Piping Stress Integration with SAP2000 Structural Analysis Models
- Darrell
Below is a CSiPlant piping model example (model can be downloaded from the link) used to import and auto-connect with a structural analysis model from SAP2000. Users must have both CSiPlant and SAP2000 installed on their PC in order to import SAP2000 structural models into CSiPlant. The import procedure will tie up a SAP2000 cloud license only for 3 or 4 minutes, so it's not as if users need a dedicated SAP2000 license to integrate with CSiPlant piping models.
Using Edit menu>Add from SAP2000 model, CSiPlant can import detailed SAP2000 structural models including load assignments, releases, and mass model definitions, and auto-connect with the piping model using 2-point pipe supports for combined nonlinear pipe/structure analysis to obtain more realistic reactions and stresses. When importing SAP2000 structural models, CSiPlant maintains the same pipe support properties and labels used in the piping-only model, automatically converting 1-point pipe supports (connected to ground) into 2-point supports connected to supporting frame elements. CSiPlant can import SAP2000 models from V21 to current version. User will be prompted to select which SAP2000 model to import, and this will launch SAP2000 using the open application programming interface (OAPI).
Before the import is complete, 4 screens will appear. Depending on the version of the SAP2000 model, the first screen may display an UNIFHT function "error" which can safely be ignored since UNIFHT is a SAP2000 default power spectral density loading function which is not available in CSiPlant. The second screen in this example displays no errors but warnings which can also be ignored. CSiPlant does not yet have special rebar or tendon elements like SAP2000, and those discrepancies trigger warning messages even though the imported SAP2000 model did not contain any rebar or tendon elements.
The third screen is usually the most important to pay attention to, especially the Auto-connect supports tab shown below. If coordinates match between the piping model and structural model, CSiPlant default settings usually work very well. The default -Z direction tells CSiPlant to search for beams underneath the pipe support locations in order to auto-connect. In cases such as rod hanger supports where the piping is supported from above, users can switch direction to +Z if most piping is supported from above. However, if only a few are supported from above, go ahead and use the default -Z and then come back later and assign individual pipe supports to frame members above using Assign>Support connection procedure. The fourth and final import log screen has no errors, and its warnings in this example can be ignored.
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.
Selected pipe support reactions from the combined model can be automatically exported back into the SAP2000 structural model, thereby saving time, confusion and errors associated with marked-up isometrics, spreadsheets, and other manual methods of communicating pipe support reactions from piping stress to the structural team. See Define menu>Support reaction export request to view options to selectively export by Load case, pipe section, Pipeline label, design request, and by support. CSiPlant can export individual load cases as well as load cases that include a combination of loads. After defining a support reactions export request and analyzing the model, in order to export pipe support reactions to SAP2000, go to Analyze menu>Export support reactions where you will be prompted to select the Export request as well as select the SAP2000 model to which the support reactions will be exported.
Other considerations
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.
Structural analysis models typically include preliminary piping weight, wind, and friction loads. Smaller diameter piping (<12" NPS) is modeled using distributed gravity-direction loads on the beams based on tributary area, and larger diameter pipe loads are usually assigned as concentrated point loads or equivalent frame point loads. Structural models treat pipe friction as an assigned load, typically 10% of the weight load. Friction is not treated as a load in piping stress models, but as a nonlinear boundary condition which is a more rigorous approach. All of these preliminary piping loads in the structural model ignore load redistribution due to thermal displacement and other applied loads, and they ignore placement of heavy valves. That's why structural engineers need realistic pipe support loads from the piping stress team which account for thermal displacements and nonlinear pipe support behavior.
It's important to be aware of these preliminary piping loads in the structural model to make sure that pipe loads and mass are not double-counted in the combined piping + structural model, particularly with larger diameter piping. Preliminary assigned piping loads from the structural model, both weight and friction loads, should be adjusted (reduced or deleted) to account for pipelines which are explicitly modeled with piping elements in the combined model.
It’s not uncommon for pipe rack structures to be designed for lateral drift of Height/100 or Height/200 (Height of pipe rack) under wind or seismic loading which means 5" of allowable lateral deflection for a 42 ft. tall pipe rack, lateral deflections which can cause significant imposed displacements at pipe support locations. If it’s important, for example, to consider local nozzle/vessel flexibilities with a 3/4” imposed displacement at an equipment connection, then it’s also important to consider support structure flexibility and imposed displacements at pipe supports from the structure, particularly support structure displacements from lateral loads. CSiPlant and SAP2000 make it straightforward and reliable to rigorously consider the effects of pipe-structure interaction.
