Calibration of AASHTO LRFD for Filled Grid Decks Based on Historical Performance

1. Calibration of AASHTO LRFD for Filled Grid Decks Based on Historical Performance Christopher Higgins and O. Tugrul Turan School of Civil and Construction Engineering Oregon State University and Mark Kaczinski and Phil Gase Bridge Grid Flooring Manufacturing Association International Bridge Conference June 8, 2011

2. Introduction & Background • Widely used in practice • Light weight compared to conventionally reinforced decks • Two way bending (orthotropic behavior) Source: www.bgfma.org Source: www.bgfma.org Main Bars (Strong Direction) Cross Bars (Weak Direction)

3. Introduction & Background • Orthotropic Thin Plate Theory • Non-homogenous biharmonic equation. • Stiffnesses can be determined experimentally

4. Introduction & Background

5. Introduction & Background D = ∞ D = 0 D = 2 • D = 0, plate acts like a one –way slab or beam. • D = ∞, plate behaves like a collection of separate strips.

6. Introduction & Background AASHTO-LRFD (2004) section 4.6.2.1.8 Higgins 2003, Higgins 2004 , ,

7. Introduction & Background • One-way slab, (Prior to AASHTO-LRFD, 1994) • Orthotropic Thin Plate Theory (AASHTO-LRFD, 1994) , • Single patch at the center • Orthotropic Thin Plate Theory (AASHTO-LRFD, 2004), • Tandem axle and multiple patches, • Fatigue Limit State • Deflection equations

8. Introduction & Background C=0.8 C=1.0

9. Introduction & Background • Many of the decks were constructed more than 30 years ago and AASHTO-LRFD(2004) not calibrated against historically successful performance • BGFMA selected 26 decks, design details and supporting information provided • Min. 10; max. 51 years in service.

10. Comparison of AASHTO LRFD 2004 and AASHTO LRFD 1994 Strength Limit State Comparison Moment main bars transverse to traffic Region generally used in practice

11. Comparison of AASHTO LRFD 2004 and AASHTO LRFD 1994 Strength Limit State Comparison Moment main bars parallel to traffic Region generally used in practice

12. Comparison of AASHTO LRFD 2004, AASHTO LRFD 1994 and Table A4 AASHTO-LRFD (2004), AASHTO-LRFD (1994) moment values for D=1.0 and C=0.8, and AASHTO-LRFD (2004) deck slab design table positive moment values (A4)

13. Comparison of AASHTO LRFD 2004, AASHTO LRFD 1994 and Table A4 AASHTO-LRFD (2004), AASHTO-LRFD (1994) moment values for D=1.0 and C=0.8, and AASHTO-LRFD (2004) deck slab design table negative moment values (A4).

14. Comparison of AASHTO LRFD 2004 and AASHTO LRFD 1994 Strength Limit State Comparison, 26 Decks Demands somewhat higher now.

15. Comparison of AASHTO LRFD Design Demands with Available Resistance Strength Limit State (Positive Moment) Max= 2.32 Min=1.27 Mean=1.64 58% above 1.5 M+ “Capacity” is adequate

16. Comparison of AASHTO LRFD Design Demands with Available Resistance Strength Limit State (Negative Moment) Max= 2.77 Min=1.08 Mean=1.48 35% above 1.5 M- “Capacity” is adequate

17. Strength Limit State with FEA: Superstructure and Distributed Stiffness Super structure flexibility: Slightly reduced negative moments, slightly increased positive moments for strength. Distributed stiffness due to cracking not significant.

18. Deflection Criteria

19. Fatigue Limit State (Positive Moment) SR<5ksi Inf. Life

20. Fatigue Limit State (Negative Moment)

21. Fatigue Limit State, 26 Decks

22. Fatigue Limit State

23. Fatigue Limit State Elkins and Higgins, 2006

24. Fatigue Limit State Elkins and Higgins, 2006

25. Fatigue Limit State : Transverse M- N= 2 Big 1 Small Rainflow counting! N = 4 Moderate 1 Small Rainflow counting! • SR/2 (transverse)

26. Fatigue Limit State: Parallel M- N = 2 Moderate 6 Small Rainflow counting N = 4 Big 2 Small Rainflow counting • SR/2.5 (parallel)

27. Fatigue Limit State: Parallel M- • SR/2.5 (parallel)

28. Fatigue Limit State : Design Section M2=0.9xM1 Normalized negative moment (from Table A4-1 AASHTO-LRFD)).

29. Fatigue Limit State, 26 Decks • SR/2 (transverse) • SR/2.5 (parallel) • 3 in. away from the CL of the support (SRx0.9) • 9/26 less than years in service

30. Fatigue Limit State Fatigue Limit State If: Fatigue cracking over the supports C=0.8 All the main bars cracked over the continuous supports Strength Limit State C=1.0 • Negative fatigue moment could be ignored

31. Limits on Possible Span Lengths • Theoretical spans were determined • Strength: C=1.0; M+ only with first yeild limit • Deflection: AASHTO-LRFD Prescribed deflection • Fatigue: C=1.0; AASHTO-LRFD Prescribed fatigue SR (Strength/3) to limit of 5 ksi • L/800 was the most conservative • New service level stresses were determined for L/800

32. Conclusions and Recommendations • Current AASHTO-LRFD moment provisions are not substantially higher than those specified for RC decks in traditional design • Suite of decks not controlled by the strength or positive fatigue moment • All 26 decks are limited by negative fatigue moment • Negative fatigue moment can be reduced by a factor of 2.2 (for design say 2) for transverse to traffic and 2.8 (for design say 2.5) for parallel to traffic cases • Additional analyses and/or tests around the negative moment region may help identify additional load distribution that may reduce stress range over the support for fatigue design • Design approach would be: use the current design for Strength I with C=1.0, detail to obtain infinite life for positive fatigue moment, and limit the service level deflections to L/800