| Applies To | |||
| Product(s): | AutoPIPE | ||
| Version(s): | 2004, XM, V8i | ||
| Area: | Analysis | ||
| Original Author: | Bentley Technical Support Group | ||
| Date Logged & Current Version | Aug. 2015 09.06.02.06 |
Problem:
Can you please elaborate as to why the Longitudinal stress calculation on the intermediate soil points results in over-stress? Are there any governing factors that AutoPIPE considers when calculating Equiv1 at a weld between 2 different pipe properties during AutoPIPE analysis of underground piping?
Solution:
The response is based on a specific user model, this may or many apply in your situation:
a. The first and the obvious is that section modulus and cross sectional area of the pipe on both sides of the transition are different. This will affect the calculated longitudinal stress values and bending stress values even if the force/moment values remain constant on both sides of the transition. This effect is observable in the user model with sudden difference on the minus and plus face of the node at which the transition is occurring for example A138.
b. The main axial force that develops in the user pipeline is due to thermal expansion. If two pipes say pipe1 with smaller wall thickness and pipe2 with larger wall thickness are both restrained at one end and free to displace at the other end – the displacement at the free ends of both the pipes will be the same if expansion and modulus of the pipes are the same. However, greater force will be required to prevent that displacement in case of thicker pipe as compared to the force required to prevent displacement of thinner pipe. With the two different section pipes connected together, pipe2 with larger wall thickness will be able overcome the opposing forces from the pipe1 with thinner wall and displace (contracting pipe1).
c. If we look at the displacement and soil report – the maximum displacement value can be observed at the interface of the two pipes. The soil springs will tend to resist this displacement of the pipe – resulting in increased pipe forces at the locations with maximum pipe displacements. Also note that the pipe forces will increase until it reaches a point with zero axial displacement and a consequent change of direction of the pipe displacement.
d. There are no specific factors that would govern the calculations of the stress value at such locations. For stress calculations, AutoPIPE will simply use the forces and moments values of the pipe and the pipe cross sectional properties to calculate the stress values.
See Also
CAN/CSA-Z662 Piping Code Calculations