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- Pressure thrust/End Cap type load effects.
- Poisson contraction/elongation load effects.
Both sets of loads are applied simultaneously, whenever there is a pressure assignment other than ambient. The distribution of internal forces and deformations depends on the structure flexibility and boundary conditions.
Pressure Thrust/End Cap
Pressure thrust/end cap loads at self equilibrating loads. This means that for any given piping component, the distribution of pressure load is in equilibrium with the pressure load at the inlet/outlet. The pressure load at the inlets/outlets is commonly referred to as "End-Cap forces" calculated as shown in Figure 1 and follows a sign convention that a positive end-cap force induces tension on the pipe. Note that for the calculation of analysis loads, no material allowances are considered.
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Due to Poisson effect, there will be a corresponding change in length (contraction for internal pressure and elongation for external pressure). The unrestrained pressure elongation strain, εp, is proportional to the average hoop strain by Poisson ratio (≈ 0.3 for most metals). Figure 4 illustrates the relationship between changing pipe radius and the corresponding change in length (for internal pressure only).
Figure 4:
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Poisson effect on pipe length due to internal pressure
For systems ARE free to deform (no restraint), pressure stresses are from the end-cap/pressure thrust forces as the Pressure elongation can occur. Examples of systems that can be considered unrestrained include:
- Straight pipe fixed/line stop at one end, free or guided at the other;
- Straight pipe fixed at the center and both ends free or guided.
- Non-buried piping with elbows or other components that allow the elongation to occur.
For systems NOT free to deform (restrained), pressure stresses are from the restrained Pressure elongation. Examples of systems that can be considered unrestrained include:
- Straight pipe fixed or with line stops at each end.
- Buried piping where the soil is sufficiently stiff to prevent the pressure elongation from occurring.