STRUCTURAL ANALYSIS WITH SAP2000

By Mr. Linzhong Deng and Prof. Michel Ghosn

Department of Civil Engineering

The City College of New York

CUNY

 

SAP2000 is a general purpose finite element program which performs the static or dynamic, linear or nonlinear analysis of structural systems. It is also a powerful design tool to design structures following AASHTO specifications, ACI and AISC building codes. These features, and many more make SAP2000 the state-of-the-art in structural analysis program.

The SAP2000 graphic user interface (GUI) is used to model, analyze, design, and display the structure geometry, properties and analysis results. The analysis procedure can be divided into three parts:

    1. Preprocessing.
    2. Solving.
    3. Postprocessing

 

 

Part I. Preprocessing.

In preprocessing, the following information is needed by SAP2000.

    1. Choosing the units for this project.
    2. Setting up geometry.
    3. Defining material and member section properties.
    4. Assigning member section properties and element releases.
    5. Defining load cases.
    6. Assigning load magnitudes.
    7. Assigning restraints.

 

  1. Choosing units.
  • From the combo (i.e. the drop down list) in the main window’s status bar, choose the units for this project.
  • II. Setting up structure’s geometry.

    There are two ways to set up the structure’s geometry: The first is from the SAP2000’s templates. The other is by creating a completely new model.

    When creating from a template, follow these steps:

      1. From File menu, choose New Model from Template… This will display the Model Template dialog box.
      2. In this dialog box:
      1. Click on the template which most closely resembles the structure you want to analyze. This will display the template dialog box.
      2. In this dialog box, choose the appropriate parameters.
      3. Click OK button.

    The screen will refresh and display 3-D and 2-D views of the model in vertically tiled adjoining windows. You can activate the one you plan to work in by clicking the window’s title. You can any one of the two windows if you wish.

    When creating from a new model, follow these steps:

      1. From the File menu, choose the New Model… This will display the window of coordinate system definition.
      2. In the window of coordinate system definition, enter the appropriate grid information. The cross points of the grid will define the necessary joints of your structure. This will display the 3-D and 2-D view window with grid.
      3. 3-D and 2-D views of the model are displayed in vertically tiled adjoining windows. You can close the 3-D windows if you wish. Active the 2-D view by click the x-y button in toolbox or by clicking any point inside the 2-D window.
      4. From draw menu, choose "Draw From Element". This will change your mouse point from " " to " " in the area of 2-D view.
      5. Draw your structure in the grid based on the grid spacings defined in step 2. Click your left mouse button to define the joints. Every joint needs one click. SAP2000 will connect the joints automatically. Double click the left mouse button to stop the action of connection. When you draw something wrong, click the " " inside the floating toolbox situated in the lower part of your screen. Then click the members which need to be deleted. Then from the edit menu, choose "delete". To see the modified structure, from the display menu, choose the show undeformed shape.

     

    III. Define material and structural section properties.

    In this step, we are going to define all the material types and all section properties which are present in this structure. This requires the following steps:

      1. From the define menu, choose material… This will display the window of define material.
      2. If your material is standard steel or concrete, you can click modify/show material button and use the library supplied properties. Click button OK to accept appropriate properties. Otherwise click the button add new material to define a new material’s properties, or the button modify/show material to change the library’s data according to your material’s properties.
      3. From the define menu, choose Frame sections… This will display the Frame sections dialog box.
      4. In this dialog box, you can define a new section type, import a section’s geometry from the SAP2000’s library, or modify a section’s geometry from the default values. Suppose you have two rectangular sections, you need the following steps to define these sections.
      1. Highlight the FSEC1 in the box frame name.
      2. Click the modify/show section button. This will display the window of rectangular section dialog box.
      3. In this box, choose the corresponding material from the material combo, type in the number in the section’s width and height’s text box. Click ok to terminate this dialog box and return to Frame sections dialog box.
      4. Choose Add rectangular in the second combo box. This will display the rectangular section dialog box.
      5. repeat step c to define the properties of section section.
      6. If you want to delete a section type, you highlight the section’s name which is to be deleted, then click the button delete sections.
      1. Click on the OK button to return the main window.

     

    IV. Assigning member section properties.

    There are three selection methods used by SAP2000 to assign member properties, support restraints, loads... For clarity and convenience, these three selection methods are summoned here. The first is to click the members one by one after you click on the pointer tool button (" ") on the floating toolbox. The second way is to drag a rectangular box after you click on the pointer tool button on the floating toolbox. All of the objects inside this rectangular will be selected simultaneously. The third way is to draw a straight line after you click the "Sect intersecting line select mode" button on the floating toolbox. All the objects intersecting the line you draw will be simultaneously selected.

    You need the following steps to assign member section properties:

      1. Select a group of members which have the same sections by one of the 3 selection methods described above.
      2. From assign menu, choose frame, then sections… from the submenu. This will display the define frame sections dialog box.
      3. In the name area of this dialog box, click the section corresponding to this selected group (e.g. FSECT1 or FSECT2, etc).
      4. Repeat steps a, b and c until you have assigned a section for every member of the structure.
      5. Select a group of members which will be assigned the same member releases.
      6. From assign menu, choose frame, then release… from the submenu. This will display the frame release dialog box.
      7. Choose the appropriate release parameters for the already selected members. If these members are truss members, click the check-boxes of torsion-start, moment22-start, moment22-end, moment33-start and moment33-end.
      8. Repeat steps e, f and g until you finish to assign release properties for all the necessary members.

