STAAD PRO Bridge Modelling

Bridge Design using the STAAD.Pro/Beava AASHTO Code

By

IEG Group, Bentley Systems Bentley Systems Inc.

March 12, 2008

TABLE OF CONTENTS 1.0 Introduction………………………………………………….1 2.0 Creating the Bridge Geometry/Structural Analysis…………2 3.0 Generate AASHTO 2002 Maximum Response……………13 4.0 Bridge Design as Per AASHTO…………………………...31 5.0 Conclusion…………………………………………………36

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1.0 Introduction The combination of STAAD.pro and STAAD.beava can make your bridge design and analysis easier. STAAD.pro is first used to construct the bridge geometry and STAAD.beava is used to find the AASHTO 2002 load positions that will create the maximum load response. The maximum load response could be any of the following:

1. Maximum plate stresses, moment about the local x axis of a plate (Mx), moment about the local y axis of a plate (My) etc. used to design for concrete deck reinforcement.

2. Maximum support reactions to design isolated, pile cap, and mat foundations.

3. Maximum bending moment or axial force in a member used to design members as per

the AASHTO code. 4. Maximum deflection at mid span.

These loads that create the maximum load responses can be transferred into STAAD.pro as load cases to load combinations for further analysis and design. Figure 1 shows the bridge design procedure discussed above.

Figure 1: Bridge Design process in STAAD.pro

The purpose of this document is to explain these steps in more detail.

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2.0 Creating the Bridge Geometry/Structural Analysis Figure 2 shows a bridge with the dimensions.

Figure 2: Bridge Dimensions

Figure 3: Completed Bridge Model in STAAD.pro

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1. Open STAAD.pro with the default units of Kip-Ft and use the Space option.

2. Click the Next button and select the Add Beam mode. Click Finish.

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3. The goal of the next few steps is to draw the stick model of the Bridge Structure (i.e. the beams and the girders). Select the X-Z grid option. Create two grid lines in the x direction at 80ft spacing. Create four grid lines in the z direction at 10ft spacing.

4. Click on the Snap Node/Beam button and draw the beams and the girders as shown

below. First draw five 160ft girders. Then draw the 40ft beams in the z direction. 5. Click on Geometry->Intersect Selected Members->Highlight. STAAD.pro will highlight

all the beams that intersect each other no common nodes. To break these beams at the intersection point, click on Geometry->Intersect Selected Members->Intersect.

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6. The beams have been created. The columns will now be created using the translational

repeat command. Select nodes 6, 14, and 7 using the nodes cursor as shown below. The node numbers may vary depending upon how the model was constructed. Select Geometry->Translational Repeat from the menu. Select the y direction for the translational repeat and select enter a Default Step Spacing of -25ft as shown below.

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7. The deck of this bridge structure will be created in the following steps using the Generate

Surface Meshing Tool. In reality, spacing of beams etc. may not be regular and hence it may become difficult to create the deck of the bridge using the Generate Surface Meshing Tool. The Parametric Meshing Mode could become very useful in these circumstances.

8. Select the Geometry->Generate Surface Meshing tool from the menu. Select four nodes that outline the 160ft x 40ft deck. Simply click on node 1, 3, 11, and 10 and select node 1 again to complete the command. Select the Quadrilateral Meshing option.

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9. 2ft x 2ft element size is adequate for this type of model. Hence input the parameters as

shown below.

10. The mesh will be created. To view the mesh properly, you will need to click on the Setup

Control Tab on your left. 11. You should note that the girders and beams are automatically broken down into smaller

elements. In reality, the girders are physically attached to the deck hence it is ok to mesh them. The concrete beams parallel to the z-axis are not attached to the deck. The load from the deck is transferred to the 40ft steel girders. The load from the girders is then transferred to the concrete beams. Hence, we should merge the beams in the z direction. Select the Select->Beam Parallel To->z from the menu.

12. Select Geometry->Merge Selected Members to merge the split concrete beams. Select each entry and press the Merge button.

