1 NCHRP 12-94 LRFD Minimum Flexural … · NCHRP 12‐94: LRFD Minimum Flexural Reinforcement...

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

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Outline

• Goal and Objectives• Literature Review• Preliminary Results• Ongoing Research• Conclusions

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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

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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

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Measure of Adequate min

• Ductility related to safety

• Maximum crack width related to serviceability

• Brittleness factor related to fracture energy

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Literature Review7

Force, F

Displacement,

acceptable

acceptableacceptable/unacceptable?

unacceptable

• Minimum ductility• Correlation between required ductility capacity and

hardening ratio

What is not well defined?


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

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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

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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.

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Literature Review - fr

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Holombo and Tadros (2009)‐ Non‐moist cured samples


Literature Review - Depth influence• Cracking strength 1/beam depth

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Holombo and Tadros (2009)


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

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• 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

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Lit. Review - Codes/Standards


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

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Based on recommendations from NCHRP 12‐80


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

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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

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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

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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


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

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Completed Tests

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

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Typical Test Setup22

ActuatorLoad Cell

Spreader Beam

Steel Rod

Neoprene Pad


Reinforced Concrete Girder RC123

Span length: 32 ftDepth: 4 ft


• 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

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Span length: 20 ftDepth: 2 ft 6 in



• Failure load is 42 kip• Failed in compression• Net tensile strain in the reinforcement was 10.5 mε

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Pretensioned Girder A3427

Span length: 34.17 ftDepth: 2 ft 8 in



• 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

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Pretensioned Girder A34


Pretensioned Girder BTC6029




• Failure load is 119 kip• Sudden failure at midspan• Net tensile strain in the

reinforcement was over 12 mε


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Pretensioned Girder BTC60


Pretensioned Girder BTE7031




• Failure load is 151 kip• Failed in tension• Net tensile strain in the reinforcement was over 16 mε


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• Debonding of tendons was observed during the test

• Analyses in Abaqus were carried out with and without debonding of tendons to demonstrate the effect

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34Segmental Unbonded Post-Tensioned Girder UNB1


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Epoxy Tension TestCracking within the concrete laitance


Segmental Unbonded Post-Tensioned Girder UNB1

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Span length: 66 ftDepth: 3 ft


Segmental Unbonded Post-Tensioned Girder UNB1

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• 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


Test Observation39


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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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

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