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Department of Civil Engineering
CIVL 3121- SAP2000 Tutorial for Truss Analysis
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The following is a step-by-step procedure for analysis a two-dimensional truss structure using SAP2000.  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.

truss_example.gif (4721 bytes)


When you start SAP2000 Educational Version 7.4 you should see the following interface window:

interface.jpg (34425 bytes)


Step 1: Set Problem Dimensions - On the bottom on the interface window, set the desired units for the problem using the pull-down menu. In this example, the units are feet and kips.

units_pull_down.jpg (5219 bytes)


Step 2: Grid Spacing - Determine the appropriate number of grid line and grid spacing to locate the joints of the truss. The grid spacing is set by defining a new problem.  To create a new problem, select New Model under the File menu.

new_problem_memu.jpg (49885 bytes)

 

When you select New Model on the menu, the Coordinate System Definition window will appear (see the figure on the right).

Remember, that SAP2000 assumes that your two-dimensional structure resides in the x-z plane.

Define your grid system by entering data on the Coordinate System Definition window. For the truss shown above, the the grid spacing in the x and z-directions is 20 feet. The number of grid spaces in the x and z-directions are 4 and 1, respectively. No y-direction grid line are necessary for this problem.

When you click OK, SAP2000 generates the grids line 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.

coordinate_system.jpg (25394 bytes)

 

sap_grid_line_1.jpg (53941 bytes)
tool_bar.jpg (12687 bytes)


Step 3: Locate Truss Joints - To define the joint locations, select the Draw Special Joint button joint_button.jpg (1022 bytes) on the lower tool bar. Click on grid intersection lines to define joints. For this problem the joint locations are shown below:

sap_joints.jpg (55352 bytes)
tool_bar.jpg (12687 bytes)


 

Step 4: Draw Frame Elements - To define each frame element, select the Draw Frame Element button element_button.jpg (909 bytes) on the lower tool bar. 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:

sap_elements.jpg (61358 bytes)

tool_bar.jpg (12687 bytes)


 

Step 5: 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 yellow "X" should appear at the joint to indicate that it is currently selected. Next click on the Joint Restraint button support_button.jpg (946 bytes) on the bottom tool bar.

The 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 support_button.jpg (946 bytes) Fast Restraints button. On the Fast Restraints menu select the pin button pin_button.jpg (961 bytes) and click OK.

Next, select the lower right-hand joint with the pointer and click on the support_button.jpg (946 bytes) Fast Restraints button. On the Fast Restraints menu select the roller button roller_button.jpg (980 bytes) and click OK.

joint_restraint_menu.jpg (18543 bytes)

After the supports have been defined the truss problem should appear in the SAP2000 interface window as follows:

sap_supports.jpg (62141 bytes)

tool_bar.jpg (12687 bytes)


Step 6: Apply Forces at Joints - To apply forces at a joint, select the joint with the pointer and click on the Assign Joint Loadings button joint_loading_button.jpg (949 bytes).  The following menu will appear:

 

In this example, there are three 3 kip 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 -3.0 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.

joint_forces.jpg (29516 bytes)

sap_forces.jpg (62255 bytes)
tool_bar.jpg (12687 bytes)


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 bottom tool bar. Next click on Assign menu and select Frame then Releases and the and Frame Releases window will appear.

sap_assign_frame_releases.jpg (72098 bytes)
tool_bar.jpg (12687 bytes)

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:

frame_releases_menu.jpg (18810 bytes)

sap_frame_releases.jpg (62282 bytes)
tool_bar.jpg (12687 bytes)


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:

define_materials_menu.jpg (17847 bytes)

On this menu you can change the properties of materials. In this example, select the OTHER material and click on the Modify/Show Material button.

 The Material Property Data window will appear.

 

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 OTHER 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 Coeff 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.

materal_property_data_menu.jpg (34329 bytes)


 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 and then click on Frame Sections. The Define Frame Sections window will appear as shown below:

define_frame_sections_menu.jpg (22275 bytes)

The default Frame Section label is FSEC1. To change the properties of the frame section click on the  on the Modify/Show Material button. The Rectangular Section window will appear.

rectangular_section_menu.jpg (37000 bytes)

To the material of this frame section click on the Material pull-down menu and select our weightless material OTHER. Click OK to return to the Define Frame Sections window and than click OK again. If you are interested in computing deflections in the truss, then you must define the Depth (t3) and Width (t2) of the cross-section. 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 important.


Step 10: Assign Frame Sections - To assign the cross-section 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 and then click on Frame Sections. You can assign the same section properties multiple elements by selecting all the elements that share the same properties. 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:

sap_frame_sections.jpg (63505 bytes)
tool_bar.jpg (12687 bytes)


Step 11: Set Analysis Options  and Run Analysis - 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 Options. The Analysis Options menu will appear as follows:

analysis_options_menu.jpg (39546 bytes)

To restrict SAP2000 to variables in the x- plane, select the Plane Frame button and click OK. The truss structure is now ready for analysis. To analyze the model press the Run Analysis button run_analysis_button.jpg (903 bytes).

If the analysis is successful, the Analysis Complete window will appear and report the the analysis is complete.  Click OK and the SAP2000 interface window will display an exaggerated deflected shape of the modeled structure.

sap_analysis_complete.jpg (31435 bytes)

If the window reports that the analysis is incomplete, make sure that the moments have been released and that the analysis options have been set correctly.

After the Analysis Complete window has been closed, typically SAP2000 displays the deflected shape of the structure as shown below:

sap_deflected_shape.jpg (61390 bytes)
tool_bar.jpg (12687 bytes)


Step 12: Print Truss Forces - To get a quick feel for the relative magnitude of the forces in the truss, select the Member Force Diagram for Frames button member_forces_button.jpg (988 bytes) along the bottom tool bar. The Member Force Diagram for Frame menu will appear as follows:

member_force_menu_1.jpg (23162 bytes) 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 yellow.  

 

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).

 

sap_member_forces_2.jpg (66091 bytes)
tool_bar.jpg (12687 bytes)

To print the results to a file click on the File menu and select Print Output Tables and click the Print to File box. 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 Set Elements 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.

 


This website was originally developed by Charles Camp for CIVL 3121.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are welcomed.

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Department of Civil Engineering | 104 Engineering Science Bldg |Memphis, TN 38152 | Phone: 901/678-2746 | Fax: 901/678-3026 | Last updated: 09/08/2016