13. By merging the beams together, the concrete column to beam connectivity is lost.

Hence, click on Geometry->Intersect Selected Members->Highlight. STAAD.pro will highlight all the beams that intersect each other no common nodes. To break these beams at the intersection point, click on Geometry->Intersect Selected Members->Intersect.

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14. The geometry has been created. The properties and specifications have to be assigned.

Concrete Deck Plate elements - 12in thick Material - Concrete

Steel Girders – Parallel to x axis 40ft Beam elements – W24X103 Material – Steel Offset = 0.5 x Depth of Beam + 0.5 thickness of slab = 0.5 x 2.044167 ft + 0.5 x 1ft = 1.5220835 ft at both ends.

Concrete Beam – Parallel to the z axis

2ft x 2ft Sections Material – Concrete Offset = 0.5 x Depth of Concrete Beam + Depth of Steel Beam + 0.5 thickness of slab = 0.5 x 2ft + 2.044167 ft + 0.5 x 1ft = 3.544167 ft at both ends.

Concrete Columns 2ft Circular Sections Material – Concrete Offset = Depth of Concrete Beam + Depth of Steel Beam + 0.5 thickness of slab = 2ft + 2.044167 ft + 0.5 x 1ft = 4.544167ft at the end connected to the concrete beams.

15. Click on the General->Property control tab on your left and click on the Section

Database button. 16. Select the W24x103 section from the American W shape database and click on the Add

button. 17. Select the W24x103 section that has been created. Click on the Select the Select-

>Beam Parallel To->X from the menu and click on the Assign button on the right. 18. Select the Define button on the right and select the Circle section profile. Input a 2ft

diameter and press the Add button. 19. Select the Rectangle section profile and input 2ft in the YD and ZD input boxes. Press

the Add button. Press the Close button. 20. Select the Cir 24 section that has been created. Click on the Select the Select->Beam

Parallel To->Y from the menu and click on the Assign button on the right. 21. Select the Rect 24.00x24.00 section that has been created. Click on the Select the

Select->Beam Parallel To->Z from the menu and click on the Assign button on the right.

22. Press the Thickness button the right and input 1ft in the Node 1 input box and press the Add button.

23. Select the newly create Plate Thickness entry in the Properties dialog box. Select all the plates using the Plates Cursor and press the Assign button.

24. Select the Spec sub-control tab on the left. 25. Press the Beam button. Select the Offset tab. Select the start option from the location

selection box. Select the local option from the Direction selection box. Input an offset of -1.5220835 in the Y input box. Press the Add button.

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26. Press the Beam button. Select the Offset tab. Select the End option from the Location selection box. Select the Local option from the Direction selection box. Input an offset of -1.5220835 in the Y input box. Press the Add button.

27. Assign these specifications to the steel girders parallel to the x-axis. Select the START LOCAL 0 -1.52208 0 specification that has been created. Click on the Select->Beam Parallel To->x from the menu and click on the Assign button on the right.

28. Select the END LOCAL 0 -1.52208 0 specification that has been created. Click on the Select the Select->Beam Parallel To->x from the menu and click on the Assign button on the right.

29. Repeat Steps# 25 to 28 for the concrete beams but use a y-offset of -3.544167 ft at both ends. For Step# 28, you will need to select beams parallel to the z-axis.

30. For the columns assign a local x-offset of 4.544167ft at the end connected to the concrete beams.

31. Click on the View->3D Rendering in the menu.

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32. Select the General->Support tab on the left and click on the Create button in the right hand side Data Area. Click on the Add button (i.e create a fixed support entry).

33. Select the newly created S2 Support 2 entry and using the nodes cursor select nodes 16, 17, and 18. Click on the Assign button.

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34. Select the General->Load tab on the left and click on Load Case Details on the right hand side in the Data Area.

35. Click on the Add button on the right. Input the “Dead Load” in the Title input box. Select “Dead” in the Loading type selection box. Press the Add button. Click the Close button.

