Thrust restraint of short offsets of mechanically restrained ductile iron pipe

[TLHS]

I was contracted to design some thrust blocks for a waterworks system. There is an area where they have been requested by the system engineer that seems difficult from a constructability standpoint (the required gravity block where the thrust is upward is much bigger than sketched). Looking at it, I suspect that thrust blocks are not required, but I am not familiar enough with standard practice in this area to be confident. At most, I would be requesting that the client relook a this area, and wouldn't make a judgment on whether they can remove the blocks, given that it's outside my scope of service to evaluate the system as a whole.

This situation is a few close fittings heading into an existing line. It looks to me that the thrust forces should cancel out and you should be able to get away without the thrust blocks or needing to work out effective restraint lengths for the mechanically restrained pipe. The displacement needed to engage the fittings fully and allow the pressure to self cancel should be small. Does this make sense, or are thrust blocks required because the effective restraint length isn't available?

 

[TLHS]

Note that the pipe to the right is an existing legacy line and is not mechanically restrained.

 

[bimr]

If the piping has mechanical joints, you do not need thrust blocks.

Most experienced designers would rarely if ever use thrust blocks. The reasons for not using thrust blocks are explained here:

[URL unfurl="true"]https://daviddkent.wordpress.com/2013/01/22/thrust...[/url]

If you are installing the pipe lateral on the main line, the soil has already been disturbed so it would be difficult to design a thrust block for it anyway. Another problem with thrust blocks is when piping is adjacent to another pipeline. Thrust blocks make it difficult to work on adjacent pipes.

If you tried to use a thrust block, the geotechnical engineer would recommend ridiculous soil pressures (to cover his butt) causing the thrust block to be oversized and then the contractor doing the installation would not install it properly. Many contractors just throw a bag of masonry on the pipe elbow instead of constructing the thrust block.

That is why mechanical joints are the preferred method of controlling thrust.

 

[TLHS]

I recognize why one wouldn't use thrust blocks. They have some legacy systems that are old cast iron and are hard to get restraint on to without doing a bunch of work, so there are tie ins and caps for those that require thrust blocks. I just can't figure out why that's here, given that it's new pipe and mechanically restrained.

That being said, I'm seeing a lot of typical details online for swing connections (which this is) that have mechanical restraint combined with thrust blocks, which seems needlessly onerous. I'm concerned, though, that I'm missing something to do with line deflection or rotation at the joints?

 

[bimr]

Your drawing is not very clear as to the type of joints. If you have mechanical joints on the entire lateral starting at the large tee, there will be no deflections or rotation at the joints.


Joint Restraints Offer Alternative to Thrust Blocks

 

[hydrae]

Standard mechanical joints do not retain thrust (contrary to bimr's statement)

Most mechanical joints installed today are upgraded

Thrust restraining followers can replace the standard followers and then the joint is restrained.

There are several brands of follower upgrades the most common being EBBA Mega-lug

Once a piping assembly has restrained joints throughout its length, then if there is not enough
length for net change in direction/size before the next unrestrained joint then a thrust block will be needed

In your case since the legacy pipe is not restrained I would be concerned about forces to the left on
the legacy pipe which could compress the bell joint an unknown distance to the left,
which is a concern if the pipe is gasket PVC or fiber concrete
but not a concern for Ductile or Cast Iron.

Is the pipe going in the Z direction new? if so then the restrained length calculation begins there
if not then the thrust block on the right is needed and a gravity block but the gravity block can be moved to the right and reducing it's size based upon the moments calculated.

Hydrae

 

[bimr]

A restrained joint is a special type of mechanical joint that is designed to provide longitudinal restraint.

DIPRA