LRFD Minimum Flexural Reinforcement Requirements

Transcription

1 NCHRP LRFD Minimum Flexural Reinforcement Requirements Presentation to AASHTO T 10 Committee Spokane, WA June 12, 2017

2 2 Project Personnel Principal Investigator: Sri Sritharan, Iowa State University (ISU) Members: Jay Holombo, T. Y. Lin International Sami Megally, Kleinfelder Hartanto Wibowo, ISU Michael Rosenthal, ISU Jacob Eull, ISU Ryan Bodendorfer, ISU

3 3 Outline Goal and Objectives Literature Review Preliminary Results Ongoing Research Conclusions

4 4 Goal and Objectives Goal: Verify AASHTO LRFD Bridge Design Specifications (AASHTO Specs) and improve effectiveness of minimum flexural design requirement Objectives: Complete analytical and experimental studies using RC and PC with CIP and segmental constructions and verify current AASHTO Spec requirements Predict performance and improve analysis capabilities Develop recommended changes for AASHTO Specs based on results from this project

5 5 Additional Information Why specify a minimum reinforcement ( min )? to provide flexural members with sufficient strength and ductility past the cracking limit state prevent brittle failure of the member immediately after cracking NCHRP projects mainly focused on analytical study has analytical and experimental components

6 6 Measure of Adequate min Ductility related to safety Maximum crack width related to serviceability Brittleness factor related to fracture energy

7 7 Literature Review Force, F What is not well defined? acceptable acceptable acceptable/unacceptable? unacceptable Displacement, Minimum ductility Correlation between required ductility capacity and hardening ratio

8 8 Implication of a conservative min Increase in cost Potential congestion Reduced ductility: member may fail in shear or concrete compression in a brittle manner Could make prestress less effective in prestressed concrete members: overreinforced condition and compressioncontrolled failure

9 9 Literature Review Exp. Study Mostly RC beams at smaller scales Variables investigated: depth, deflection, brittleness number, crack width, etc. Emphasis on minimum reinforcement to ensure ductile performance beyond yielding Concrete members could have sufficient ductility even when designed with the minimum reinforcement Evidence of depth influence on MoR

10 10 AASHTO Requirements Date Adopted Prior to 2005 Sectional Requirements ( ) Flexural Cracking Strength (psi) ( ) Over Demand Requirements ( ) Description M n 1.2M cr 7.5 f c M n 1.33M u Based on historical modulus of rupture value M n 1.2M cr 11.7 f c M n 1.33M u Higher limit introduced to reflect research results on high strength concrete, as endorsed by ACI Committee current M n 3 ( 1 f r + 2 f cpe )S 1 = 1.6 Cracking factor 2 = 1.1 Prestress factor 3 = f y /f u (1.0 prestress) 7.5 f c M n 1.33 M u Compares ultimate instead of nominal moment capacity. Effects of flexural cracking and prestress are factored separately, per NCHRP

11 Literature Review - f r 11 Holombo and Tadros (2009) Non moist cured samples

12 12 Literature Review - Depth influence Cracking strength 1/beam depth Holombo and Tadros (2009)

13 13 Lit. Review - Codes/Standards A wide range was covered: AASHTO, ACI, Japanese Code, Japanese Highway Specs, British Standards, Eurocode, Norwegian Code, FIB, New Zealand Standards, and Leonhard method Minimum reinforcement is to ensure ductile response beyond cracking Variations of minimum reinforcement requirements for a RC beam and a PS girder

14 14 Lit. Review - Codes/Standards Key variables that affect applicability: compressive strength of concrete concrete cracking strength type of cross section amount of prestressing in the member effects of creep and shrinkage use of unbonded tendons load combinations

15 15 Lit. Review - AASHTO Specs (2012) Minimum reinforcement required to ensure that the amount is adequate to develop a factored flexural resistance, M r, at least equal to the lesser of: 1.33 x the factored moment required by strength load combination or 1 Based on recommendations from NCHRP 12 80

16 16 Lit. Review - Conclusions Results from previous studies are inconclusive and there has been no agreement on rational unified minimum flexural reinforcement requirements Researchers have used fracture mechanics approach to characterize behavior of beams with minimum reinforcement assuming that the provided reinforcement will reach the yield limit state, focusing more on the additional response beyond the state of yielding rather than cracking Need to develop acceptable minimum reinforcement ratio that will ensure development of ultimate moment with sufficient margin beyond the cracking limit state

17 17 Progress of Research Analytical and experimental studies have been carried out Experimental study consists of static tests of various girders designed with a reinforcement ratio of 75% AASHTO minimum Effects of span to depth ratio and deviator are evaluated

