3D Analysis Of Truss Bridges

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  • Tension and compression as opposed to bending and shear Go over components Usually consists of two parallel trusses Two Types Through Truss – more common, roadway near the bottom of truss Deck Truss – Deck is placed on top of truss Fracture critical because they have steel members in tension Rarely load path redundant because there are typically only two load path to the substructure, 3 are needed to be considered redundant
  • Timber was abundant and works well in tension or compression, many times covered to protect wood Iron rods replaced vertical members as railroads needed higher capacity and as iron became available Cast Iron replaced all timber and steel replaced iron Truss bridges were common from the 1870’s through the 1950’s 6920 Truss Bridges Remain, 146 in Minnesota MnDOT has rated all of its TH Bridges and is in the process of rating local truss bridges Pratt, Howe, Warren and Parker are the only engineers to become famous
  • A bridge load rating calculation provides a basis for determining the safe load capacity of a bridge Rated according to AASHTO’s Manual for Condition Evaluation of Bridges Inventory – corresponds to design level of stresses but reflects the existing condition of the bridge Operating – corresponds to the maximum permissible live load allowed, used for looking at infrequent regulated loads HS20-44 and five permit vehicles Load Factor Design
  • A bridge load rating calculation provides a basis for determining the safe load capacity of a bridge Rated according to AASHTO’s Manual for Condition Evaluation of Bridges Inventory – corresponds to design level of stresses but reflects the existing condition of the bridge Operating – corresponds to the maximum permissible live load allowed, used for looking at infrequent regulated loads HS20-44 and five permit vehicles Load Factor Design Discuss Pedestrian Load
  • Fall of 2007 MnDOT asked us to propose on a project to load rate truss bridges We were selected and asked to rate three bridges on the TH system as part of the first contract with a later contract where we rated three bridges on the local system The Hastings Bridge was constructed in 1951 and consists of thirteen spans with a total length of 1,857 feet. The truss portion is 964 feet in length. The main river crossing is a three span continuous riveted steel through arch.
  • The Winona Bridge was constructed in 1941 and consists of 24 spans with a total length of 2,289 feet. The main river crossing is a three span riveted steel through truss and is 930 feet long.
  • The Red Wing Bridge was constructed in 1958 and consists of 29 spans with a total length of 1,631 feet. The main river crossing is a continuous cantilever Warren steel through truss and is 864 feet long.
  • Bridge 4955 was constructed in 1930, and consists of an 80’ long single span riveted pony truss. Bridge 6646 was constructed in 1949, The main river crossing consists of two 200 foot long Parker High Truss spans. Bridge 6676 was constructed in 1952. The main river crossing consists of two 200 foot long Parker High Truss spans.
  • We proposed using a three dimensional approach and MnDOT accepted Typically modeled in 2 dimensions Our models include the deck, floor beams and stringers and bracing Discuss steel density Discuss fixity of decks
  • Gathered all the information for the bridge Original Drawings As-builts Shop Drawings Inspection reports Bridge Inventory Previous ratings Field mesurements Overlays, section loss, damage, added sidewalks, fixity of decks
  • Created our own set of summary drawings showing the truss dimensions and member sections
  • Truss members are many times built up using angles, channels, cover plates and lacing. Some sections are no longer used, challenge to find properties Modified the density to account for lacing and hardware
  • Influence Surface = Function that represents the load effect at a point in the structure as a unit load moves over the surface. Eliminates need to do live load distribution
  • Load cases were generated from the influence surface One load case for each member based on maximum response
  • Load cases were generated from the influence surface One load case for each member based on maximum response
  • Amount of Data a stumbling block
  • Able to incorporate section loss and capacity reduction factors. Factors are based on age and condition of structure.
  • MathCAD used to verify results
  • Challenge to come up with coincident loads
  • Drawings of each individual gusset plate were created.
  • Gusset plates did not control in the bridges we rated Inspection results were accounted for Fieldwork was completed to verify analysis
  • Most smaller truss bridges ultimately are rated based on floor beams and stringers. The larger truss bridges were rated based on the members of the truss. Reports were 100-200 pages long even though we are only interested in the inventory and operating rating.
  • Don’t have to use AASHTO live load factors which are approximate Can use the model later
  • Collaborative Effort
  • 3D Analysis Of Truss Bridges

