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Vertical displacements displayed below for weight case only (selfweight including fluid). Right-side window is weight case with consideration of P-delta with large displacements with max vertical displacement of -8.69". Left-side window are displacement results from the same weight case, but ignoring P-delta effects, with a calculated max vertical displacement of -41.5", which is what other legacy piping stress programs will report. With a long flexible pipeline like this, there is catenary behavior that needs to be accounted for in which axial load is being distributed to the anchors to help support the pipeline. P-delta with large displacements accounts for this catenary behavior which made quite a difference in this example.
Using the same long span model, we load sequenced an additional Modal case to be based on the end state of the GR-PD load case, which is the weight case analyzed with P-delta large displacements. On the left window we have Modal results from the default unstressed Modal case. On the right window we have have results from the load sequenced Modal case which includes the effects of P-delta large displacements. Consideration of P-delta with large displacements made a 300%+ difference in fundamental period/frequency results. The catenary behavior of the long flexible pipeline distributed axial load to the anchors, and P-delta accounted for changes in pipeline stiffness due to those axial loads, not unlike the tightening of a guitar string.
Unstressed Modal case fundamental period/frequency is 1.828 seconds/0.547 Hz "Prestressed" Modal case accounts for P-delta large displacements fundamental period/frequency 0.587 seconds/1.702 Hz
You can create this example with your own piping stress program and compare with CSiPlant by downloading a free fully-functional trial license by clicking the "Trial" link here https://www.csiamerica.com/products/csiplant
The CSiPlant model we used can be downloaded here