18 18 Analytical Study Numerical models were developed using Response 2000 (sectional analysis) and Abaqus (member analysis) Section analyses do not reflect the true toughness of the member Analyses with measured material properties generally show good agreements with the experimental results Variability in material properties is an issue

19 19 Test Variables Type Bonded Pretensioned Unbonded Post Tensioned Bonded Post Tensioned Reinforced Concrete Purpose Span to Depth Ratio / Depth Effect Span to Depth Ratio Influence of Deviator Span to Depth Test Number X X X X X X Ratio Span to Depth Ratio X X X X X X

20 20 Test Variables and Matrix Test Numbe r Type Section Depth (without deck) Span Length (ft) Span to Depth Ratio 1 BTE70 5' 3" Bonded 2 BTC60 3' 9" Pretensioned 3 A34 2' 8" Deviato r f' c (ksi) N/A 6 4 UNB1 3' 0" Unbonded UNB2 3' 0" Type 1 Post 6 Tensioned UNB3 4' 6" UNB4* 3' 0" Type 2 8 Bonded Post BON1 3' 0" Tensioned BON2 4' 6" N/A 10 Reinforced RC1 4' 0" Concrete RC2 2' 6" 20 8 N/A 5 ρ design 75% AASHTO Min

21 21 Completed Tests 2 Reinforced Concrete Girders (RC1 and RC2) 3 Pretensioned Girders (A34, BTC60, and BTE70) 1 Segmental Unbonded Post tensioned Girder (UNB1)

22 22 Typical Test Setup Load Cell Actuator Spreader Beam Neoprene Pad Steel Rod

23 23 Reinforced Concrete Girder RC1 Span length: 32 ft Depth: 4 ft

24 24 Reinforced Concrete Girder RC1 Cracking load is 20 kip corresponding to 4.3 Failure load is 65 kip Failed in compression Net tensile strain in the reinforcement was 10.5 mε

25 25 Reinforced Concrete Girder RC2 Span length: 20 ft Depth: 2 ft 6 in

26 26 Reinforced Concrete Girder RC2 Cracking load is 18 kip corresponding to 5.9 Failure load is 42 kip Failed in compression Net tensile strain in the reinforcement was 10.5 mε

27 27 Pretensioned Girder A34 Span length: ft Depth: 2 ft 8 in

28 28 Pretensioned Girder A34 Cracking load is 33 kip corresponding to 7.1 Failure load is 79 kip Test terminated due to excessive support movement and actuator stroke capacity Net tensile strain in the reinforcement was over 12 mε Possible debonding of strands

29 29 Pretensioned Girder BTC60 Span length: 60 ft Depth: 2 ft 9 in

30 30 Pretensioned Girder BTC60 Cracking load is 54 kip corresponding to 4.3 Failure load is 119 kip Sudden failure at midspan Net tensile strain in the reinforcement was over 12 mε Possible debonding of strands

31 31 Pretensioned Girder BTE70 Span length: 70 ft Depth: 5 ft 3 in

32 32 Pretensioned Girder BTE70 Cracking load is 75 kip corresponding to 5.5 Failure load is 151 kip Failed in tension Net tensile strain in the reinforcement was over 16 mε Possible debonding of strands

33 33 Pretensioned Girder BTE70 Debonding of tendons was observed during the test Analyses in Abaqus were carried out with and without debonding of tendons to demonstrate the effect

34 Segmental Unbonded PostTensioned Girder UNB1 34

35 35 Epoxy Tension Test Cracking within the concrete laitance

36 Segmental Unbonded Post- Tensioned Girder UNB1 36 Span length: 66 ft Depth: 3 ft

37 Segmental Unbonded Post- Tensioned Girder UNB1 37

38 Segmental Unbonded Post- Tensioned Girder UNB1 38 Cracking load is 39 kip, corresponding to 4.8 Failure load is 45 kip Failed in tension Tested on 6/5/2017 and 6/6/2017 data processing is ongoing

39 39 Test Observation

40 40 Ongoing Tasks Fabricate and test remaining segmental girders with unbonded and bonded tendons Carry out refined analyses using updated material properties after the test Propose recommendations to revise the current AASHTO LRFD requirements

41 41 Conclusions Tested beams show sufficient ductility beyond experiencing flexural cracking despite using 75% of AASHTO minimum reinforcement Deeper beams show a trend of having lower modulus of rupture Lower l means Less number of cracks Possible debonding of bars/strands Cracking in the concrete laitance Higher than the assumed reinforcement properties Premature fracture of strands at anchorage Section analyses do not adequately reflect the brittleness of the member