Cracking,%20Deflections%20and%20Ductility%20Code%20Provisions%20and%20Recent%20Research - PowerPoint PPT Presentation

Title: Cracking,%20Deflections%20and%20Ductility%20Code%20Provisions%20and%20Recent%20Research


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Cracking, Deflections and DuctilityCode
Provisions and Recent Research
Serviceability and Ductility The Other Limit
States

• October 2006

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Cracking, Deflections and DuctilityCode
Provisions and Recent Research

• Overview
• Code provisions for ductility
• Background to the study
• Codes AS3600, AS5100, EC2, BS5400, BS8110,
  CEB-FIP 1990, ACI 318
• Background to prediction of cracking and
  deflections
• Code provisions for crack widths and stress
  limits
• Code provisions for deflections
• Recent research
• Conclusions

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Code provisions for ductility
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Background to the study

• Prediction of cracking and deflection
• Why is it important?
• Why is it difficult?
• What do the codes say?

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Prediction of Cracking and DeflectionsWhy is it
important?

• Second order effects
• Load distribution and transfer
• Loads on non-structural members
• Durability
• Code compliance
• Contract conditions
• Client expectations
• Aesthetics
• Clearances, ponding etc.

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Why is it difficult?

• Uncertain or unknown material properties
• Inconsistent and incomplete code provisions
• Inherently random nature of cracking
• Unknown manufacture procedures and construction
  programme
• Variations in curing procedures and environmental
  effects
• Complex loading history

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Uncertain or unknown material properties

• Concrete tensile strength creep rupture?
• Concrete stiffness under tension non-linearity?
• Concrete creep and shrinkage properties
• Concrete behaviour under unloading/ reloading

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Inconsistent and incomplete code provisions

• Tensile strength of concrete
• Effect of shrinkage on tensile strength
• Tension stiffening
• Loss of tension stiffening
• Effect of uncracked parts of structure
• Effect of shrinkage

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Unknown manufacture procedures and construction
programme

• Concrete age at loading?
• Time before application of loads or restraints?
• Effect of steam curing
• Locked in thermal stresses?
• Storage, curing
• Differential shrinkage?

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Complex loading history

• Critical sections subject to may be sagging,
  hogging, sagging, hogging
• Effect of axial load
• Calculation of non-recoverable deflections (eg
  creep)

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What do the codes say?

• Compare AS3600, AS5100, EC2, BS5400, BS8110,
  CEB-FIP 1990, ACI 318
• Differing and inconsistent provisions
• No one code covers all significant effects

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Background to prediction of cracking and
deflections

• Formation and propagation of cracks
• Relationship between cracking and section
  stiffness
• Tension stiffening
• Loss of tension stiffening
• Time related effects
• Creep
• Shrinkage
• Differential shrinkage
• Calculating deflections from section stiffness

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Background to prediction of cracking and
deflections

• Recommended readingConcrete Structures
• Stresses and Deformations
• Ghali Favre and Elbadry

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Formation and propagation of cracks
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Relationship between cracking and section
stiffness

• Tension stiffening
• Displacement of Neutral Axis
• Loss of tension stiffening

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Time related effects

• Creep
• General agreement on mechanism and analysis
  approach
• Amount and rate of creep variable
• Shrinkage
• Affects both section curvature and effective
  cracking stress
• No agreed approach to analysis of either effect
• Differential shrinkage
• May have a large effect on section curvature and
  deflections
• Not specifically covered by any of the codes
  studied

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Calculating deflections from section stiffness

• Two approaches in codes
• Effective stiffness approach (ACI and
  Australian codes) - Branston equation
• Average of cracked and uncracked section
  stiffness.
• Integrate section curvature along the length of
  the member.

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Code provisions for stress limits

• AS 3600, AS 5100 and EC2
• Crack control by stress limits governed by bar
  diameter and spacing
• AS 5100 has much lower stress limits applicable
  to stresses due to permanent loads in exposure
  classifications B2, C or U
• EC2 limits related to specified crack widths
  under quasi-static loading
• AS 3600 limits similar to EC2 limits for 0.4 mm
  crack width for bar diameter, and 0.3 mm for bar
  spacing
• AS 5100 limits for exposure classification B2 and
  higher similar to EC2 limits for 0.2 mm crack
  width
• The specified stress limits will result in
  substantially higher design crack widths with
  increased cover.

