In actual piping systems, loads are applied sequentially rather than simultaneously. Piping is first subjected to its self‑weight during installation, followed by internal pressure during commissioning and thermal expansion during operation. Additional environmental loads, such as wind or seismic effects, may act later. AutoPIPE reflects this real‑world behavior by applying loads in a defined sequence, typically Gravity → Pressure → Temperature → (Wind or Seismic), with gravity always applied first.
This sequential approach is particularly important when the model includes non‑linear features such as support gaps, friction, or soil interaction. In such systems, the structural response depends on load history: supports restraints may engage or disengage, friction forces may activate or release, and soil resistance may evolve as loading progresses. Applying all loads simultaneously, as done in some analysis programs, cannot accurately capture these effects.
When support gaps, friction, or soil properties are defined in a piping model, the use of AutoPIPE’s non‑linear analysis is strongly recommended. This methodology, developed by a Professor Emeritus from the University of California, Berkeley, employs an Operational Condition Approach in which individual load cases are applied incrementally. The system state from each load step is carried forward to the next according to a defined load sequence, allowing non‑linear effects to be properly captured. AutoPIPE also provides the ability to include or exclude friction on a load‑by‑load basis to comply with applicable design code requirements, such as neglecting friction in seismic load cases.
Two other alternative non‑linear analysis methods are discussed in an ASME paper published in 2009. The paper suggest the only method that meets the code philosophy and and reqirements is the Operational Condition Approach.
How to perform a Non-Linear Analysis?
On the Static Analysis Sets dialog screen, enable boxes in the "Non-Linear" column
.
OR
Select an Analysis set and press the Modify button to open the dialog. Where he option "Gaps/Friction/Soil" can be enabled / disabled as needed and press OK button.
Review Analysis Summary output report:
Here one can clearly see that the Load Cases analyzed are Gr, T1, E1, and W1. Also that the load sequence was:
1st, analyze load case GR.
2nd, from the displaced pipe location due to Gravity, add load case T1.
3rd, again, from the displaced pipe location due to Gravity, add load case E1.
Lastly, again, from the displaced pipe location due to Gravity, add load case W1.
Conversely, pipe displacement and support load results can be significantly different if the Wind loads were applied to a piping system that had already been displaced by operating conditions, example OP1:
Again, one can clearly see that the Load Cases analyzed are the same: Gr, T1, E1, and W1. However, notice the load sequence is now different:
1st, analyze load case GR.
2nd, from the displaced pipe location due to Gravity, add load case T1.
3rd, again, from the displaced pipe location due to Gravity + T1, add load case E1.
Lastly, again, from the displaced pipe location due to Gravity + T1, add load case W1.
How to define Non-linear options?
Only by opening the Static Analysis set dialog, enabling the "Gaps/Friction/Soil" checkbox and pressing OK button, will the NonLinear Analysis dialog appear (refer to program help for details on this dialog screen).
Note: Why does NonLinear Analysis provide the following options on the dialog above:
How to control load Sequence?
There are 2 settings to control load sequence both options are accessed by Non-Linear Analysis options dialog mentioned above.
1.Initial case for Occ. loads
2. Use default sequence
For more complicated load sequence, uncheck the "Use Default Sequence" box to modify the Load sequence and press OK button as shown below:
Now manually adjust the load sequence as needed. There are some limitations to load sequence:
1. Initial state for occasional load cannot be another occasional load case.
2. Initial state for occasional load cannot be a User load case.
3. Initial state for User load cannot be another User load case.
etc...
Attention:
Item #1: Review AutoPIPE help section:
Help > Contents> Search Tab> enter "Understanding Load Sequencing" (include the quotes), press List Topics button, double click on the selected topic from the list provided to see more information select the hyper link for "Understanding Load Sequencing".
Note:
V-stop lift-off message is displayed by AutoPIPE when a pipe is resting on a V-stop in sustained/dead load condition i.e. gravity (which means that the support force displayed in the grid in the Global vertical axis would be negative, meaning that force is acting downward on the v-stop), but the pipe lifts-off from the support (Zero force acting on the support in global vertical axis, displacement of the point would be positive in global vertical axis) under operating loads i.e. Pressure or temperature.
The maximum possible frictional force that can be developed for a support depends on the bearing (vertical) force applied to that support point, forces applied in the orthogonal direction to bearing and the friction coefficient.
Item #2: Examples:
01. Non-Linear Analysis - Gap Support
02. Non-Linear Guide vs Inclined Supports Explained in AutoPIPE
03. Non-Linear Analysis - Wind Load
04. Non-Linear Analysis - Pipe / Support Friction Force - GrT1P1
05. Non-Linear Analysis - Pipe / Support Friction Force - GrT1P1E1
Item #3: Output report
Item #4: General methods used by Stress Analysis computer programs
ASME published a paper that compare 3 methods used by most computer programs for stress analysis - click here for summary