Anchor Block Sizing Factor?

Is the PE pipe welded or sliding joints? If welded blocks are not required.

It looks like the FOS is just a recommended or arbitrary value based on soil considerations. This is more of a civil engineering issue and therefore the question should be posed in the civil engineering forum.

As a mechanical engineer we would provide the forces on the block to a civil/structural engineer to design, as the mechanical engineers gave to you

That being said, the poisson effect is the fact that as the pipe expands radially due to hoop stress it contracts axially since the wall mass volume remains constant. Therefore if the pipe is anchored at the ends it cannot contract and therefore in reality it is stretched due to being anchored. The axial stress in the pipe wall due to this axial stretching is the poisson constant times the hoop stress. This contraction causes a pulling force on the anchor. The thermal force can be a pulling or pushing force on the anchor. In the paper you attached it is considered a pulling force due to contraction of the pipe as the pipe is installed in the sunlight up to 100 F and then buried where it is indicated temperature may get close to freezing, in this case pipe shrinks relative to the installation length and pulls on the anchor so the poisson force and thermal force adds as indicated in the paper

If the anchor is at the end of the line where the piping elbows up vertically to above ground, this last anchor before pipe transitions to above ground experiences the longitudinal axial force due to longitudinal pressure force on the elbow of 0.5 x hoop stress x area of pipe, as longitudinal pressure stress is always 1/2 of the hoop stress. This acts on the side of the anchor where the pipe goes up to underground, but not the other two forces described above. The other two forces as described above act on the other side of the anchor only. For the next anchor down the line the two forces indicated above act on both sides of the anchor so theoretically they balance and therefore there is a net zero net force on intermediate anchors which is why they are typically not required for buried piping, but are required for above ground piping to prevent buckling. Note that for above ground piping the thermal expansion force dominates and is in tension during summer months so compression exist in long above ground pipelines.

As indicated by kevin anchors are usually not required on HDPE fusion joint pipe. However at the end of line where it goes to above ground it may be required to handle large movements of the underground piping effecting the above ground piping. I had to install such an anchor on HDPE once. I used a manufactured structural anchor assembly, not a concrete block.

If you have connection to underground bell and spigot pipe like shown in the paper then looks like you will need an anchor as they show.

Honestly no idea if it’s sliding or welded joints. I appreciate the responses very much thanks.

mqr7 said:

Hi all, first my background is structural. It’s the first time I have been requested to design an anchor block. The piping is 48” dia. HDPE, all buried. Pipe rxn is 440 kips due to Poisson’s effect? (Not sure what that is but I leave that to the piping folks). We have a geotech report and the soils are primarily clay, with some layers of cohesionless material in there.

I have been following guidelines from PPI (see here: https://www.plasticpipe.org/common/Uploaded files/1-PPI/Divisions/Municipal and Industrial/Division Publications/Anchor Block Report.pdf)

It has been helpful but not sure about some areas being applicable to my situation, so wanted to ask for some guidance. In that document, they use a 1.5 FOS to size the anchor block. They also consider only cohesionless soils in their report, and assume typical values of Ka, Kp, etc (vs. having the actual values in my case) to determine soil pressures acting on either side of the block. I am wondering where that 1.5 factor is derived, and whether it is being used to size the block due to some unknowns since they don’t have exact values and are providing generic guidance. If yes, does it make sense to include it in a situation where a geotech has provided those values in a report (and those factors therefore, inherently have margin built into them)?

I just want to make sure I am not being more conservative than I need to be since the block size is already quite large, but most importantly I want to make sure I am doing this correctly. Any advice or guidance is appreciated.

Click to expand...

There are 3 design criteria for the design of blocks. First is the bearing criteria, in which we make sure that the size of the block and service loads applied should be less than allowable bearing capacity. Second criteria is to ensure sliding due to horizontal thrust has a safety factor of 1.5. Similarly for the third criteria the overturning of the block needs to have 1.5 safety factor as well. For detailed calculations you can refer to some good design example.

Thank you @mamax123 I appreciate you responding. I was thinking the 1.5 was possibly to preclude sliding and overturning, as you described. Thanks for confirming. Do you have an example or any literature you can share that shows that?

https://www.plasticpipe.org/common/...s/Chapter 6 - Design of PE Piping Systems.pdf

Refer to page 352:

https://www.plasticpipe.org/common/...es/Chapter 9 - PE Pipe Joining Procedures.pdf

Thank you all