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Cracking, Deflections and Ductility Code Provisions and Recent Research

Serviceability and Ductility The Other Limit States. Cracking, Deflections and Ductility Code Provisions and Recent Research. October 2006. Cracking, Deflections and Ductility Code Provisions and Recent Research. Overview Code provisions for ductility Background to the study

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Cracking, Deflections and Ductility Code Provisions and Recent Research

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  1. Serviceability and Ductility The Other Limit States Cracking, Deflections and DuctilityCode Provisions and Recent Research October 2006

  2. 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

  3. Code provisions for ductility

  4. Background to the study • Prediction of cracking and deflection: • Why is it important? • Why is it difficult? • What do the codes say?

  5. 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.

  6. 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

  7. 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

  8. 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

  9. 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?

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

  11. 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

  12. 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

  13. Background to prediction of cracking and deflections • Recommended reading:Concrete Structures • Stresses and Deformations • Ghali Favre and Elbadry

  14. Formation and propagation of cracks

  15. Relationship between cracking and section stiffness • Tension stiffening • Displacement of Neutral Axis • Loss of tension stiffening

  16. 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

  17. 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.

  18. 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.

  19. Code provisions for stress limitsStress Limits for Maximum Bar Diameter

  20. Code provisions for stress limits Stress Limits for Maximum Bar Spacing

  21. Code provisions for stress limits Design crack widths for maximum stress

  22. Code provisions for stress limits Design crack widths for maximum stress

  23. Code provisions for stress limits Design crack widths for maximum stress

  24. Code provisions for crack widths • AS 3600 and AS 5100 • No requirement for calculation of crack widths

  25. Code provisions for crack widths • EC2

  26. Code provisions for crack widths -EC2

  27. 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)

  28. Code provisions for crack widths Design surface crack width: BS8110 BS 5400

  29. 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)

  30. 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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