How does the program internally calculate the section cut forces?

Expanded Question:

When section cut forces are determined by hand calculations, they correspond to the exact forces at the given location and they represent the effect of structure on one side of the cut acting on the structure on the other side of the cut. However, in SAP2000, the results seem to be affected by discretization and by the groups used define the section cut. The F1 context help (topic "Section Cut Output Conventions") mentions that "section cut forces are reported as forces acting on the objects that make up the group that defines the section cut". Could you please clarify what exactly is meant by this and what is the exact procedure to determine the section cut forces? Below is a list of several additional specific questions:

• Does the algorithm differ for section cuts defined by group and for section cuts defined by cutting planes?

• What is the procedure for area objects? Are area joint forces used to calculate the section cut forces?

• What is the procedure for frame object? Are element joint forces used to calculate the section cut forces?

• How are the section forces converted or transformed to the desired location (default or user-specified)?

Answer:

Section cuts can be defined before or after the analysis is run. What is the difference in internal working of the program for these two cases?

Are the section cut forces calculated during the analysis, or on the fly when the results are requested?

I have a bridge model developed using the bridge modeler. Do I still need to specify to save section cuts or are they saved automatically?

Section Cut Defined By Group

Can I define the cutting plane to cut though any joints?

For section cuts defined by quadrilateral cutting planes, SAP2000 calculates the section cut forces by summing the element joint forces from the frame, shell and link members that are fully cut by the cutting plane and included in the group that defines the section cut. The joints that are considered are those that (1) belong to the section cut group; (2) are assigned to the fully cut elements; and (3) are on the specified side of the cutting plane. In order for the program to determine on which side of cutting plane the joints are, the cutting plane should not directly pass through any joints, if possible.

Section Cut Defined in Graphical User Interface

The magnitude of the Integrated Forces for the right side is different from the magnitude of the integrated forces for the left side. Why?

Expanded Question: After using "Draw > Draw Section Cut", the magnitude of the Integrated Forces for the right side is different from the magnitude of the integrated forces for the left side? Based on equilibrium, should not the magnitude of these two sets of forces be the same?

Answer: The section cut forces on the left side and on the right side can be different, because the section cut forces are based on the forces at the joints (of the element cut by the section cut) at each side of the section cut. Create simple example of cantilever wall with two member and determine the section cut forces at the middle of each member.

When section cut is drawn in 3D view only X and Y coordinates are provided. How exactly is the section cut plane defined for this case?

For the "Draw > Draw Section Cut" menu command, the forces are reported in the section cut coordinate system, which is defined by three axes (1,2,Z) as follows:

• section cut 1 axis is located in a plane that is parallel with global XY plane; it is rotated by user-specified "Angle (X to 1)" counterclockwise from the global X axis
• section cut 2 axis is located in a plane that is parallel with global XY plane; it is rotated 90 degrees counterclockwise from section cut 1 axis
• section cut Z axis is parallel with global Z axis

Integrated forces are reported either on the left side or on the right side of the section cut using right hand rule. For example, if the section cut is drawn in a horizontal plane from left to right in an XZ view, then integrated forces on the left side represent the effect from the structure above the section cut, expressed in the section cut (1,2,Z) coordinate system, using right hand rule. Similarly, integrated forces on the right side represent the effect from the structure below the section cut, expressed in the section cut (1,2,Z) coordinate system, using right hand rule.