Precast/Prestressed Girder Help

Start of Tutorial

  1. Start Precast/Prestressed Girder; by default, the program is installed in Programs/Bentley.
  2. The Project tab will now be displayed, as shown in Figure 1. In the Project tab, enter the general information in the fields as shown in Figure 1. Select Standard (LFD) under Design Code, U.S. Units under Units, and Multi-Span (Continuous) under Span Type. All superimposed dead and live loads will now be analyzed on a continuous beam model.

    Project Tab

  3. Click the Geometry tab to display the Geometry screen, as shown in Figure 2. Enter the information for bridge layout, curb data, topping data, and lane data.

    Geometry Tab

  4. Click Add/Edit Multi-Span (on the Geometry screen) to bring up the Multi-Span screen, as shown in Figure 3. You will be entering three spans, as follows.

    Figure 3 Multi-Span

    1. For the first span, input “117” in the Pier to Pier field, “116” in the Precast Length field, “115” in the Bearing to Bearing field, “0.5” in the Pier CL Precast field, and “116” in the Release Span field. Click Add. It will appear in the list on the screen.
    2. Repeat the above steps to input the remaining two spans as shown in Figure 3.
    3. To modify a span, highlight it in the list, make the necessary changes, and click Modify. To delete a span fromthe list, highlight it and click Delete.
    4. When done, click OK to return to the Geometry screen.
    Note: Topping data can only be defined in the first span and will be considered the same for all spans (i.e., cannot be edited in subsequent spans). The program will automatically update the number of lanes based on the lane width; however, if necessary, you can edit this value.
  5. On the Geometry screen, select the following:
    1. Select 01 from the Span list.
    2. I-Girder from the Beam Type list.
    3. BT-72 from the Beam ID list.
  6. If you are manually inputting the data, click Generate and the Generate Beams screen displays, as shown in Figure 4. This button allows the program to automatically generate the beams for a specific span. For this tutorial, generate six beams for span 1. If you are working from the input file (Tutor1.csl), first delete the current beam information for all spans before generating the beams.

    Generate Beams

    1. Type 6 in the Quantity field and “4.25” in the Overhang field.
    2. Click OK. Precast/Prestressed Girder will automatically generate the beams in Span 1 and return to the Geometry screen.
    The beams for Span 1 will appear in the list under Beam Type/Location, as shown in Figure 4.
  7. Copy this same configuration to all of the spans (i.e., number of beams, beam types, spacing, etc.). Click Copy To All. This will automatically copy the beam configuration for span 1 to spans 2 and 3. By selecting the spans from the Span list, check that the configuration has been copied to both spans. Each span should have the same configuration.
  8. Select the Image option from the Show menu, or its respective icon from the toolbar, to activate the Image screen, as shown in Figure 5.

    Image

    You should see a 3-D representation of the structure on the screen. Use the buttons on the left side of the Image screen to manipulate your view of the image (for example, use Rotate to manipulate the image to the view shown in Figure 5).

  9. Select Show > Model, or select its icon on the toolbar, to bring up the Model screen (Figure 6). A 3-D model will be displayed. The type of model displayed can be selected from the top of the Model screen. Use the buttons on the left side of the screen to manipulate the view of the model.

    Model

  10. Click the Materials tab to display the Materials screen, as shown in Figure 7.
    1. Input the data shown in the figure for Concrete and Tension Rebar
    2. select 1/2-270K-LL-1 in the Tendon ID list
    3. Under Pattern, select the Draped/Straight option
    4. Type 0.4 in the Depress Pt. Fraction field and 2 in the Midspan Increment field
    5. Type 36 in the Debonding Length Increment field
    6. Under Maximum Auto-Debonding Percentage, type 75 in the Per Row field and 50 in the Total% field

