Thrust block design

[Luke54]

I am trying to design a thrust block for a 450mm diameter water main but its turning out to be very big as its in sandy clay soil. Right now I'm just using the passive pressure on the face of the block, but apparently a large component of the resisting force is generated by the passive pressure and frictional resistance on the pipe. I can work out the equations for these per metre length of pipe, but I need to know what length of pipe I can apply them to. Any help would be appreciated.

 

[paddingtongreen]

When I have needed a thrust block, it has been because the pipe joints are some kind of bell and spigot joint that cannot hold any force. What kind of pipe? What kind of joints? Do you have some kind of surge to resist? or is it a constant force?

The alternative with the bell and spigot pipes was to put clamps at each end with tension rods between them to take the force.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ToadJones]

Pad-
I'm not sure exactly why, but on the project I am working on now, we poured thrust blocks where ever the water main changed directions. The CM told me it was for when the water main was turned on and the rush of water hits the joints

 

[SlideRuleEra]

See "Thrust Restraint Design for Ductile Iron Pipe" (free .pdf download) from the Ductile Iron Pipe Research Institute. Even if your pipe is not ductile iron, the math is the essentially the same:

http://www.dipra.org/pdf/thrustRestraint.pdf

However, don't rule out thrust blocks completely. Just because it may be large does not mean that it will not be cost effective.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[ToadJones]

Great point by SlideRuleEra- concrete is pretty dang cheap. It will offer a nice insurance policy. Digging up water lines is $$$.

 

[cvg]

first - all pipelines experience surge at some point in time, there is no such thing as constant force. Transient forces will occur which require joint restraint.

it may be slightly more cost effective to install thrust blocks than joint restraints but most agencies are moving towards restraining the joints because of the continuing, long term headaches of dealing with large masses of concrete buried in their right of way.

EBA Iron has software for calculating this

http://www.wateronline.com/ecommcenter.mvc/ebaairon?VNETCOOKIE=NO

DIPRA also has good software to do this:

http://www.dipra.org/index.cfm

 

[cvg]

joint restraint vs thrust blocks

http://www.wateronline.com/download.mvc/Article-Joint-Restraint-vs-Thrust-Blocks-0001

 

[paddingtongreen]

The point is that Luke54 wants to use the friction on the pipe in addition to the thrust block. I didn't think it was iron pipe but I had to ask the question, just in case.

To my way of thinking, the block has to move before the friction is engaged, the soil being elastic.

Off track, Toad, have you had the job of restraining the main steam blowdown line? Great "fun", it is distantly related to this.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[csd72]

Luke54,

as is noted on sliderules reference, thrust blocks are not just about a change in the velocity of the pipe but are also about the pressure along the axis of the pipe.

In fact I had a discussion on this with a very experienced water engineer and he claimed that less than 10% of the thrust force was due to the flow of the water and the rest was water pressure.

 

[cvg]

you can't rely on the friction on the pipe unless the joints are fully restrained. Therefore your two options are to a)restrain enough joints on either side of the bend or valve to develop enough friction to counteract the thrust or b) put an anchor or thrust block on the end large enough to handle the thrust.

Transient surges generally result in the largest thrust on a pipeline, not the constant working pressure. These surges may occur on a frequent basis when a pump turns on or off, valves are opened or closed etc.

 

[PSlem]

I installed tie downs to increase frictional resistance on a thrust block. Might be an option.

 

[ToadJones]

Pad-
Never had to design specifically for the restraint of the main steam blow-down; thankfully that was handled by others on the team, but I have seen the loads.
I have done some "Top Steel" boiler support design (giant trusses and box girders). Just the dead loads from the Main Steam line were mind boggling. I found myself checking the loads I received from the piping guys.