Split Buttress Design

[bsmarks]

We are working on a 20" DIP water main that is being partially being replaced. The main will be cut and capped temporarily while the new pipes are being installed. My manager has asked me to design a split buttress for hold down the pipe during construction. I'm looking for guide on how to design such a thing. My understanding is that it has two concrete blocks on either side of the pipe and beam runs between the two blocks. See the link provided for an example. The county which we are designing this for has standards for up to 8" mains. Any info would be very much appreciated.

 

[JedClampett]

You're better off getting someone with a structural background on this. Basically you need to figure out the pipe plus contents weight for the lenght you're supporting, add in any thrust due to the cap (if the pipe is restrained the thrust is distributed through the length) and design two beams to support it. You could borrow the big steel book (AISC 13th Edition), but I'd hate to simplify this too much into a just steel problem. There's concrete, soil, steel unbraced lengths and just the geometry of fitting everything together to think about.
A lot of your scan is cut off, but it looks to be appropriate for a much smaller pipe than 20 inch. So all the blocks, embedments, beams, etc. will have to be upsized.

 

[rconner]

While I'm not sure enough information is provided to fully describe what the original designer or your manager intends, it appears the partial view provided of this this device could indicate intention to (temporarily?) restrain e.g. a mechanical joint cap or plug (placed over a plain end that they wish to remain in pressurized service at a cut location, e.g. vs some magnitude of pressure thrust at the cut-in location?), and then at some time in the future they may want to go in and cut or remove the cross-beams e.g. to connect to and extend the 20" line in the 2'-6" wide trench?
It appears also the designer wants two cross-beams that span across some sort of access plumbing (to the cap etc. for by-pass, filling and/or testing purposes?) It furthermore appears that the designer wants the sort of saddlebag concrete anchors or buttresses on each side to be installed against "undisturbed" soil, in effect thus outside the existing trench. While the forces against a 20" cap will be considerably higher than those against an 8" cap(if that is what is pictured), I have seen this sort of basic thing being done before over the years, and even for quite large pipes and high pressure.

While the publication at

http://www.dipra.org/wp-content/uploads/Design-Thrust-Restraint.pdf

will probably be helpful in giving you an idea of the kind of force(s) involved, and also the aggregate bearing area of concrete required against undisturbed soil, it will not design this special thing for you. Obviously, the (steel?) cross-beams will also have to be structurally adequate for applied bending and shear forces. I hope this information is helpful.

 

[bsmarks]

Thank you for the information. It gave me a better idea. The provided drawing was just an example. The intention is to use this for temporary restrain of the pressurized main. Once the replacement pipe is installed it will be connected to the existing pipe and this system would be removed. The main is being replaced because it is from 1850. My understanding is that this kind of thing is used often for temporary restraint of pipes. I've attached the full details but I am not looking for a taping system. Both the existing and replacement pipe are 20" so the two beams that hold the pipe would be on a 20" pipe.

 

[rconner]

Good information. I guess that makes existing/now working 20" main 162 years old. Unless you have some old submittals or drawings there unfortunately may be little dependable data on the precise dimensions (outside diameter/OD and inside diameter etc.) of such a very old (I assume gray cast iron) main, and you have not provided the pressure conditions on the main (that may at least provide some clue as to the provided thickness etc.?) However, in any case I suspect this is probably and old "bell and spigot (caulked) joint" system (such joints e.g. per rough sketch I've attached available since they were invented by a Chelsea Water Company guy in England ~1785). That being the case, you will not have an existing 20" MJ pipe bell to stick a plug in like your drawing, as that joint was not invented by AMERICAN until the 1920's and not immediately adopted by some users for many years thereafter.
While maybe not the only way to do this thus (cutting into and?) assembling a dependable bulkhead onto the end of such old pipe will likely require some sort of mechanically-joined cap, or instead a sleeve with plug bolted in to the outside end of same to be slid up over the old cut plain end and then that socket bolted down to the old cut end with a stuffing-box type gasket sealing on the outside of the old pipe.
Of course to be able to comfortably slide something over the cut end of this old pipe and get a gasket to seal, knowledge of the actual outside diameter of the old main where you want to cut as well as the condition of the OD surface where the gasket needs to seal will certainly be good to have. Any chance to make a small >pothole size excavation safely around old pipe to check this out before this buttress work? Once you know plain end dimensions and assuming suitable surface/condition, it is quite possible the old main can be capped (with either modified ductile iron or a custom-fabricated steel coupling closure and the buttress installed

 

[bsmarks]

The high hydraulic grade is 211ft, invert elevation of 80ft and surge pressure of 120lb/in². So the design pressure would be approximately 177psi. Unfortunately the as-built has no information about the pipe material but as you said I would also assume gray cast iron. Multiplying the cross sectional area of the pipe by the design pressure give approximately 64,760 lbs of thrust force.

 

[rconner]

Notice in Table 12-1 of the historical site at

http://www.american-usa.com/system/assets/150/original/Section 12_RB.pdf

that if original designer knew this was going to be a 177 psi (near 400 ft head) main, and they had access to eventual "AWWA Standard" design principles, they would have probably called for a "Class D" dimensioned pipe with 22.06" OD. This OD pipe would result in a bulkhead force based on that piston OD of near 68K lb.
Unfortunately, I don't think "AWWA" really came until a half century or more after your main installation date, so head may not conclusively nail down pipe dimensions - in any case truly interesting testimony that such a large iron main of such high pressure has endured for 162 years. If/when dug up please let us know what you find.