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LRFD Minimum Flexural Reinforcement Requirements
Presentation toAASHTO T‐10 Committee
Spokane, WAJune 12, 2017
NCHRP 12-941
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
2
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Outline
• Goal and Objectives• Literature Review• Preliminary Results• Ongoing Research• Conclusions
3
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
4
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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– 12‐80 mainly focused on analytical study– 12‐94 has analytical and experimental components
5
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Measure of Adequate min
• Ductility related to safety
• Maximum crack width related to serviceability
• Brittleness factor related to fracture energy
6
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Literature Review7
Force, F
Displacement,
acceptable
acceptableacceptable/unacceptable?
unacceptable
• Minimum ductility• Correlation between required ductility capacity and
hardening ratio
What is not well defined?
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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: over‐reinforced condition and compression‐controlled failure
8
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
9
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
AASHTO RequirementsDate
Adopted
Sectional Requirements (5.7.3.3.2)
Flexural Cracking
Strength (psi) (5.2.4.6)
Over‐Demand Requirements (5.7.3.3.2)
Description
Prior to 2005 Mn 1.2Mcr 7.5f’c Mn 1.33Mu
Based on historical modulus of rupture value.
2005 – 2011 Mn 1.2Mcr 11.7f’c Mn 1.33Mu
Higher limit introduced to reflect research results on high strength concrete, as endorsed by ACI Committee 363.
2011 –current
Mn 3(1fr+2fcpe)S1 = 1.6 Cracking factor2 = 1.1 Prestress factor3 = fy/fu (1.0 prestress)
7.5f’c Mn 1.33 Mu
Compares ultimate instead of nominal moment capacity.Effects of flexural cracking and prestress are factored separately, per NCHRP 12‐80.
10
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Literature Review - fr
11
Holombo and Tadros (2009)‐ Non‐moist cured samples
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Literature Review - Depth influence• Cracking strength 1/beam depth
12
Holombo and Tadros (2009)
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
13
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
• 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
14
Lit. Review - Codes/Standards
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Lit. Review - AASHTO Specs (2012)
• Minimum reinforcement required to ensure that the amount is adequate to develop a factored flexural resistance, Mr, at least equal to the lesser of:
1.33 x the factored moment required by strength load combination
or
1
15
Based on recommendations from NCHRP 12‐80
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
16
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
17
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
18
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Test Variables19
Type PurposeTest Number
1 2 3 4 5 6 7 8 9 10 11
Bonded PretensionedSpan‐to‐Depth Ratio / Depth
EffectX X X
Unbonded Post‐Tensioned
Span‐to‐Depth Ratio X X X
Influence of Deviator X X
Bonded Post‐Tensioned
Span‐to‐Depth Ratio X X
Reinforced ConcreteSpan‐to‐Depth
Ratio X X
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Test Variables and MatrixTest
Number
Type SectionDepth
(without deck)
SpanLength
Span‐to‐Depth Ratio
Deviator
f'c ρdesign
(ft) (ksi)
1Bonded
Pretensioned
BTE70 5'‐3" 70 13.33
N/A 6
75% AASHTO Min
2 BTC60 3'‐9" 60 16
3 A34 2'‐8" 34.17 12.81
4Unbonded
Post‐Tensioned
UNB1 3'‐0" 66 22
Type 1
6
5 UNB2 3'‐0" 54 18
6 UNB3 4'‐6" 54 12
7 UNB4* 3'‐0" 54 18 Type 28 Bonded Post‐
TensionedBON1 3'‐0" 54 18
N/A9 BON2 4'‐6" 54 12
10 Reinforced Concrete
RC1 4'‐0" 32 8N/A 5
11 RC2 2'‐6" 20 8
20
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Completed Tests
2 Reinforced Concrete Girders (RC1 and RC2) 3 Pretensioned Girders (A34, BTC60, and BTE70) 1 Segmental Unbonded Post‐tensioned Girder (UNB1)
21
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Typical Test Setup22
ActuatorLoad Cell
Spreader Beam
Steel Rod
Neoprene Pad
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Reinforced Concrete Girder RC123
Span length: 32 ftDepth: 4 ft
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
• 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
24
Reinforced Concrete Girder RC1
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Reinforced Concrete Girder RC225
Span length: 20 ftDepth: 2 ft 6 in
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
• 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
26
Reinforced Concrete Girder RC2
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Pretensioned Girder A3427
Span length: 34.17 ftDepth: 2 ft 8 in
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
• 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
28
Pretensioned Girder A34
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Pretensioned Girder BTC6029
Span length: 60 ftDepth: 2 ft 9 in
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
• 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
30
Pretensioned Girder BTC60
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Pretensioned Girder BTE7031
Span length: 70 ftDepth: 5 ft 3 in
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
• 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
32
Pretensioned Girder BTE70
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
33
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
34Segmental Unbonded Post-Tensioned Girder UNB1
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
35
Epoxy Tension TestCracking within the concrete laitance
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Segmental Unbonded Post-Tensioned Girder UNB1
36
Span length: 66 ftDepth: 3 ft
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
Segmental Unbonded Post-Tensioned Girder UNB1
37
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
• 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
38Segmental Unbonded Post-Tensioned Girder UNB1
NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
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NCHRP 12‐94: LRFD Minimum Flexural Reinforcement Requirements
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
41