1. Load Rating of Precast Box Culverts
Brian Barngrover, P.E.
January 18,2018

2. What is a Load Rating?
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3. What is a Load Rating?
“Units of live load that can be supported by a bridge before the structure reaches a defined limit state”
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4. What is a Limit State?
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5. What is a Limit State?
(from LRFD) – “A condition beyond which the bridge or component ceases to satisfy the provisions for which is was designed.”
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6. Simple example:
Simple example of a bridge.
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7. Simple example: Rigid Rugged Resilient
You have determined that your limit state is a crack at the centerline.
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8. Simple example: Rigid Rugged Resilient
Your unit of live load is an elephant
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9. Simple example: RF = 2.0 Rigid Rugged Resilient
Based on your calculations, placing two elephants on this bridge is just enough to cause that centerline crack. Therefore, for this bridge, the RF = 2.0
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10. Where are Load Ratings defined?
TODO: Need picture of cover of load rating manual here
In my opinion load ratings should be a chapter of LRFD Bridge Design Specification, as all elements of the rating equation are defined therein.
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11. Rating Equation (from MBE):
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12. Rating Equation: RF = (C – DL) / LL
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13. Rating Equation: RF = (C – DL) / LL
C = Capacity of the bridge element (limit state)
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14. Rating Equation: RF = (C – DL) / LL
C = Capacity of the bridge element (limit state)
DL = Dead load effects (essentially all non-live loads)
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15. Rating Equation: RF = (C – DL) / LL
C = Capacity of the bridge element (limit state)
DL = Dead load effects (essentially all non-live loads)
LL = Live load effects (based on one ‘unit’ of live load)
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16. Which culverts need to be rated?
Only bridges are load rated
Bridges were defined as having clear spans > 20’
The definition of a bridge has been changing
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17. Required Load Ratings
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18. Required Load Ratings
Inventory Rating
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19. Required Load Ratings Inventory Rating Operating Rating
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20. Trucks used in Rating Factors
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21. Vehicles used in Rating Factors
Design Vehicle
Typically HL-93, “notional” truck, not a real one, used to design the bridge, millions of cycles
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22. Vehicles used in Rating Factors
Design Vehicle
Permit Vehicle
Represents real trucks, typically defined by the state, far fewer number of cycles, allows for lower load factors
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23. Vehicles used in Rating Factors
Design Vehicle
Permit Vehicle
Legal Vehicle
Also represents real trucks, not typically required for design, usually reserved for existing bridges that do not rate using Design Vehicles, used for posting of load limits, can be used for design if required by a state
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24. Load Rating Culverts Which limit states are checked?
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25. Load Rating Culverts Which limit states are checked? Flexure
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26. Load Rating Culverts Which limit states are checked? Flexure Shear
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27. Load Rating Culverts Which limit states are checked?
Flexure
Shear
Where are these limit states checked?
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28. Load Rating Culverts Which limit states are checked?
Flexure
Shear
Where are these limit states checked?
At the critical sections
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29. Where are the critical sections?
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30. Where are the critical sections?
For flexure, there are three possible locations (+ mid-span)
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31. Where are the critical sections?
For flexure, there are three possible locations
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32. Where are the critical sections?
For flexure, there are three possible locations
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33. Where are the critical sections?
For flexure, there are three possible locations
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34. Where are the critical sections?
For shear, there are two possible locations (+ mid- span)
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35. Where are the critical sections?
For shear, there are two possible locations
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36. Where are the critical sections?
For shear, there are two possible locations
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37. Getting to 1.0 Maximum moment/maximum shear Axial loads Steel yields
Critical section locations – can you move them?
Haunches in the structural model
Always distribute wheel loads
Always check iterative beta
Critical section depths
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38. Problems with Rating Culverts
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39. Problems with Rating Culverts
Older culverts design using LFD or ASD
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40. Problems with Rating Culverts
Older culverts design using LFD or ASD
Culverts at depths where LL can be neglected
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41. Live ETCulvert Demo
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42. Questions?