    Materials Tab

  11. Click the Loads tab to activate the Loads screen. Input the loads shown Table below.
    Span Beam Component Type Magnitude
    1 1 Prec. DL Line Load 0.180 klf
    1 2 Prec. DL Line Load 0.180 klf
    1 3 Prec. DL Line Load 0.180 klf
    1 4 Prec. DL Line Load 0.180 klf
    1 5 Prec. DL Line Load 0.180 klf
    1 6 Prec. DL Line Load 0.180 klf
    1 Comp Comp. ADL Area Load 0.0305 ksf for 51 ft.
    2 1 Prec. DL Line Load 0.180 klf
    2 2 Prec. DL Line Load 0.180 klf
    2 3 Prec. DL Line Load 0.180 klf
    2 4 Prec. DL Line Load 0.180 klf
    2 5 Prec. DL Line Load 0.180 klf
    2 6 Prec. DL Line Load 0.180 klf
    2 Comp Comp. ADL Area Load 0.0305 ksf for 51 ft.
    3 1 Prec. DL Line Load 0.180 klf
    3 2 Prec. DL Line Load 0.180 klf
    3 3 Prec. DL Line Load 0.180 klf
    3 4 Prec. DL Line Load 0.180 klf
    3 5 Prec. DL Line Load 0.180 klf
    3 6 Prec. DL Line Load 0.180 klf
    3 Comp Comp. ADL Area Load 0.0305 ksf for 51 ft.

    Start by inputting the precast loads as follows:

    1. Select 01 from the Spans list and 1 from the Beams list, to enter a load for Span #1, Beam #1.
    2. Select Precast DL from the Components list, Line Load from the Types list, and enter 0.180 in the Mag. klf field.
    3. Click Add. The load will appear in the list on the screen.
    4. Now select the load you just entered and click Copy To.
    5. The Copy To screen will display, as shown in the following figure. Since you will be using this load on all beams and all spans, select the All Beams and All Spans check boxes.

      Copy To

    6. Click OK and return to the Loads screen. The program will automatically copy the load to all spans and beams. To display all the loads on all spans in the list, select the All Spans check box.
  12. To enter the composite loads:
    1. Select 01 from the Spans list
    2. Select Composite ADL from the Components list
    3. Select Area Load from the Types list
    4. Type 0.0305 in the Mag field and 51 in the Width field.
    5. Click Add to add to the list.

    Loads Tab - Entering Composite Load

    This load will be used on all spans, so click Copy To to activate the Copy To screen. Select the All Spans check box and click OK and return to the Loads screen. The load will automatically be copied to all spans. Once all loads have been entered, the Loads screen should resemble the previous figure.

    Loads Wizard

    Note: Although not used in this tutorial the Loads Wizard is a useful tool which should be explored
  13. Under Live, on the Loads, highlight H/HS25 Truck in the Available list and click the >> button. It will appear in the Selected list, as shown in Figure 9. (Note this is the only truck you will be using for Tutorial 1.) Highlight H/HS25 Lane in the Available list and click the >> button to add it to the Selected list.
    Note: To verify the default live load library and path, select Setup from the Libraries menu. For the LFD design code, Precast/Prestressed Girder uses default.cs4. Click Close to return to the Loads screen.
  14. Click the Analysis tab to activate the Analysis screen. Click Analysis Factors to bring up the Analysis Factors screen, as shown in the following figure. The Distribution tab should be selected.

    Analysis Factors - Distribution Tab

    1. For Dead Load, Live Load - Girder, and Impact Factor, select the Computed option to have the program calculate the factors. Under Impact Factor, select the Shear If same as Moment check box. Change the option to Distributed Dead Load by setting it to be based on Tributary Fraction.
    2. Click the Gamma/Beta Factors tab to display the appropriate screen, as shown in Figure 12. The default values displayed are as specified in AASHTO LFD Table 3.22.1A. Select only the Group 1 option.

      Figure 12 Analysis Factors - Gamma/Beta Factors Tab

    3. Click OK to accept all data in both tabs and return to the Analysis. Note if you modify any information in the Analysis Factors, or click OK, a message will appear asking if you want to keep the existing design pattern; click Yes to retain the pattern.
  15. Click Project Parameters to activate the Project Parameters. The Allowable Stresses tab should be selected, as shown in Figure 13.

