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The following is a step-by-step procedure for analysis a two-dimensional truss
structure using SAP2000 (v20). The order of some of these steps is not critical; however,
all step should be completed before execution of the the analysis. If you have any
questions, or you you find any of these instructions unclear or inaccurate, please contact
Dr. Charles Camp.
To help students become familiar with some of the numerous aspects and features of SAP2000, the following tutorial will focus on determining the forces in each member of the
roof truss shown below. Assume all
members are pin connected.
When you start SAP2000 Educational Version 20 you should see the
following interface window:
Step 1: New Model - To
start a new problem, select New Model
under the File menu.
The New Model
window gives many different templates
for general structures. In this example we will
use the Grid Only template.
To use the grid, determine the appropriate number of
grid lines and grid spacing to locate the joints of the truss.
On this menu, you can
select you units for the problem, the default is
lb, in, F. You can change the unit when
necessary and SAP2000 will convert the values
for you. In this example, the default units are
acceptable.
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you select Grid Only on the menu, the Quick Grid Lines
window will appear (see the figure on the right).
SAP2000 assumes that your two-dimensional structure resides in the x-z plane.
Define
your grid system by entering data on the Quick Grid Lines
window. For the truss shown above, the the grid spacing in the x and z-directions is 240
in. The number of grid line in the x and z-directions are 5 and 2,
respectively. Only one
y-direction grid line is necessary for 2D problems.
When you click OK, SAP2000 generates the grids lines you have just
defined and shows you the grid system in the SAP2000 interface window.
By default SAP2000 show two views of your problem, typically a 3-D view and an x-y
plane view. To adjust the views, select an window and click on the appropriate view button
located along the top edge of the interface window. |
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Click on the window label and delete the
left-hand side window so that you have a general
3D view and then click on the xz
button on the top bar to see a 2D view of the
structure.
Step 2: Locate Truss Joints - To define the joint
locations, select the Draw Special Joint button  on the
left side tool bar. Click on grid intersection lines to define joints. For this problem the
joint locations are shown below:
Step 3: Draw Frame Elements - To define each
frame element, select the Draw Frame Element button
 on
the left tool bar. The Properties of Object window will appear.
At this point, we can select a frame object you want to draw, or use the default
and update the frame element properties later.
In this example, we will just
use the default properties. Close the
Properties of Object window and begin
to draw frame objects. To define an element, click on a joint at the beginning of the element
and than on the joint at the end of the element. To end a series of element definitions,
simply double-click on the final joint. For this truss problem, the frame elements are
shown below:
Step 4: Define Structural Supports - To define
the location and type of structural support, select the support location by clicking on
the joint with the pointer. A blue "X" should appear at the joint to indicate
that it is currently selected. Next, click on the the Assign
tab at the top of the SAP2000 interface, then click on Joint, and then Restraints
... button on
the bottom tool bar.
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The Assign Joint
Restraints menu will appear as shown on the right. In most cases, the directions 1,
2, and 3 listed on the menu correspond to the x, y, and
z directions. When working on two-dimensional structures, the Fast Restraints
button may be used for most problems. If the support conditions for your problem are not
listed in the Fast Restraints section of the menu, you should select the
appropriate combination of restraints.
In the truss example, select the lower-left hand
joint with the pointer (an "X" should appear at the joint) and then click on the
the pin
button 
and click OK.
Next, select the lower right-hand
joint with the pointer and Fast Restraints menu select the roller
button
 and click OK. |
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After the supports have been defined the truss problem should appear in the SAP2000
interface window as follows:
Step 5: Apply Forces at Joints - To apply forces at a
joint, select the joint with the pointer and click on the Assign,
then Joint Loads, and then Forces. The following menu will appear:
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In this
example, there are three 3,000 lb. forces acting along the bottom cord of the truss. Remember
that the truss was modeled in the in the x-z plane, therefore the forces are acting in the
negative z-direction. Enter -3000 in the Forces Global Z input field and
click OK.
The forces should be should be displayed on the truss (proper
direction and magnitude) in the SAP2000 interface window. |
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Step 6: Set Analysis Options - In
this example, the truss structure is modeled in the x-z plane. To limit analysis to
variables in the x-z plane click on the Analyze menu located along the
top the SAP2000 interface window and then click on Set Analysis Options. The Analysis
Options menu will appear as follows:
To restrict SAP2000 to variables in the x-z plane, select
the Plane Frame button,
uncheck the RY box, and
click OK. The truss structure is now ready for analysis.
Step 7: Release Internal Moments at Joints - SAP2000
assumes that all structures are frames. Therefore, to analyze a truss structure we should
convert each joint from a fixed connection to a pin connection.
To ensure that every joint
in the structure is pin connected, select all the members by clicking the Select
All button on the left-side tool bar. Next click on the Assign menu,
then select Frame, then Releases/Partial
Fixity..., and then and
Assing Frame
Releases and Partial Fixity window will appear.
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In this
example, the structure is a truss, which by definition has no moment capacity at each joint.
