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If the combined model requires analysis of static accelerations (static seismic or transportation loads) or any dynamic analysis load case, users will usually need to modify the default Mass Source in the combined model in order to obtain realistic reactions and stresses. The default mass source for CSiPlant includes piping and frame member selfweight, fluid contents, insulation, inner liner, and cladding if modeled. However, structural analysis models also account for heavy equipment, cable trays, and other objects with significant weight as assigned gravity-direction (-Z) distributed loads and/or gravity-direction concentrated point loads. Those assigned loads need to also be considered in the mass model if the user is going to analyze static accelerations or any dynamic analysis load cases.
The Mass source feature enables users to convert selected gravity-direction assigned loads into mass in all 3 translational directions (X, Y, and Z). With most other pipe stress programs, if you were to assign a 50,000 lb. gravity-direction load to the piping or structure, it would not change the calculated natural frequencies or static seismic loads even one tiny bit, and that limitation can be a significant problem if you want to realistically consider combined pipe/structure interaction.
Below left is the default Mass source (MSSRC) after importing a SAP2000 model. This CSiPlant default Mass source ignores the gravity-direction assigned loads included in the SAP2000 model. In the screenshot below right we change the default Mass source to the one imported from SAP2000 (MSSSRC1). As you can see, this Mass source, like the default Mass source in CSiPlant, includes "Element self mass" which is the mass of piping and structural elements selfweight based on the sections and material as well as mass from fluid contents and insulation. However, the Mass source from SAP2000 also includes mass from gravity-direction assigned loads for Equipment weight, cable trays and small diameter piping, and it could also easily include a percentage of live loads, snow, and other applicable loads which need to be accounted for in the mass model. Heavy equipment such as air cooled heat exchangers can weigh over 150 kips each, and a battery of 4 or more air coolers on top of a pipe rack is not unusual. Mass for all that equipment and other objects need to be considered in static acceleration and dynamic analysis load cases.
After importing the SAP2000 structural model, go to Define>Load cases to modify the GR weight case to specify the Mass source, and also add gravity-direction load patterns imported from SAP2000 which can potentially affect piping vertical displacements. Equipment loads, for example, are assigned to the structure, not piping, but they can nonetheless cause vertical (and lateral) displacements on the support structure which affects supported piping and therefore can affect piping code stress calculations and end reactions.
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It's almost always easier to work with piping and structural models positioned near the origin from a graphics performance standpoint. Use Select options and Edit>Move to move the piping and/or structural model in order to synch their coordinates. You can use that same Edit>Move operation if you need to later move the model back to plant coordinates.
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