pipeline), trust force is diminished by development of the friction factor between the pipeline and the embeddment soil. Certain surface (length of pipe) is needed on each pipe leg to provide enough friction force to contract with thrust force depending on:
- pipe diameter
- pipe test pressure
- angle of break
- pipe installation depth (weight over the pipe)

The formula is mentioned in AWWA M11 for water industry. If you don't have this minimum length on each side of your bend, then extra anchor should be provided by thrust block.
please note that most often this length is few meter but in certain cases (large diameter cross country pipelines with high pressure) it could reach to 200-400 meter. Then one should consider thrust block for a restrained joint pipeline.

 

[waterpipe]

I meant friction force not friction factor ...

 

[stanier]

Waterpipe,

A fully welded pipe takes the pressure by stress in the pipe wall. Do you think it would be any different if the pipe sat on a pipe rack? you dont see concrete thrust blocks up in the air and the pressure x area is certainly not transferred to the structure.

A fully welded pipe does not rely upon any soil friction to restrain the pressure.

You only need thrust blocks when the pipe is not restrained as in rubber ring jointed systems or unrestrained bellows.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[waterpipe]

Dear staneir,

I do agree with you that a fully welded pipe takes the pressure by stress in the pipe wall. That's exactly the reason that unless the pipeline is properly restrained to resist the unbalance forces, either separation can occur at the pipeline joints or pipe wall stresses could approach yield strengths. In large diameter pipelines with hight bend angles or pressure (test pressure better to say), one should control that pipe stress does not get close to the yield strength. If it gets close then extra resistance to movement should be used by increasing the bearing area (a thrust block).

A fully welded pipe (restrained joints) may resist the trust force by relying on soil friction to restrain the pressure. In fact, the trust force is resisted through the development of friction between the pipe and the soil surrounding it. According to AWWA M11, the thrust force resisted by restrained or harnessed joints is the force acting along the longitudinal axis of the pipe. So if the restrained joint is used a sufficient length of pipe must be restrained by welding or harnessing to resist the unbalanced thrust force. Here's a force diagram as per AWWA M11:

Normally, in cross country pipelines; there are few hundered meter distance between bends that means enough restrained length. However, in my opinion, the correct way would be to control either the stress or the required restrained lengths for your welded pipeline (better to say for pipelines with restrained or harnessed joints) to make sure that you are on the safe side. Otherwise, thrust blocks might be required even for the welded steel pipeline. This could be the case for large diameter and high pressure pipelines.

 

[stanier]

Waterpipe,

The pipe wall thickness is selcted to contain the pressure. Ie the hoop stress determines the wall thickness. This PD/2t. The longitudinal stress occurring inthe pipe wall is half of this value PD/4t.

Ergo, if the pipe can withstand the pressure there will never be a need for thrust blocks as you describe.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[waterpipe]

Thank you for your respond stanier.

But still I am thinking about the force (the summation of the 2 force vectors parallel to pipe shown in the previous post) that is exerted at the bend.
Consider the pipeline shown on the previous figure to be a free body. The thrust force with NE direction wants to move the bend toward NE. If the pipeline is hanged by ropes from a ceiling, then the bend moves toward NE. But since it's buried and the joints are restrained, the friction between the pipeline surface and soil develops and avoids the bend to move. More thrust force then means more required restrained length. This is what AWWA addresses in its formula.

Recently, I've read an article from ASCE, pipeline 2010 written by ASCE Task Committee on Thrust Restraint Design of Buried Pipelines. It says:
"Current restrained joint design theories for different pipe materials depend on two primary forms of resistance mechanisms to resist the unbalanced thrust forces that arise at a horizontal bend, valve, tee or other similar sources of thrust in a buried pressure pipeline. The first and most commonly recognized form of resistance is from friction and adhesion resistance at the pipe-soil interface; and the second form is from passive and/or bearing resistance of the pipe against the soil surrounding the pipe."

