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Just as the tightening of guitar strings changes the lateral stiffness and natural frequency, axial tension load in piping and structural frame elements increases lateral stiffness of those elements, whereas compressive loads soften lateral stiffness. Legacy pipe stress programs totally ignore these real world P-delta effects on element stiffness. P-delta also accounts for loading about the deflected position of the piping which is ignored by legacy pipe stress programs.
Another important consideration ignored by legacy pipe stress programs is the sequence or order of the applied loads. Whenever friction, gaps, or soil is modeled, the order of the applied loads can and will make a difference in design calculations. To take one example, proper analysis of a winter shutdown load case scenario should not start from ambient temperature, the shutdown should realistically start from the operational state where the piping is already in a stressed and deflected position, and then transition from there to winter shutdown temperature/pressure with removal of fluid contents load, if applicable. Legacy pipe stress programs cannot properly model that load case scenario.
It doesn't take a pipe stress or structural expert to see that pipe/structure interaction can often have a profound impact on design calculations as compared to piping-only models with rigid pipe supports and anchors. Not only consideration of support structure flexibility which is important, but also consideration of imposed displacements at pipe support locations caused by structural frame displacements. Pipe racks are often designed to accommodate lateral deflections of Height/100 or Height/200 under wind or seismic load. That means 2", 3" or more imposed displacements at some pipe support locations. Sometimes even small imposed displacements can be quite impactful for design calculations. There is also a coupled analysis effect to consider, particularly with rack structures supporting large diameter piping in which the cross sectional area of some pipes can be larger than the supporting columns.
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