Load Testing of Concrete Bridges North Carolina’s Experience Presented by: Tom Drda, P.E. FHWA Senior Bridge Safety Engineer
Load Testing of Concrete Bridges North Carolina’s Experience
Presented by: Tom Drda, P.E.
FHWA Senior Bridge Safety Engineer
Load Testing of Concrete Bridges in NC
Presentation Overview• NC Bridge Background Information
• Why Load Test?
• Results of Load Testing
• Conclusions
Load Testing of Concrete Bridges in NC
NC Bridge Background
18,100 Bridges total –
780 Local Bridges
4,320 Bridges load restricted
Rate each bridge after biennial inspection
Staff of 12 Load Rating Engineers
Load Testing of Concrete Bridges in NC
Why?
• NC General Statue 20‐118 resulted in an increase in legal load
• GS 20‐118 Passed July 2008, implemented September 1, 2008
Load Testing of Concrete Bridges in NC
Impacts
• Increase in force effects 5 to 60%
• Resulted in over 2,900 Load Posting Notification
• Resulted in 400 bridges on primary routes being posted.
Load Testing of Concrete Bridges in NC
Plan of Action
• Prioritize bridges likely to see new vehicles
• Identify bridges which require posting
• Perform refined analysis
• Diagnostic Load Test
Load Testing of Concrete Bridges in NC
Goal of Load Testing
• Verify and document reserve capacity in concrete bridges
• Verify refined load rating results (distribution)
• Avoid load restricting bridges on Primary Routes
Load Testing of Concrete Bridges in NC
Load Testing of Concrete Bridges in NC
Example Bridge
4 span simply supported reinf. concrete T‐ Beam
Built in 1931
H‐15 Design Load
Span Lengths 46’, 47.5’, 47.5’, 46’
90 degree skew
Good
condition
Load Testing of Concrete Bridges in NC
Load Testing of Concrete Bridges in NC
Traditional Load Rating Results
Inventory
SV 20 Tons (42 legal)
TTST 22 Tons (42 legal)
Operating
SV 33 Tons (42 legal) TTST 37 Tons (42 legal)
Interior beam controls
Load Testing of Concrete Bridges in NC
Finite Element Load Rating Results
Inventory
SV 27 Tons (42 legal)
TTST 29 Tons (42 legal)
Operating
SV 45 Tons (42 legal) TTST 49 Tons (42 legal)
Exterior beam controls
Load Testing of Concrete Bridges in NC
Load Testing of Concrete Bridges in NC
Adjusted Finite Element Load Rating Results
Inventory
SV 41 Tons (42 legal)
TTST 45 Tons (42 legal)
Operating
SV 68 Tons (42 legal) TTST 76 Tons (42 legal)
Exterior beam controls
Load Testing of Concrete Bridges in NC
Summary of Findings
• Exterior beam was controlling member 70% over traditional load rating
• Stresses were much lower than predicted by traditional load rating and FEM
• Cracked sections see higher stresses
Load Testing of Concrete Bridges in NC
Conclusions/ResultsLoad Testing verified the results of refined load ratings ‐
Distribution
2D modeling ineffective for refined load ratings on skew
Load tested eliminated posting requirement for bridges tested
Process reduced the number of Primary Route load restricted bridges from 400 to 140
Load Testing of Concrete Bridges in NC
Acknowledgements:
North Carolina Department of Transportation – Bridge Management Unit
URS Corporation
Satrajit
Das, PhD, PE
Questions?