Why does adding a node point on segment with soil settlement (impose displacement) already defined make a big difference in the results. See following image and results At node A02 & B02.
Note: Soil settlement (impose support displacement) was only modeled for the center elements on both segments:
Segment A and B were identical, then node points A04 and A05 were added. Now the results are very different.
The way AutoPIPE stores soil information and how imposed support displacements are applied to soil points explains the issue mentioned above. Looking at the updated model below:
AutoPIPE stores the soil information for an element with the preceding node i.e. soil information for the segment B, element B02 - B03 is stored at node B02.
When the intermediate point A05 is inserted between A02 and A03, the impact of imposed support displacement applied at A02 is limited to intermediate soil points up to A05. Whereas in segment B, the impact of imposed support displacement applied at B02 continues up to point B03.
Fix, need to apply another imposed support displacement at point A04 and A05 that corresponds with the same displacement found in segment B at the same location.
Assuming soil point B02 +3 is the corresponds with A05 on Segment A, Select A05 and add an impose support displacement = -0.262"
If we apply this resulting displacement value at the newly inserted point A05, we would have slightly better matching results:
The applied displacement value was a result value, and not really the actual imposed displacement, however we can see that we get a much better result now
Question: Why can Impose Support Displacement be modeled at node points that does not have a visible anchor / support?
Behind the User Interface, soil is applied as 3 individually defined incline supports (up/Down, Left/Right. Forward/Backward) located at each Node point and intermediate soil points. These incline support have the defined soil stiffness setting from the Soil properties dialog. Thus impose support displacement can be applied at any node point that has soil properties defined, but cannot be applied at a node points that does not have an anchor, support, or soil defined.
Assuming that the pipe is extremely flexible and Soil properties are extremely rigid. Simple test with a short pipe segment [anchor (A00), and 3 node points (A01 A02 A03)] 50 feet apart.
1. Impose support displacement applied to one node point (EX. A02) and not adjacent node points up (ex. A01) or down (ex. A03) stream.
Results, displacement will be linearly distributed to all intermediate soil points both up (ex. A01-A02) and down (ex. A02-A03) stream of the imposed displaced node point (ex. A02) to the next adjacent node points.
2. Impose support displacement applied to 2 node points adjacent to one another (ex. A01 & A02) and no other node points up or down stream (ex. A00 & A03).
Result, Again as mentioned above, impose displacement is linearly applied to all intermediate soil point up (ex. A00 - A01) and downstream (ex. A02 - A03) of the paired node points (ex. A01 - A02). However the same amount of impose displacement will be applied to all of the intermediate soil node point between the paired node points (ex. A01 - A02).
Model Soil Properties with Soil Calculator and Underground Piping