    Figure 13 Project Parameters - Allowable Stresses Tab

    The fields are grayed out because the Use factor to calculate check box is selected. When this option is selected, the program calculates the factors using the default stress factors, as specified in Article 9.15. The next step explains how you can modify these factors.

    Note: The factors specified on this screen are the general project level factors applicable to all beams in the project. You can, however, modify individual design level factors at the beam level for specific beams in the Beam tab.
  16. To edit the stress values, click Edit Factors to activate the Allowable Stress Factors, as shown in Figure T14. Input the data as shown in the figure and click OK. The program will calculate the new factors and return to the Project Parameters.

    Figure 14 Allowable Stress Factors Screen

    Note: The program calculates the new factors by multiplying them by the concrete strengths. For example, theallowable compression at bottom at release is 3000 psi, which is obtained by multiplying the concretestrengths at release, 5000 psi, by the corresponding factor, 0.6. Remember that Precast/Prestressed Girder calculates theallowable stress factors using the values specified in the Allowable Stress Factors screen if the Use factorto calculate check box is selected.

    To manually edit the allowable stresses, click the Use factor to calculate check box. Click OK to return to the Analysis.

  17. At this point, it is recommended to save the project. Select the Save As option from the File menu. The Save As screen will open. Enter the file name (e.g., MyTutor1) and click Save. The default extension is *.csl.
  18. To perform the analysis click Run Analysis on the Analysis. Once the analysis is completed, the results will display on the screen, as shown in Figure 15. Use the lists at the top of the screen to view individual load case results and envelope results.

    Figure 15 Analysis Screen - After Analysis is performed

  19. After reviewing the analysis results, click the Beam tab to activate the Beam screen, as shown in Figure 16. First, select the beam you want to design. There are two ways you can select the beam: 1) from the Span and Beam lists, or 2) clicking the cross-section of the beam displayed at the top of the screen. For this tutorial, select 1 from the Span list and 2 from the Beam list.

    Figure 16 Beam Tab

    Note: The buttons under Data Modification contain the same information as the Project Parameters (from the Analysis screen). By default, Precast/Prestressed Girder designs the beam based on the data from this screen (i.e.,project level); however, you can modify the values for a specific beam. Any changes will effect only theselected beam.
  20. Click Loss Data to activate the Loss Data screen, as shown in Figure 17. Select “AASHTO” under Methods and enter 18 in the Release Time field and 75 in the Relative Humidity field. Click OK and return to the Beam screen.

    Figure 17 Loss Data Screen

  21. To automatically generate the strand pattern click Strand Pattern and click Auto Design, as shown in Figure 18. You can also manually input the strand pattern. For this tutorial, the program will generate a draped pattern using 36 strands.

    Figure 18 Strand Pattern

  22. A message will appear to inform you that one or more sections fails under current design/analysis data. Click OK to view the Design Status, as shown in Figure 19. Review the results of the strand pattern. Red text indicates any sections which fail under the current design.

    Figure 19 Design Status - Enhanced Report Viewer

    When you are finished reviewing the results, click the X in the top right corner of the screen to close this screen and return to the Strand Pattern screen. Click OK to return to the Beam screen.

  23. To view graphs of your project select Diagrams from the Show menu. The Diagram screen will appear, as shown in Figure 20.

    Use the lists to view the various diagrams of your project (including moments, shears, and stresses). Experiment with the various options available to become more familiar with their functions.

    Figure 20 Diagram

This completes Tutorial 1. To print the output of the project, select Print from the File menu. The Print screen will display. Select the appropriate options and click OK. Exit the program by selecting Exit from the File menu.

Parent topic: Continuous 3-Span Bridge with Bulb Tee Girder T1 and Continuity Design (U.S. Units) (LFD)