     

    V. Defining load cases.

    Now, it is time to give SAP2000 the applied load’s information. The steps are:

      1. From Define menu, choose Static load cases… This will display the define load case dialog box.
      2. This dialog box will display the default load, LOAD1, with type set to Dead, and self-weight multiplier set to unity. This will automatically include the self-weight of structural members in the analysis based on preset specific weights given in function of the material type. We don’t have to change anything for this first load case. But if you wish to enter the weight by your self and put it as joint load, or if you went to ignore the offset of the dead weight, then you should change the self-weight multiplier to 0 to avoid count the self weight twice.
      3. Define additional load cases, change the LOAD1 to LOAD2 (or the case you defined), select load type from the Type drop-down list box, change the self-weight multiplier to appropriate number. In most times, you change the self-weight multiplier to 0 because dead load already count dead load in LOAD1). Then click on the Add new Load button to notify SAP2000. Repeat this step until you define all the load cases.
      4. Finally, click OK to back to main window.

    In the following section of assigning joint load cases, you must assign a numerical volume and the location of each joint loads for every load cases.

     

    VI. Assigning loads.

    For simplicity, we just talk about assigning joint loads. If you wish to apply a distributed load on a member, you can refer to SAP2000 manual for detail. To assign joint loads execute the following steps:

      1. Select the joints which have the same joint loads. You can use one of the three selection methods used previously to select members.
      2. From the Assign menu, choose Joint Static Loads, then Forces… from the submenu. This will display the Joint forces dialog box.
      3. In this dialog box, accept the default load case name as LOAD1, enter the corresponding joint force components in the Load area. Click OK to accept the above joint loads.
      4. Repeat steps a, b and c until you assign all the joint loads of this load case defined to this structure.
      5. Repeat steps a, b, c and d until you finish every load case’s load assignment.

     

    VII. Assigning restraints.

    It is very important to assign restraints to your structure. Otherwise your structure will become unstable or it becomes a free body and it cannot be solved by SAP2000. Applying joint restraints requires the following steps:

      1. Click the Pointer Tool button ( i.e. ) in the Floating Toolbar.
      2. Click the joints which have the same restraints.
      3. From the Assign menu, choose the Jointà Restraints… from the submenu. This will display the joint restraint dialog box.
      4. In this dialog box, choose appropriate restraint parameter. Then click OK to accept this assignment.
      5. Repeat steps a, b, c and d until you finish the restraint assignment.

    PART II. SolvingIn this part SAP2000 will assemble and solve the global matrix. The following steps are needed:

    1. From the Analysis menu, select Set Option… This will display the Analysis Option dialog box.
    2. In this dialog box, check the available DOF. If you are analyzing a plane truss, check UX and UY, leave the UZ, RX, RY and RZ blank.
    3. Click OK to accept what you choose.
    4. From the analysis menu, select Run. This will display the Save Model File As dialog box.
    5. In this dialog box, save the model under a filename. No extension is necessary.
    6. Click the OK button, the analysis will begin. A top window is opened in which the various phases of analysis process are progressively reported. When the analysis is complete, the screen will display the message "ANALYSIS COMPLETE".
    7. Click OK button in the top window to close it.

     

     

     

     

     

     

    PART III. Postprocessing.

    The main options in postprocessing are:

      1. Displaying the deformed shape.
      2. Displaying the member forces.
      3. Printing the results.
      4. Designing the structural members and checking the safety of a design.
      5. Modifying the structure.

    For simplicity, we just discuss the three fundamental options: displaying the deformed shape, displaying the member forces and printing results here.

    1. Displaying the deformed shape.

    After the analysis is complete, SAP2000 automatically displays the deformed shape of the model for the default load case, LOAD1, in the active display window. We can now display the deformed shape for another load case in one of the two view windows.

    1. Activate one of the two view windows by clicking anywhere inside that window.
    2. Click the display deformed shape button on the floating toolbar. This will display the deformed shape dialog box.
    3. In the drop down list in the load area of this dialog box, select the load case to be displayed, then click OK button. The deformed shape will show.
    1. Displaying the member forces.
    1. From the Display menu, click the Show element forces/stressesà frames, this will display the member force diagram dialog box.
    2. In this dialog box, select the component which need to display (for truss, choose Axial force) in the Component area, and click OK button. The axial force diagram for the entire truss is displayed. By moving cursor to a specific location, we can read the values of the force at that point.
    1. Printing the results.
    1. From File menu, select Print Output Table… In the display dialog box, click OK to accept the default setting. The detailed output results will be printed.
    2. From File menu, select Print Input Table… In the display dialog box, click OK to accept the default setting. The detailed input information will be printed.

    You can also get the detailed results in another way. When we analyze a structure, by default, SAP2000 will create three output files: filename.out, filename.log and filename.EKO. The output file filename.out stores the output of your analysis. The output file filename.EKO stores the input information for this structure. The output file filename.log take all of the running information. These files are text files. You can print these files using computer operating system. For example, we can print these files from Notepad. The steps are:

      1. Open Notepad by double click the Notepad icon on the main window.
      2. From File menu, choose Open. This will display a standard Microsoft file selection dialog box.
      3. In this dialog box, choose the drive and subdirectory where your file is located.
      4. Click on the file name you want to display and print. (i. e. any one of filename.out, filename.EKO, or filename.log.)
      5. Click OK to terminate this dialog box. Your file will display by Notepad.
      6. Review the file to make sure your results are correct.
      7. From File menu, choose print… This will display the print dialog box.
      8. Click OK to accept the default print setting. Your file will print on background.
      9. Repeat steps b, c, d, e, f, g and h to print another file.
      10. Close Notepad by choosing Exit from the File menu.