Dear researchers
The modeling of P-Delta for the structural elements (specially columns) reduce the stiffness of the structure and consequently increase the period of vibration. Now, these changes are negligible when the structure and P-Delta effects are modeled in OpenSees software. Instead, the mentioned changes are more noticeable when modeling is perform in the softwares such as Etabs and Sap2000.
How to apply P-Delta effects is the reason for difference changes due to P-Delta modeling in different softwares.
Now with all these interpretations, what is the most accurate method for modeling P-Delta effects? In other words, In which software are stiffness changes calculated more correctly due to modeling of P-Delta effects?
As you know, In Etabs and Sap2000 software, P-delta effects are applied to the structure by defining a gravity load combination, while this procedure is performed in OpenSees using geometric transformation (second-order P-Delta effects) and its effects on stiffness matrix.
Your question itself seems to have the answer...
" P-delta effects are applied to the structure by defining a gravity load combination, while this procedure is performed in OpenSees using geometric transformation (second-order P-Delta effects) and its effects on stiffness matrix."
.. and that's why the difference
Opensees is more accurate. because it updates the stiffness matrices using geometric transformation based on the updated deformed shape of the element.
I agree with Ali.. because that's the design code defined meaning of 'P-Delta'.. Actually you can have either of the approaches in SAP: The stiffness-reduction method or the classical P-Delta method
More on it: differences between the two FE packages can be due to many factors. For example, SAP models mimic rigid panel zones at beam-column junctions, but STAAD, for example, doesn’t. I suppose OpenSEES too doesn’t, unless you do them manually.
Concerning the two approaches (actually described here based on ETABS, which is very similar to SAP in menus and dialog boxes):
1) Stiffness-reduction method: Do as you have already done – “...P-delta effects are applied to the structure by defining a gravity load combination,...”
2) Classical P-Delta method: Define a new load case, and provide settings as
• Load Case Type = Nonlinear Static
• Loads Applied: Include both vertical as well as lateral loads in it
• Geometric Nonlinearity Option = P-Delta plus Large Displacements (even P-Delta alone will give results very close to this)
• ...and run the load case.
Rahul Leslie
At first, I just said that different entrance methods is applied to model P-Delta effects in each software. But in fact both methods are inwardly similar to each other and should lead to the same effects on the structure.( In opensees, gravity loads also affect geometric transformation).
Secondly, the mentioned stiffness changes of the structure between to software is just due to modeling of P-Delta effects. because all possible influencing factors on stiffness of the structure are modeled like each other in both software (such as rigidity of floors, end length offset, panel zone, etc.)
I guess in opensees only P small delta effects are considered and the difference in changes should not be so great between two mentioned software.
- Badges
- Science topic
- Similar topics
- Engineering
- Modeling