This page contains frequently asked questions related to *section cuts*.
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h1. General FAQ
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h2. 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)?
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h2. 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?
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h2. Can the program display force diagrams based on a sequence of section cuts?
*Extended Question*: I have developed a 3d stick model of a slab bridge with the use of shell elements to model the slab. I have made and saved various section cuts through the slab and wanted to view the shear and bending moment diagrams with quantitative values. Is this possible?
*Answer:* While there is no built-in capability within the program to display shear and moment diagrams for a series of section cuts as of V14.0.0, you can obtain these diagrams outside of the program with some additional pre-processing and post-processing.
(1) Use interactive database editing ("Edit > Interactive Database Editing") to conveniently define multiple section cuts. The data can even be imported from Excel in which you can setup formulas to automate defining the sections cuts at desired intervals along the span.
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(2) Run the analysis and use "Display > Show Tables > ANALYSIS RESULTS > Structure Output > Other Output Items > Section Cuts > Table: Section Cut Forces - Analysis" menu command to display the section cut forces in a tabular format. Then you can export the data to Excel for post-processing and plotting the moment and shear diagrams.
This may require significant effort if done manually, but it would be a reasonable approach if it is automated using Application Programming Interface (API). The discretization should be refined as needed in order to adequately define the section cuts.
Please note that a new feature is planned (collection of section cuts along a line) that will enable you to do this within the program.
h1. Section Cuts and Moving Load Analysis
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h2. When Results Saved For Section Cuts Option is checked on the "Moving Load Case Results" saved form, what data are exactly saved?
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h2. Exactly what data are being saved when the Save Section Cuts options is checked?
*Extended Question:* Does the program create influence surface for each section cut force component for section cuts defined prior to running the analysis? For section cuts defined or drawn after the analysis is run, does the program determine the section cut forces from influence lines for the joint forces for all joints that are included in the section cut group? If so, these influence surfaces are calculated as a part of the moving load analysis, so could you please clarify what additional information is being saved?
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h2. Are the section cut forces calculated during the analysis, or on the fly when the results are requested?
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h2. 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?
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h1. Section Cut Defined By Group
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h1. Section Cut Defined by Cutting Plane
h2. What is the orientation of positive 3 axis of quadrilateral for quadrilaterals located in vertical plane?
The positive 3 axis refers to the positive 3 axis of the quadrilateral, using the same convention as for shell elements. Therefore, the positive 3 axis is always uniquely defined. {verify}
h2. 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.
h1. Section Cut Defined in Graphical User Interface
h2. 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.
h2. 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.
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h2. For draw section cut option, why are the integrated forces shown in 1, 2 and Z directions? Why not 1, 2 and 3?
For the section cuts drawn in the graphical user interface, the section cut 3 axis is always parallel with global Z. Therefore the integrated forces are shown in 1, 2 and Z directions.
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Additional FAQ drafts are listed in the article.
Info
This page is devoted to frequently asked questions (FAQ) related to section cuts.
On this page:
Table of Contents
General FAQ
General frequently asked questions which concern section cuts are as follows:
How are story forces and story shears reported?
Extended Question: I am running a seismic analysis on a multi-story braced-frame project with rigid diaphragms. How are story forces and story shears reported for each braced frame?
Answer: This can be done using section cuts. Display the elevation view of the framing of interest, display the frame force to be reported, then draw a section cut at any elevation within the story by selecting Draw > Section Cut. Frame shear force will correspond to the section-cut force in the 1 direction.
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Related Incidents:
Incident 47967: Frame Shear Output
Incident 64021: Obtaining story shears in SAP2000 via a single section cut or from a difference of two section cuts.
How do I save a drawn section cut?
Extended Question: When I try to save a section cut, I receive an error message which reads: Section Cut cannot be saved. Currently only cuts in undeformed 2D Global views can be saved. How is this resolved?
Answer: Two options are available for saving drawn section cuts. These methods are described as follows:
Display frame forces within the 2D global view, draw the section cut, then save.
Can force diagrams be displayed for a sequence of section cuts?
Extended Question: I have made and saved section cuts along the length of a bridge slab. Can I view the shear and bending-moment diagrams for this series of section cuts?
Answer: This series of diagrams may be obtained with additional pre-processing and post-processing, described as follows:
Define multiple section cuts using interactive database editing, available through Edit > Interactive Database Editing. Data may also be imported from Excel, where section-cut data may be automated over a series of intervals. These controls are shown in Figure 1:
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Figure 1 - Section-cut definition tables
Run the analysis, then display section-cut forces in tabular format by selecting Display > Show Tables > Analysis Results > Structure Output > Other Output Items > Section Cuts > Table: Section Cut Forces - Analysis.
