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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 effect of tightening of a guitar string.
Unstressed Modal case fundamental period/frequency is 1.828 seconds/0.547 Hz Load sequenced Modal case fundamental period/frequency 0.587 seconds/1.702 Hz
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