The following basic outline can be used for modeling buried pipe:
1. At least 200-300 feet of the buried pipe should be included in order to model the effect of a cumulative anchor
2. Nodes (and associated restraints) should not be spaced further than 20 diameters apart for pipe greater than 12 inch in diameter, or greater than 30 diameters apart for 12 inch and under diameters.
3. Contributory area should be calculated for each node as:
4. The subgrade modulus of elasticity for the type of soil is then determined, preferably from actual soil tests. In lieu of better information, the following data is available:
5. The effective soil restraint stiffness for each node is calculated by multiplying the contributory area for each node times the soil subgrade modulus.
6. Lateral restraints (and possibly rotational restraints representing restraint force couples) with the calculated stiffnesses should then be inserted into the piping model at the appropriate nodes.
7. Next the density of the pipe should be set to zero, since the weight of buried pipe is uniformly supported along its length. Weight loads in buried pipe do not cause deflections, stresses, or forces in the pipe. (Note that this step should be skipped when doing dynamic analysis of underground pipe, since the mass distribution is important in the dynamic analysis.)
8. Any axial stops in the form of large flanges or concrete anchors, designed to resist the thermal expansion of buried pipe, should then be coded into the model. If these are present, it is recommended that the restraint stiffnesses calculated above be reduced by approximately 25% in order to yield more conservative anchor loads.