Export data to Excel for post-processing and plotting of shear and moment diagrams.
A new feature, collection of section cuts along a line, is in development. This control will enable the plotting of response quantities for a series of section cuts within the software interface.
Section cuts defined by a cutting plane
For section cuts defined by a cutting plane, frequently asked questions are as follows:
What is the orientation of the positive 3 axis for quadrilateral elements?
Answer: The positive 3 axis of quadrilateral elements follows the same convention as that for shell objects in which axis orientation is normal to the element area, and positive projection follows the cross product of axes 1 and 2.
Can the cutting plane be defined through any joints?
Answer: If possible, the cutting plane should not pass directly through joints such that the software may determine the side of the cutting plane on which joints are located. For section cuts defined by a quadrilateral cutting plane, SAP2000 calculates section-cut forces by summing the joint forces within frame, shell, and link objects which are entirely within the cutting plane and included in the section-cut group. Joints considered include those which 1). belong to the section-cut group; 2). are assigned to objects entirely within the cutting plane; and 3). are on the specified side of the cutting plane.
Section cuts defined using the graphical user interface
For section cuts defined using the graphical user interface, frequently asked questions are as follows:
Why is the magnitude of integrated forces different for the left and right sides?
Expanded Question: According to equilibrium, should not the magnitude of integrated forces be equivalent for both left and right sides?
Answer: Section-cut forces may be different on either side of a section cut because they are based on joint forces which occur in the global system. Globally, these forces are in equilibrium, but their relation to a section cut is arbitrary and does not necessarily serve the function and conditions of a free-body diagram. As an example, consider a section cut located in the middle of a cantilever beam modeled by a single frame element and loaded by its self-weight only. The results on the side of the section cut that includes the fixed end will represent the beam reactions, while the results on the side of the section cut that includes the free joint will be zero.
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How is the plane of the section cut defined when drawn in a 3D view?
Expanded Question: When a section cut is drawn in a 3D view, only the X and Y coordinates are provided. How is the section-cut plane defined?
Answer: When using the Draw > Draw Section Cut command, 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 within the plane parallel to the global X-Y plane, and rotates counterclockwise from the global X axis according to the user-defined parameter Angle (X to 1).
Section-cut 2 axis is also located within the plane parallel to the global X-Y plane, though it is oriented 90° counterclockwise from the section-cut 1 axis.
Section-cut Z axis is parallel to the global Z axis.
Integrated forces are reported either on the left or right side of the section cut, according to the right-hand rule. Take, for example, a section cut drawn from left to right along the horizontal plane within a X-Z view. Integrated forces on the left side represent behavior above the section cut, while those on the right side, below. Using the right-hand rule, these forces are expressed in the section-cut coordinate system (1,2,Z).
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Why are section-cut integrated forces shown in 1,2,Z and not 1,2,3 directions?
Answer: For section cuts drawn within the graphical user interface, the section-cut 3 axis is always parallel to the global Z axis, therefore integrated forces are shown in 1,2,Z directions since this notation is more precise.
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Related Incidents:
Incident 41578: Section cut results for response spectrum analysis
Email Section cuts for response spectrum load case: (ok → hf, 1/12/2012, ID 8094858)
What is the section-cut force output convention when drawn in the graphical interface (Draw > Draw Section)?
Answer:
The Draw > Draw Section cut works in local planes and the details are shown as follows:
For cuts in plan (slab results) the default 1 axis is in the direction of the cut and
F1 is in-plane shear
F2 is axial force
FZ is out-of-plane shear
M1 is out-of-plane (bending) moment
M2 is twisting moment
MZ is in-plane (drilling) moment.
For horizontal cuts in elevation of XZ walls (pier results) the default 1 axis is in the X direction and
F1 is in-plane shear
F2 is out-of-plane shear
FZ is axial force
M1 is out-of-plane (bending) moment
M2 is in-plane (drilling) moment
MZ is twisting moment.
For horizontal cuts in elevation of YZ walls (pier results) the default 1 axis is 90 degrees to X direction and
F1 is in-plane shear
F2 is out-of-plane shear
FZ is axial force
M1 is out-of-plane (bending) moment
M2 is in-plane (drilling) moment
MZ is twisting moment.
For vertical cuts in elevation of XZ walls (spandrel results) the default 1 axis is in the X direction and
F1 is axial force
F2 is out-of-plane shear
FZ is in-plane shear
M1 is twisting moment
M2 is in-plane (drilling) moment
MZ is out-of-plane (bending) moment.
For vertical cuts in elevation of YZ walls (spandrel results) the default 1 axis is 90 degrees to X direction and
