EXAMPLE TRUSS Consider the pin-jointed steel truss shown in Figure 1. The members consist of universal angle (UA) sections as follows: • • •
Rafter: UA 200x100x15 Main tie: UA 125x75x12 Internal members: UA 90x90x10
These sections can be found in the cross-section library of the STAAD.PRO program. The Young’s modulus of steel is E=210x106 kN/m2.
Determine the axial forces in the members of the truss, reactions at supports and the node displacements of the truss.
15 kN 15 kN
15 kN 7 6
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7
8
10
5
12
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9
1
11
1
4
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2 2.5 m
3m
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2.5 m
2.5 m
Figure 1.
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2.5 m
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We will now create the structure in STAAD.Pro. After launching STAAD.Pro, the new model wizard will appear allowing you to create a new file or open recent files. Select Truss as the structure type. Then input a file name for the structure such as “Truss”. You need to select a location to have the file created and saved in. You are advised to save the file in your home area on the network.
Select Open Structure Wizard to select one of the prototype models already existent in the library of the program.
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Select the Howe Roof truss model.
Input the length, height and the number of bays (spans) along the length of the truss.
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The truss will appear on the screen as shown below. The transfer the truss to the main window of the program, close the window “default.stp –StWizard”.
Select Yes.
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Paste Prototype Model – press OK. The Truss will now be shown in the main window of the program.
To change the orientation of the truss, select View, then Orientation.
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Set Elevation Angle and Rotation Angle zero, then select Apply and Close.
Define supports. First select the Support tab on the left-side on the window. Click on Create so you can add a support to the support list.
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A window will appear allowing you to create several different types of supports. Make sure the Pinned tab is selected, then click on Add.
Repeat the command by selecting Fixed But to create the roller support. Select FX and MZ as releases and then click on Add.
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The two types of supports will appear in the list of supports.
Assign each type of support to the appropriate node of the truss - simple support at node 1.
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Assign roller supports at node 5 of the truss.
The truss has now both supports.
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Define the properties (cross-sections) of the truss members. First click on the General tab on the leftside on the window. Notice that the Property tab is already selected. Now click on Section Database to define properties of the truss members.
The The Section Profile Tables will appear on the screen. Choose Steel, British and then Angle. Scroll down through the list and select the appropriate cross sections for the members. For the rafter members, select UA200x100x15 from the list. Un-tick the Material option (below the list of cross-sections). The material property (Young’s modulus) will be defined separately. Then click Add Add to add this section to the list of cross-sections for the truss members.
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For the elements of the main tie, select UA125x75x12 from the list of cross-sections and then click Add.
For the internal members of the truss, select UA90x90x10 from the list of cross-sections and then click Add.
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The three selected cross-sections are added to the list of sections for the truss.
Select the first cross-section from the list and assigned it to the rafter’s members.
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Select the third cross-section from the list and assigned it to the internal members.
Select the second cross-section from the list and assigned it to the main tie’s members.
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Define the Young’s modulus of elasticity of the material. Select Command , then Material Constants followed by Elasticity.
Enter the value of the Young’s modulus of elasticity for steel.
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Define the loads acting at the nodes of the truss. First select the Load tab on the left-side of the window. A window will appear including the “Load Cases Details” option. Click on the Load Cases Details tab, then click on Add, to create a new primary load case.
A window will appear allowing you to input a title such as “Load Case 1” in the title field (Note: inputting a title is optional). When you are done, click on Add and Close to create a new primary load case.
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Select Load Case 1 , then click on Add to select the load.
A window will appear allowing you to create several different types of beam load. Select Nodal Load, then Node. Input -15 for FY and then click on Add and then Close.
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A window will appear showing the defined vertical point load of 15 kN under the Load Case 1.
We will now assign the point load to the nodes 6, 7 and 8. Select FY -15 kN under Load Case 1, select Use Cursor To Assign as assignment method and finally click on Assign for the assignment method. Your mouse cursor will now change into a load icon. Click on node 6, then 7 and 8 to assign the point load. Note that there is a blue arrow pointing downwards at nodes 6, 7 and 8. This indicates that a point force has been applied at that node.
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Since the arrow is too big, this can be scaled down to a reasonable size. To do that, click on View, then Structure Diagrams..
Choose Scales and change the value under the Point Force tab until you get a reasonable size for the arrow of the point load. For example, choose 10 for the scale factor, then click on Apply and then OK.
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The size of the arrow changed.
We will now prepare the structure for analysis. First click on the Analysis / Print tab. A window will appear allowing you to select the type of analysis to perform. Make sure the Perform Analysis tab is selected. Select All for the print option. The print option allows you to select optional outputs to print when the analysis is performed. When you are done, click on Add and then on Close.
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The following window will appear.
We will now add post-print commands. Click on the Post-Print tab on the left side of the window then click on Define Commands … to set up commands to print support reactions, member forces and joint displacements.
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Select the Member forces tab, select Global and then click on Add.
Select Joint displacement tab and click on Add ant then Close.
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Select Support reactions tab and then click on Add.
All the three print commands are shown now in the Post Analysis Print list.
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Click on PRINT JOINT DISPLACEMENTS in the Post Analysis Print list. Select Assign To View for the assignment method in order to print member forces for all beam elements and then click on Assign. A message box will appear asking you to proceed. Click on Yes.
The yellow question mark for PRINT JOINT DISPLACEMENTS DISPLACEMENTS will change into a green tick.
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Repeat the command for the support reactions and the members forces.
Now perform the analysis of the structure. First click on Analyze and then on Run Analysis.
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A window will appear allowing you to select the analysis engine. Leave STAAD Analysis selected for the analysis option then click on Run Analysis.
Select the Save option to save the file.
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A window will appear showing the progress of the analysis. Once the analysis is completed, we can view the results via either View Output File option or Go to Post Processing Mode option. Let’s choose Go to Post Processing Mode to view the results and click on Done.
Select Load Case 1, then OK.
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The default results which appear on the screen are Node displacements.
Displacement of the nodes of the truss. The deflected shape of the truss is shown in green.
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Since the deflections are too small, they are scaled to a reasonable size. To do that, click on Results, then Scale.
Change the value under the Results Scales - Displacement tab until you get a reasonable size for the node displacements. For example, choose 4 for the scale factor, then click on Apply and then OK.
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The deflected shape of the truss is now visible.
The view the magnitude of the node deflection at the joints of the truss, select Results then View Value..
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Choose Node and Global Y for the deflections in the vertical direction.
The vertical deflections are shown next to the joint.
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To view the axial forces in the members of the truss, select View then Structure Diagrams ..
Tick Beam Forces – Axial – Diagram. Tension is shown in blue and compression in red.
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The axial force diagram for each member of the truss is shown below.
If the size of the axial force diagram is too large, this can be scaled down. Select Results, then Scale.
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Choose, for example, factor 100 for the axial force.
The size of the axial force diagram is now smaller.
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To view the magnitude of the axial forces in the members of the truss, select Results, then View Value.
The axial force diagram for all members can be shown or only certain members can be selected. In the figure below, only the main tie members are selected.
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The magnitude of the axial force in the members of the main tie is shown next to the members.
Members 5 and 7 are selected.
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The axial force in members 5 and 7 is shown below.
Members 6 and 8 are selected.
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The axial force in members 6 and 8 is shown below.
Members 9,10 and 11 are selected.
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The axial force in members 9,10 and 11 is shown below.
Members 12 and 13 are selected.
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The axial force in members 12 and 13 is shown below.
The reactions at supports can be shown on the screen. Select Results, View Values ant then Reactions.
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The reactions at supports are shown on the screen.
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