36. Select the newly created 1: Dead Load entry in the data area. Press the Add button.

Select the Selfweight item and press the Add button. 37. Click on the Analysis/Print control tab item on the left and press the Add button. 38. Click on the Analyze->Run Analysis menu. Use the STAAD Analysis option and click

on the Run Analysis button. 39. If the analysis completed successfully, you should look at the exaggerated deflected

shape of the bridge under the action of selfweight. Try to find out any connectivity problems etc. You can go to the Post-Processing mode by clicking on Mode->Postprocessing command in the menu.

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40. You can look at the bending moment diagram for the bridge by clicking on the Beam-

>Forces control tab on your left in the Post-Processing mode.

41. You can look at the stress distribution diagram for the bridge by clicking on the Plate control tab on your left. Select the Max Absolute stress type from the Stress Type selection box and click on the Ok button.

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3.0 Generate AASHTO 2002 Maximum Response The purpose of this section is to generate the following AASHTO 2002 Maximum Response:

1. Maximum plate stresses, moment about the local x-axis of a plate (Mx), moment about the local y axis of a plate (My) in plate # 967 and 1046 that will be used to design for concrete deck reinforcement. The plates are located at the center of the two spans as shown below.

2. Maximum support reactions at the three supports which will be used to design pile cap

footings using STAAD.foundation.

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3. Maximum bending moment (Mz) in members 65, 145, 476, 555, 964, 1043, 1456, 1535,

1944, and 2023 used to design members as per the AASHTO code.

4. Maximum deflection at nodes 842 and 881 to check if the deflection of the girders is less

than L/360 = 80ft x 12in/360 = 2.7in.

STAAD.beava will be used to generate the above maximum responses (i.e. the location of the AASHTO HS-20 loading or lane loading on this bridge that will generate the maximum responses listed above).

1. Click on Mode->Bridge Deck Preprocessor menu 2. The first step in STAAD.beava is to generate a deck and define a roadway. The second

step is to generate the influence surface and view the influence surface diagrams. The influence surface diagram will give a clear picture of the distribution or stresses, moments, forces, etc. across the bridge as a result of loading a certain place with unit loading. The last step is to use the Load Generator to generate the desired maximum responses and transfer them into STAAD.pro as independent load cases for further analysis and design.

3. To generate a deck and define a roadway, select the Plates Cursor and select all plates in the model. Click on Deck->Create Deck command in the menu. To accept the default name of the deck click the Ok button. In STAAD.beava, you are allowed to create multiple decks and multiple roadways on a single deck.

4. Click on Deck->Define Roadway menu and click the New button. The next few steps will illustrate the creation of lanes on the Deck 1 that has been created. Using the Nodes Cursor, the user may find out the coordinates of the deck as shown below. The origin of the deck is at the top left hand side corner.

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5. The Define Roadway dialog box contains three tabs. Namely, Straight, Curved, and

Custom. The Straight option allows users to simply define the outer curb origins and STAAD.beava will calculate the lane widths automatically.

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If the designer has no idea about the lane widths of this bridge, he/she can simply enter the following inputs. The meaning of the Curb A and Curb B Origin input parameters can be best understood from the above figure. The Angle input box simply allows the user to define a roadway which is placed at an angle in the Global XZ-plane.

The Curved tab is used to create a curved lane on any deck.

For example, in this case, if curved lanes were desired, the engineer first has to know the center of the curve. The Angle input parameter will control the starting point of the lanes on the bridge. For Example, if the lane starts at the center of the bridge, the engineer may enter

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-270 degrees for both Curb A and Curb B. The Angle is measured with respect to the line parallel to the global x axis which passes through the center of the circular lane. In the example shown in the following figure curved lanes have to be generated. Suppose the lanes are only to be generated on the region that lies between 0 and 65.4 degrees angle in the anticlockwise direction (plan view). Curb A radius is 10ft and Curb B radius be 100ft.

