How safe are our highway structures? Optimised assessment of bridges

1. How safe are our highway structures? Optimised assessment of bridges Aleš Žnidarič Slovenian National Building and Civil Engineering Institute

2. Contents • General about bridge assessment • Bridge WIM, including French experience • SAMARIS project • Load testing • Examples of improved assessment • Conclusions

3. Why optimised bridge assessment?

4. Why optimised bridge assessment? • new bridges are designed conservatively: • uncertainty about increases in loading • inexpensive to add capacity • assessment of existing bridges should be less conservative: • very expensive to strengthen/replace or post a bridge • capacity and current loading can be measured and monitored

5. Bridge design: high uncertainty of data: conservative analysis for long-term safety design loading schemes  high safety factors unnecessary: costly rehabilitation measures load limits Efficient assessment: better defined inputs: carrying capacity realistic, even site-specific loading  lower safety factors optimised: cheaper rehabilitation measures posting of bridges Design vs. Assessment

6. Why optimised assessment? • to select optimal rehabilitation measures: • do nothing • protect • repair • strengthen • replace

7. Why optimised assessment? • to select optimal rehabilitation measures: • do nothing • protect • repair • strengthen • replace

8. Assessment of existing bridges • factors taken into account: • condition, level of damage • structural safety: • carrying capacity • loading (dead, traffic, dynamic loading) • reliability of data • serviceability (clearances, traffic, obsoleteness) • service life, importance of structure • 5-level assessment

9. Questions for assessment • What is its carrying capacity (age, condition, drawings…)? • How does the bridge really behave: • What are the influence lines? • How the traffic load distributes? • What is the real loading: • in a country, type of road, specific bridge • dynamic amplifications (Bridge)WIM measurements

10. SAMARIS project

11. SAMARIS Project • Sustainable and Advanced MAterials for Road InfraStructure • EU 5th FW programme • 2003-2005 • 4,5 million € (5,4 million USD) • 23 partners, 16 countries • 2 subprojects (streams): • Pavements (FR) • Structures (SI)

12. SAMARIS Structures Objectives: • To develop guidelines and specifications for use of innovative intervention techniques (materials). • To provide an updated inventory of highway structures in selected EEA and CE countries and tools for their optimised assessment.

13. Traffic Loading

14. Traffic loading • Design loading schemes • Assessment (rating) loading schemes • Site-specific loading (WIM data) • Dynamic loading

15. Truck histograms from Europe

16. Truck histograms from Europe

17. Traffic load modelling

18. Comparison of sites in NL and SI

19. Site-Specific Load Assessment Better assessment load models needed: • for more accurate assessment of bridge safety • to save many bridges,particularly on less heavily trafficked roads • to account for number of vehicles and overloadedvehicles

20. Site-Specific Load Assessment There is an urgent need for effective overload enforcement – better compliance with legal limits will greatly reduce traffic loading on bridges.

21. Dynamic Loading

22. Dynamic loading • Problem: combining the extremes of dead load and dynamic effects => (too) high DAF • SAMARIS experiment: • 31-m long span • to assess influence of pavement unevenness • to evaluate DAF for 1000’s of vehicles • upgraded SiWIM system

24. Dynamic loading • Before resurfacing

25. Dynamic loading • After resurfacing

26. Dynamic loading Reduction of DAP due to resurfacing:

27. Dynamic loading Reduction of DAP due to resurfacing:

28. Conclusions on loading •  factors from Eurocodes can be lower if used for assessment of existing bridges • traffic patterns in EU, EEA and CEC are different • dynamic amplification factors for the extreme load cases are considerably lower than specified in the design codes

29. Load Testing

30. Load testing • on bridges that seem to carry out normal traffic satisfactorily, but fail to pass the assessment calculation • the available model of the bridge does not perfectly match with the real bridge itself To optimise bridge assessment by finding reserves in load carrying capacity Very different situation around Europe

31. Load testing • benefits: • less severe rehabilitation measures • less traffic delays • tremendous savings • drawbacks: • very costly • danger of damaging the structure

32. Load testing • best candidates: • difficult structural modelling • lack of documentation (drawings, calculations,…) • when savings are greater than the cost of load test

33. Load testing • Types of load test: • proof • diagnostic • “soft”

34. “Soft” load testing • the lowest level of load application • to supplement and check the assumptions and simplifications made in the theoretical assessment • use bridge WIM to provide: • “normal” traffic data • information about structural behaviour of the bridge • influence lines • statistical load distribution • impact factors) from normal traffic. • no need for pre-weighed vehicles • the bridge need not be closed to traffic • no risk of overloading and potential damaging of the structure

35. Teoretična in izmerjena vplivnica

36. Influence lines 50 years old, no drawings Probably not simply supported but how fixed?

37. SiWIM and Soft Load Testing Soft load testing Simply supported RF = 0,58

38. SiWIM and Soft Load Testing Soft load testing Simply supported RF = 0,58 Measured RF = 1,46 Message: Check, how bridges really behave.

39. SiWIM and Soft Load Testing Simply supported RF = 0,58 Measured RF = 1,46

40. Load distribution • normally guestimation • bridge WIM can evaluate it statistically

41. Example • 13 posted bridges assessed for special transport • 11 proved safe even for a 165-tonnes special vehicle with 12 axles • for the rest no reliable data on carrying capacityavailable • on shorter bridges normal traffic can be worse than special transports

42. Conclusions 1. Bridge WIM systems: • very efficient way of collecting traffic loading data • very useful for collecting structural data: • influence lines • load distribution factors • DAF

43. Conclusions 2. Design conservatively, assess optimally: • very expensive to strengthen/replace a bridge • measurements: • capacity, • traffic (incl. dynamic) loading, • load distributions • load testing • monitoring

44. Conclusions 3. Proper assessment (with monitoring) can: • prove that many existing bridges are safe in their current condition for their current loading • justify optimal rehabilitation measures • save a lot of money

45. Thank you!