    1. 1. Barritt Lovelace, PE WSB and Associates May 20 th , 2009 Truss Bridge Load Rating Using Three Dimensional Analysis
    2. 2. Truss Bridge Overview <ul><li>Truss bridges resist forces primarily through members acting in tension or compression </li></ul>
    3. 3. History of Truss Bridges <ul><li>Early truss bridges were built with wood </li></ul><ul><li>Railways led to timber-cast iron hybrids </li></ul><ul><li>Eventually cast iron replaced all timber </li></ul><ul><li>Steel replaced iron </li></ul>
    4. 4. Bridge Load Rating <ul><li>Members rated at both inventory and operating levels </li></ul>C = Capacity of Member P = Dead, Live and Pedestrian Load I = Impact Percentage Inventory Level – Design Loads Operating Level – Infrequent Regulated Loads
    5. 5. Load Rating Vehicles AASHTO HS20-44 Loading MnDOT Permit Vehicles
    6. 6. Truss Bridge Rating Projects <ul><li>Hastings Bridge 5895 </li></ul><ul><li>TH 61 over the Mississippi River </li></ul>
    7. 7. Truss Bridge Rating Projects <ul><li>Winona Bridge 5900 </li></ul><ul><li>TH 43 over the Mississippi River </li></ul>
    8. 8. Truss Bridge Rating Projects <ul><li>Dwight D. Eisenhower Bridge 9040 </li></ul><ul><li>US 63 over the Mississippi River in Red Wing </li></ul>
    9. 9. Local Bridge Truss Ratings <ul><li>Bridge No. 4955 </li></ul><ul><li>CSAH 20 over the Lac Qui Parle River, Lac Qui Parle Township, MN </li></ul>Bridge No. 6646 CSAH 36 over the Red River of the North, Georgetown Township, MN Clay County Bridge No. 6676 CSAH 25 over the Red River of the North, Hendrum Township Norman County, MN
    10. 10. Three Dimensional Approach <ul><li>Staad Pro used to model bridges </li></ul><ul><li>Decks modeled using finite elements </li></ul>
    11. 11. Data Collection
    12. 12. Summary Drawings
    13. 13. Built Up Member Modeling <ul><li>Section Wizard Software </li></ul><ul><li>Export Properties to Staad Model </li></ul>
    14. 14. Influence Surface Generation <ul><li>Utilized StaadPro BEAVA Software </li></ul>
    15. 15. Load Case Generation <ul><li>A single load case is generated for each member we are rating </li></ul>
    16. 16. Load Case Generation
    17. 17. Analysis <ul><li>StaadPro generates a results database </li></ul><ul><li>Microsoft Access used to organize data </li></ul><ul><li>Axial loads are exported to a spreadsheet for generation of rating factors </li></ul><ul><li>Results were compared to loads on the original plans </li></ul>
    18. 18. Truss Member Rating
    19. 19. Truss Member Rating
    20. 20. Gusset Plate Rating <ul><li>Coincident axial loads are generated in excel for Gusset Plate Rating </li></ul>
    21. 21. Gusset Plate Rating <ul><li>Gusset plates were rated according to MnDOT spreadsheet modeled after FHWA rating procedures </li></ul>
    22. 22. Gusset Plate Rating <ul><li>Tension </li></ul><ul><li>Flexure </li></ul><ul><li>Shear </li></ul><ul><li>Block Shear </li></ul><ul><li>Edge Buckling </li></ul><ul><li>Buckling </li></ul><ul><li>Rivets </li></ul>
    23. 23. Results
    24. 24. Advantages of 3-D Rating <ul><li>Improved accuracy </li></ul><ul><li>Incorporated deck system </li></ul><ul><li>Incorporated bracing </li></ul><ul><li>Ability to make changes to fixity </li></ul><ul><li>Ability to model damage to truss members </li></ul><ul><li>Can easily run additional permit vehicles </li></ul><ul><li>More accurate load distribution </li></ul>
    25. 25. Project Team Members <ul><li>MnDOT </li></ul><ul><li>Yihong Gao, Dave Conkel, Gary Peterson </li></ul><ul><li>Lowell Johnson, Dave Dahlberg, Kevin Western </li></ul><ul><li>WSB </li></ul><ul><li>Barritt Lovelace, Sabri Ayaz, Jim Archer, </li></ul><ul><li>Brad Skow, Dan Gatz, Dave Vincent, Brad Robinson </li></ul>
    26. 26. Questions <ul><li>[email_address] </li></ul>

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