To release the moment capacity, click on the check boxes that are associated with the Moment
22, Moment 33, and Torsion. Torsion can only be
released at one end of the element, whereas, the other moment must be released at both the
Start and End of the element.
After the moments are released, the truss structure should appear in the SAP2000
interface window as follows: |
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Step 8: Define Material Properties - SAP2000 assumes the
loads acting on a structure include the weight of each weight. In our truss analysis, we
assume that each element is weightless. To define the properties of a material, select
the Define menu located along the top the SAP2000 interface window and
then click on Materials. The Define Materials window will appear as shown
below:
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On this menu
you can change the properties of materials. In this example, select the
A992Fy50 (steel with a yield stress of 50 ksi)
material and click on the Modify/Show Material.. button.
The Material Property Data window will appear.
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Change the
value in the Weight per unit Volume input field to zero. Click OK to return to the Define Materials window and
than click OK again. Now we have a material named
A992Fy50 that has no weight per volume.
For this example problem, the default values for the Mass per unit Volume,
Modulus of elasticity, Poisson's ratio, and the Coefficient of Thermal Expansion can
be used. For most linear elastic statically loaded structures, only values for
Weight per unit Volume and Modulus of Elasticity are required.
Step 9: Define Frame Sections - To define the cross-section
properties of a structural element click on the Define menu located along
the top the SAP2000 interface window, then click on Section Properties,
then Frame Sections..., and then the Frame Properties window will appear as shown below:
The default
Frame Section label is FSEC1. To change the properties of the frame
section click on the Modify/Show Property... button.
The I/Wide Flange Section window will appear.
To define the material
of this frame section click on the Material pull-down menu and select our
weightless material
A992Fy50. Click OK to return to the
Frame Properties
window and than click OK again. If you are interested in
computing deflections in the truss, then you must define the cross-sectional
dimensions of each frame element. In this example, we are interested only in the
axial forces in a determinate truss, so the value of the cross-sectional areas
are not required.
Step 10: Assign Frame Sections - To assign the
frame properties of a structural element, select the element with the pointer and
click on the Assign menu located along the top the SAP2000 interface
window, then click on Frame, and then Frame Sections... You can assign the same section
properties multiple elements by selecting all the elements that share the same properties.
Choose the frame element form the Assign Frame Sections window. The
frame section name will appear next to each element selected. After the frame sections
have been assigned the SAP2000 interface window will appear as follows:
Step 11: Run Analysis -
To analyze the model press the Run
Analysis button
 . The Set Load Cases to Run
menu will appear as shown below:
By default there are two
load cases: DEAD and
MODAL. More load cases can be added,
but for this example the DEAD
load case is all that is required. Click on
Run Now button If the analysis is successful, the Analysis Complete window will
appear and report the the analysis is complete. Click OK and the
Save Model File As window will
appear as shown below:
SAP2000 creates about 40 temporary files when
your run a model, so it is beneficial to choose a
special folder to store the SAP2000 files. The
Windows Desktop is not a good location. Which a
folder is selected, name the SAP2000 model file.
IN this example, the file name is truss_example.
SAP2000 will save the model information in the
file named truss_example.sdb in the folder
selected.
SAP2000 interface window will display an exaggerated deflected shape of the modeled
structure.
Step 12: Print Truss Forces - To get a quick feel for the
relative magnitude of the forces in the truss, click on the Show
Forces/Stresses pull-down menu
 at the top of the SAP2000 interface, select
Frames/Cables/Tendons..., and the Display Frame
Forces/Stresses menu will appear as follows:
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The default
values will display the Axial Forces using the Fill Diagram. If
you click OK, the SAP2000 interface window will display the relative magnitude of the
axial forces with compress forces in red and tension forces
in blue.
Another way to display force information is to unclick Fill Diagram
and click on Show Values on Diagram. In this case, the value of each
axial force will be displayed next to the member (see the figure below). |
To print the results to a file click on the File menu, select Print
Tables..., and the following menu will appear:
Fort this example, all we required are the axial
forces in the truss, so click on expand the
Element Output item under the
ANALYSIS RESULTS section and
expand the Frame Output item
and then click on Tables: Element Forces
- Frames. Also, click on the box
Print to File and the
TXT file button to define file format.
Click OK and define the name
and location of the TXT file.
There is an option for
Spreadsheet Format if desired. The default location for the file is the same
directory as the problem files. A different location can be specified by
clicking File Name and choosing the desired file location and name.
In order to correlation the results printed in the output file to frame elements
in the structure, the frame labels turned on and printed out. To display the
frame element labels click on the Show Undeformed Shape button
 on the
main interface. Next, click on the Display Options button
 and under
the Frame section of the menu click on Labels.
The frame element numbers, or any other information displayed in the main
SAP2000 interface, can be printed by clicking on the File menu and
selecting Print Graphics (the image will be sent to the default printer).
The results of the truss analysis presented in the output file are listed by
frame element number.
Note that SAP2000 list the variation of the internal forces and moments along
the element. For truss analysis there are no bending moments and shear forces.
The values listed in the "P" column are the axial forces in the truss members.
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