The engineer needs to provide the following inputs in the Define Roadway dialog box.

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Note that the Curb A radius of 60ft and Curb B radius of 100ft have been provided. The center of the circular lanes is located at x=0 and z=0 with respect to the global coordinate system. The Anticlockwise direction has been provided. STAAD.beava will automatically generate lanes for the deck region that that lies between 0 and 65.4 degrees angle in the anticlockwise direction (plan view). Suppose the lanes have to be generated for the entire circular deck. The user may enter in the following inputs. Note that in this case, the start Angle of the Curb A and Curb B are 65.5 degrees and the direction of lane generation is in Clockwise.

The Spacing Between Points input box lets the user control the increment of the moving load on the bridge deck. In the Straight and the Curved tabs discussed above, the numbers of lanes are automatically calculated by STAAD.beava. In this example, there are four 10ft wide lanes. These lanes can be created using the Custom tab.

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6. Click on the Custom tab and enter the following inputs:

7. The first lane has been created. To create the second lane click on the Add Lane to

Right Button. Enter in the following parameters in the input boxes.

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8. The second lane has been created. To create the third lane click on the Add Lane to Right Button. Enter in the following parameters in the input boxes.

9. The third lane has been created. To create the last lane click on the Add Lane to Right Button. Enter in the following parameters in the input boxes. Press the Ok button. Press the Close button for the Roadways dialog box.

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The Custom tab allows users to create lanes with/without curbs. For example, the following scenario can be created in this example. The following is just an example and you are not required to test this out on this model.

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10. Click on Loading->Influence Surface Generator. This will generate the influence surface diagrams for the entire deck. This process may take long depending upon the number of plates in the model.

11. Click on the Loading->Influence Diagram from the menu. One of the maximum response conditions is the maximum absolute stress in plate 967. The engineer can look at the influence surface diagram for this plate using the Loading->Influence Diagram from the menu. Select Plate Stress for the Diagram Type selection box, Max Absolute for the Stress Type selection box, and plate 967 for the Plate number selection box. Press the ok button. The influence surface diagram should be displayed in the graphics window along with the legend on the left hand side.

12. Click on Vehicle->Database from the menu. This menu will display the vehicle

database. Click on AASHTO HS 20-44. You will notice the point loads that are associated with this vehicle in the Vehicle Database dialog box. You may also create your own Vehicle definitions by pressing the New button.

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The Figure shown above explains the input parameters used to create the HS 20-44 vehicle. The Figure shown below explains the definition used to create the third axle which does not have a fixed location. STAAD.beava will place the third axle anywhere between14ft to 30ft whichever location generates the maximum response.

13. Click the Ok button on the Vehicle Database dialog box. Click on Loading->Run Load

Generator command in the menu.

14. Select the AASHTO ASD/LFD design code from the Design Code selection box. Select

the Ultimate Limit State for the Limit State selection dialog box. Let us instruct STAAD.beava to find the AASHTO HS 20-44 load position/conditions that will generate the maximum plate stresses, moment about the local x-axis of a plate (Mx), moment about the local y axis of a plate (My) in plate # 967 and 1046.

15. Click on the AASHTO tab in the Load Generator Tab and select the HS 20-44 in the Loading Class selection box.

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16. Click on the Plate Center Stress tab in the Load Generator Tab. Input the parameters

as shown in the following figure.

Please refer to section 1.6.1 Plate and Shell Element of the STAAD.pro Technical Reference Manual for a description of the stress types. The first instruction in the above dialog box instructs STAAD.beava find out the AASHTO load positions/conditions on the four lanes that will generate the maximum positive stress on the top side of element 967. The top side of element 967 is shown in the following figure.

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17. The second AASHTO 2002 Maximum Response Criteria is the maximum support

reactions at the three supports which will be used to the design pile cap footings using STAAD.foundation. Select the Support Reactions tab and enter the information as shown in the following figure.