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Code provisions for stress limitsStress Limits
for Maximum Bar Diameter
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Code provisions for stress limits Stress Limits
for Maximum Bar Spacing
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Code provisions for stress limits Design crack
widths for maximum stress
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Code provisions for stress limits Design crack
widths for maximum stress
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Code provisions for stress limits Design crack
widths for maximum stress
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Code provisions for crack widths

• AS 3600 and AS 5100
• No requirement for calculation of crack widths

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Code provisions for crack widths

• EC2

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Code provisions for crack widths -EC2
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Code provisions for crack widths

• EC2 - Notes
• Crack spacing is mainly related to cover depth
• Crack width is directly proportional to crack
  spacing
• Tension stiffening is limited to 40 of steel
  strain without stiffening
• Coefficient for long term tension stiffening is
  reduced by 1/3 (0.6 to 0.4)

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Code provisions for crack widths
Design surface crack width
BS8110
BS 5400
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Code provisions for crack widths
CEB-FIP 1990 (MC 90) Design crack width
Length over which slip between concrete and steel
occurs
Steel strain at the crack
Steel strain under a force causing stress equal
to concrete tensile strength over concrete
tension area x empirical coefficient
Free shrinkage of concrete (generally negative)
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Code provisions for crack widths
ACI 318 - 89, 99, Gergely-Lutz equation
ACI requirements based on stress limits derived
from the Gergely-Lutz equation
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Code provisions for deflections

• AS 3600, AS 5100, and ACI 318

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Code provisions for deflections

• AS 3600, AS 5100, and ACI 318 - Notes
• Code provisions based on the Branson Equation
  ACI 318 is differently formulated, but gives the
  same results.
• Ief is the average effective stiffness, applied
  over the full length of the member.
• Ms is determined at the critical section(s)
  specified in the code.
• AS 5100 provisions are identical to AS 3600,
  (other than a typographical mistake!)
• In the Australian codes the cracking moment is
  reduced by a factor dependent on the concrete
  shrinkage. ACI 318 makes no adjustment to the
  cracking moment.
• AS 3600 and AS 5100 provide a factor kcs to
  account for the effects of creep and shrinkage
• kcs 2 - 1.2(Asc / Ast ) gt 0.8

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Code provisions for deflections

• AS 5400 and 8110
• Deflections calculated from integration of
  section curvatures
• Cracking moment and curvature of cracked sections
  allows for a short term concrete tensile stress
  of 1 MPa, reducing to 0.55 MPa in the long term.
• Shrinkage curvature determined from the free
  shrinkage strain, and the first moment of area of
  the reinforcement about the cracked or uncracked
  section, as appropriate.
• BS 5400 tabulates factors based on the
  compression and tension reinforcement ratios.

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Code provisions for deflections

• Eurocode 2 and CEB-FIP 1990 (MC 90)
• Members which are expected to crack should behave
  in a manner intermediate between the uncracked
  and fully cracked conditions and, for members
  subjected mainly to flexure, an adequate
  prediction of behaviour is given by Expression
  (7.18)

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Code provisions for deflections

• Eurocode 2 and CEB-FIP 1990 (MC 90)

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Code provisions for deflections

• Eurocode 2 and CEB-FIP 1990 (MC 90)
• Shrinkage curvatures may be assessed using
  Expression (7.21)

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Code provisions for deflections

• Summary
• Australian and American codes based on the
  Branson equation, using a uniform average
  effective stiffness value.
• Australian codes allow for loss of tension
  stiffening through a reduction of the cracking
  moment related to the free concrete shrinkage.
• Allowance for shrinkage curvature in the
  Australian codes is simplified and will
  underestimate curvature in symmetricaly
  reinforced sections.
• British codes allow only a low tension value for
  cracked sections, which is further reduced for
  long term deflections
• European codes adopt an intermediate approach for
  cracked sections, with an allowance for loss of
  tension stiffening.
• British and European code provisions for
  shrinkage curvature are essentially the same

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Code provisions for deflections

• Summary
• None of the codes included in this study make
  specific provision for differential shrinkage for
  monolithic construction.
• Recent research suggests that loss of tension
  stiffening takes place within a few days, and
  reduced tension stiffening values should be used
  in almost all cases

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Recent Research

• Beeby, Scott and Jones
• Loss of tension occurs much more quickly than has
  previously been assumed, within 20-30 days
• Mechanism is cumulative damage, resulting from
  loss of tensile strength under load, creep plays
  an insignificant part
• Evidence that final tension stiffening may be
  largely independent of concrete strength.

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Conclusions

• Cracking and deflections may be highly variable,
  even under nominally identical conditions
• Codes do not make specific provisions for all the
  relevant factors
• AS 3600 and AS 5100 stress limits may result in
  substantially greater crack widths than allowed
  in other codes

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Conclusions

• In spite of similar approaches, different code
  methods for crack width calculation give highly
  variable results.
• Eurocode 2 appears to be the most consistent
• Predicted deflections are also highly variable.
• Shrinkage effects are significant, even in
  symmetrically reinforced sections.
• Allow for loss of tension stiffening
• Consider the possibility of differential
  shrinkage