This textbook provides a unique and comprehensive perspective on the analysis and design of building structures. Striving to bridge the gap between how engineers and architects view structural support, this book’s numerous examples employing SAP2000 illustrate how modern analysis software can provide a common language for understanding and exploring structural behavior. SAP2000 follows in the same tradition featuring a very sophisticated, intuitive and versatile user interface powered by an unmatched analysis engine and design tools for engineers working on transportation, industrial, public works, sports, and other facilities.
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.
SAP2000 Student Demo is made available to you through the College of Engineering's SAP2000 Student Demo license. The NC State College of Engineering is ordering the right for you to use SAP2000 Student Demo for academic-related purposes. Installation of SAP2000 Student Demo on a computer owned by a business other than NC State University is.
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.
| When 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. |
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.
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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.
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. |
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:
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. |
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.
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: |
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:
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. |
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.
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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:
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.
This website was originally developed by Charles Camp for CIVL 3121. |