The first instruction in the above dialog box instructs STAAD.beava to find out the AASHTO loading on the four lanes that will generate the maximum Fy reaction at support #16. 18. The third AASHTO 2002 Maximum Response Criteria is the maximum bending moment

(Mz) in members 65, 145, 476, 555, 964, 1043, 1456, 1535, 1944, and 2023 used to design members as per the AASHTO code. Select the Beam End Forces tab and enter the information as shown in the following figure.

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19. The Last AASHTO 2002 Maximum Response Criteria is the maximum deflection at

nodes 842 and 881. Select the Node Displacements tab and enter the information as shown in the following figure.

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20. Press the Ok button. You will notice the following dialog box which will summarize the STAAD.beava AASHTO 2002 Maximum Response load locations for the desired responses that we entered above. You may get accurate load positions for each lane using the Lane No. Selection box. Click the Close button.

21. The vehicle positions generating the maximum Response for each desired response can

be viewed in the STAAD.pro graphics window. 22. Click on the response selection box located on the top right corner of your screen. You

will notice a list of all the desired responses that were entered. Right click on the graphics window and select the Labels command. Select the Deck tab and check the loads and vehicles options. Click the Ok button. You will notice the vehicle positions on the deck.

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23. These loads can be transferred to STAAD.pro as individual load cases for the slab, member, and foundation design using the code check features in STAAD.pro. To transfer these loadings into STAAD.pro, select the Loading->Create Loading in STAAD.pro Model command from the menu. Click on Model->Modeling command to return to STAAD.pro.

24. In the General->Load control tab you will notice that all the AASHTO loadings have been created. These loadings do not include the selfweight of the structure that will be needed for the design. You may simply add the SELFWEIGHT Y -1 command to each load case using the STAAD.pro input file editor. You may add this load item to each load case using the STAAD.pro GUI.

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4.0 Bridge Design as Per AASHTO Steel Design as per AASHTO can be summarized in the following steps: 1. Perform Analysis 2. Code Check 3. Select 4. Grouping 5. Perform Analysis 6. Code Check We have already completed the first step. If the beams pass in Step# 2, the subsequent steps can either be used for optimization or may not be required. If the beams fail after Step#6, the entire design cycle from Step# 1 to Step# 6 have to be repeated. To minimize the number of cycles, one may take advantage of the Ratio design parameter which is not discussed in this manual. For more information on the Ratio design parameter please refer to Section 2.13.1 AASHTO (ASD) of the STAAD.pro Technical Reference Manual. After obtaining the analysis results, we could design the steel beams using the following steps: 1. Members 65, 145, 476, 555, 964, 1043, 1456, 1535, 1944, and 2023 are used to design

members as per the AASHTO code and all the load cases that we have created. Hence, in the Modeling Mode, click on the Design->Steel control tab on your left. Select the AASHTO design code in the Data Area.

2. Create a group of members 65, 145, 476, 555, 964, 1043, 1456, 1535, 1944, and 2023 using the Tools->Create New Group menu command.

3. We will accept the default steel design parameters in STAAD.pro except for members 964 and 1043 have to have proper DFF, DJ1and DJ2 parameters assigned for the deflection checks.

4. Select beams 964 and 1043 in the graphics window and click the Define Parameters button in the Data Area. The deflection of beam 964 with respect to nodes 6 and 14 have to be checked. The deflection of beam 1041 with respect to nodes 14 and 7 have to be checked.

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Hence, select the DJ1 parameter from the Design Parameters dialog box and enter node number 6 and press the Add button. Select the DJ2 parameter and enter node number 14 and press the Add button. Again, Select the DJ1 parameter from the Design Parameters dialog box and enter node number 14 and press the Add button. Select the DJ2 parameter and enter node number 7 and press the Add button. Select the DFF parameter from the Design Parameters dialog box and enter 360 and press the Add button. This is the L/360 deflection criteria. Click the Close button. Under the Parameter 1 tree item in the Data Area, you will notice the following items with question marks besides them.