Unified Approach to Thrust Restraint Design, JOURNAL OF TRANSPORTATION ENGINEERING, ASCE / JANUARY 2007, states:"While providing thrust-resisting supports in an above ground pipeline can resist thrust forces, resisting thrust forces in an underground pipeline is usually accomplished with thrust blocks, restrained joint systems, or a combination of both." And:"and "As an alternative to providing thrust restraint mechanism using thrust blocks, restrained joint systems may be used. In general, the restrained joint system is a mechanical (welded or harnessed) joint providing longitudinal restraint. The objective of the thrust restraint design using a restrained joint system is to determine the length of the pipe that must be restrained on each side of the point of action of the thrust force. The primary objective of the restrained joint system design is to transmit the unbalanced forces to the surrounding soil without over stressing the pipeline wall and without subjecting the pipeline to joint separations."

Here's a figure under "Restrained Joint System Design":

Still thinking about your explanation, I would be more than happy to see comments about this discussion from you and others since I can see many "senior" names in this post.

 

[rconner]

As previously mentioned many different thrust restraint design methods and material embodiments of same have generally done a pretty good job for a very long time. I would only say that I believe there is however increasing realization among many folks interested in this subject that pipelines are more than straight lines on paper. In other words, with even fully welded or restrained systems and horizontal bends at constant temperature(if there is such a thing)there is in fact some movement or deformation of pressurized systems, aboveground or below ground. In this regard, pipelines are like any other loaded engineering structures. Quite frankly, it is also a practical and geotechnical reality that movement or deformation(from simplistic straight line depictions)will not only occur in plan view, but instead at least to some extent in full three dimensions, as a combination of inevitably some imperfect alignment, uneven construction support, settlement, and also distortion of soil wedges(at least in shallow cover situations) etc. in the immediate bend areas. While there will also be transverse shear loads on any unblocked etc. pressurized bend system, that to some extent also add to the hoop and tensile stresses calculated by e.g. the (simplistic pipe hoop stress, wall thickness design)Barlow formula, if any deformation(e.g. bending)of the system is not pretty much freely accommodated e.g. by flexibility of the joints, this is then inevitably also accompanied by bending stresses (e.g. Mc/I etc.), in some fashion also additive to all others and variable at locations down the bend leg length. In a perfect world the piping should dependably withstand the maximum e.g. localized principal stresses, from whatever applied loads and deformations in all directions(as depicted elementally in Mohr’s circle etc. fashion), either satisfying such comprehensive design, or at least having adequate excess strength and conservatism of the piping/material employed in the absence of same. With all the varied pipe materials, joining, and soil variables potentially involved, this is truly a morass of complexity for decision-makers. In reading these and other blogs over particularly the last few years however, it appears some rigorous analysis of some welded and flanged configurations per interpretations of piping code rules e.g. by piping design (FEA etc.) software, have revealed the need to thicken some piping cylinders over simplistic hoop stress needs, due to these and other effects in some bend areas. Throw in also increasingly en vogue and in the news(and perhaps much more widespread than many folks, at least up until recently, have realized)seismic accelerations, movements, and soil behaviors during same into the mix, and this makes any attempted rigorous analysis even more fun.

I would only say in closing that while I definitely agree some modern and formidable restrained joints have indeed proven most convenient and useful, and also well proven for many years, I don’t believe I would necessarily throw any tools out of the toolbox just yet, including external anchorages like thrust blocks or batter piles etc. (well-designed and constructed buttresses might prove particularly helpful if unrestrained pipes are what a Contractor or Owner has available and based on their experience and reputation trusts, and you want to get a working pressure pipeline built!) Everyone have a great weekend.

Maybe when folks get to be truly “senior”, they’re really only in it for the “stars”?

 

[stanier]

Waterpipe,

I agree with you for rubber ring jointed pipe but not for a fully welded pipe.

For a fully welded pipe the thrust is taken in the wall of the pipe and no restraint is necessary.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/