These design parameters have to be assigned to members 964 and 1043. The DJ1 6, DJ2 14, and DFF 360 parameters have to be assigned to beam 964. The DJ1 14, DJ2 7, and DFF 360 parameters have to be assigned to beam 1043. Select the DJ1 6 parameter in the Data Area. Uncheck the Highlight Selected Geometry check box. Select beam 964 in the graphics window. Select the Assign to Selected beams option and click the Assign button. Similarly, assign the remaining design parameters using the procedure described above. 5. Select the Select->By Group Name command from the menu and select the beam

group that we have created. Click the Close button. Click on the Commands button in the Data Area and select the Check Code option in the Design Commands dialog box. Click on the Assign button. Click the Close button.

6. Click on Analyze -> Run Analysis command from the menu. Select the STAAD Analysis option and press the Run Analysis button. After the analysis is completed click on the View Output File option to view the steel design results. On the left hand side, click on the RESULTS tab and click on STEEL DESIGN.

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You will notice that all the beams have failed as per the AASHTO code checks. You can instruct STAAD.pro to perform member selection. After performing the member selection, the selection needs to be applied to all beam members. The offsets have to be manually updated. Close the output file. 7. Let us limit the depth of the girders to 2ft and 2.3ft for the selection. Create and assign

the DMIN 2 and DMAX 2.3 design parameter to members 65, 145, 476, 555, 964, 1043, 1456, 1535, 1944, and 2023 using the instructions in Step# 4 discussed above and must be placed before the Check Code command with the rest of the design parameters. The After Current check box will enable you to place the DMIN 2 and DMAX 2.3 design parameter at the correct location.

8. Assign the Select and Ratio 0.9 command to members 65, 145, 476, 555, 964, 1043, 1456, 1535, 1944, and 2023. The Ratio 0.9 command must be placed before the Check Code command with the rest of the design parameters. The After Current check box will enable you to place the Ratio design parameter at the correct location. Staad.pro has to be instructed that all members in the design group have the same section profile. Select the Check Code entry in the Data Area. Click the Commands button in the Data Area and select the Group option. Select Ax in the Property Specification dialog box and check the After Current check box. Click on the Add button in the Design Commands dialog box. Click the Close button. Select the GROUP AX MEMB design command in the Data Area and click on the Select Group/Deck button. Select the design group that you had created in Step 2. Click the Assign button. After running the analysis, as per Step 6 discussed above, you will notice that there are two sets of steel design results; one for the initial selection and the other for the member selection. In the Post-processing mode, the design results are shown in the Beam->Unity Check Tab.

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9. The last step in the design process is to check if this member selection is ok because of

the changes in the force distribution in the entire structure and the proposed new member property proposed by STAAD.pro. Open the General->Properties control tab in the Modeling mode. Select W21X275 and select the Select->Beams parallel to->x command from the menu. Select the Assign to Selected Beams option and click the Assign button. The W24X192 have a depth which is 0.03ft less than that of W24X103. Hence, it is not necessary to change the offsets of the beams and the columns in this case.

10. Using the STAAD.pro editor, comment out the following lines as follows: *RATIO 0.9 ALL *WE SELECTED W21X275 AS PER STAAD.PRO SELECTION. INITIAL SECTION WAS W24X103. *SELECT MEMB 65 145 476 555 964 1043 1456 1535 1944 2023 *GROUP AX MEMB _MIDSPANBEAMS

11. Perform the analysis as per the instructions in Step# 6 and you will notice that the beams

are passing.

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5.0 Conclusion STAAD.pro in combination with STAAD.beava can be used to analyze bridges as per the AASHTO code. STAAD.pro is first used to construct the bridge geometry and STAAD.beava is used to find the AASHTO 2002 load positions that will create the maximum load response. These loads that create the maximum load responses can then be transferred into STAAD.pro as load cases to load combinations for further analysis and design. This manual has demonstrated the design of the steel girders. A similar design approach can be used for design of concrete members, slab elements and foundations.