Masonry Code Of Practice Amp

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Masonry Code Of Practice Amp

  1. 1. Dr. A. Meher Prasad Department of Civil Engineering Indian Institute of Technology Madras email: prasadam@iitm.ac.in MASONRY CODES OF PRACTICE
  2. 2. <ul><li>ACI 530-02 / ASCE 5-02 / TMS 402-02 </li></ul><ul><li>Minimum requirements for structural design and Construction of masonry unit. </li></ul><ul><li>Allowable Stress Design (ASD) </li></ul><ul><li>Limit State Design (LSD) </li></ul><ul><li>IBC 2000 </li></ul><ul><li>NZS 4230: Part 1: 1990 </li></ul><ul><li>Eurocode 6: Design of Masonry Structures </li></ul><ul><li>IS: 1905 - 1987 </li></ul>Masonry Codes of Practice
  3. 3. Design Philosophies <ul><li>Working Stress Method (WSM) </li></ul><ul><li>Ultimate Load Method (ULM) </li></ul><ul><li>Limit States Method (LSM) </li></ul>
  4. 4. Design Philosophies … “ Limit states design” is supposed to be the most rational design philosophy. Why?
  5. 5. Design Philosophies <ul><li>Working stress method ( WSM) </li></ul><ul><li>- Behaviour under ‘service loads’ </li></ul><ul><li> - All uncertainties accommodated in ‘factors of safety’ applied to material strengths </li></ul><ul><li>Ultimate load method (ULM) </li></ul><ul><li> - Behaviour under ‘ultimate loads’ </li></ul><ul><li> - All uncertainties accommodated in ‘load factors’ </li></ul>WSM attempts to ensure adequate safety under service loads, while ULM attempts to ensure adequate safety under extreme loads. WSM does not investigate behaviour beyond service loads, ULM does not guarantee serviceability under service loads.
  6. 6. WSM and ULM WSM attempts to ensure adequate safety under service loads, while ULM attempts to ensure adequate safety under extreme loads. WSM does not investigate behaviour beyond service loads, ULM does not guarantee serviceability under service loads.
  7. 7. Limit States Method (LSM) A limit state is a state of impending failure , beyond which a structure ceases to perform its intended function satisfactorily, in terms of either strength or serviceability ; i.e., it either collapses or becomes unserviceable . Unlike WSM , which bases calculations on service load conditions alone, and unlike ULM , which bases calculations on ultimate load conditions alone, LSM aims for a comprehensive and rational solution to the design problem, by considering safety at ultimate loads and serviceability at working loads . LSM is described as a ‘semi-probabilistic’ method or a ‘Level I reliability’ method
  8. 8. Strength Design Model pdf S n R n R and S are independent random variables Probabilistic approach: nominal / characteristic values (deterministic) Moral: There is always a risk of failure. No structure is 100% safe! R  S  Structure will survive! R < S  Structure will fail!
  9. 9. F s = R n / S n P f = Prob [ R < S ] Reliability = Probability of survival = 1 - Probability of failure pdf S n R n Deterministic measure of safety: Probabilistic measure of safety: f S ( s ) f R ( r ) = Prob
  10. 10.  R k   S k Load and Resistance Design (LRFD) Format Design Resistance  Design Load Effect  S n R n  S n =  R n Load factor > 1 (‘overloading’) Resistance factor < 1 (‘understrength’)  (ULM format)  (WSM format)
  11. 11. Partial load and material safety factors Ultimate limit states – partial load factors: UL = 1.5 (DL + LL) UL = 1.5 (DL + QL) or (0.9DL + 1.5 QL) UL = 1.2 (DL + LL + QL) Note : It is not correct to apply 33.3% increase in allowable stress in WSM when only DL and QL are involved! Ultimate limit states – material safety factors : Concrete: = 1.5 Steel: = 1.15
  12. 12. Design Philosophies <ul><li>Empirical Design </li></ul><ul><ul><li>Formulae for the design developed by experience </li></ul></ul><ul><ul><li>Not a design analysis for sizing and proportioning masonry elements </li></ul></ul><ul><ul><li>For simple structures still being continued in ACI 530 – 02 and to some extent in IBC 2000 </li></ul></ul><ul><ul><li>Imposes severe limitation on building height proportions </li></ul></ul><ul><ul><li>IS 1905 – mixes empirical with allowable stress design </li></ul></ul>
  13. 13. Design Philosophies ... <ul><li>Allowable Stress Design </li></ul><ul><ul><li>All uncertainties are accommodated in Factor of safety applied to material strength </li></ul></ul><ul><ul><li>Under working loads, the stress developed in a member must be less than the permissible stress </li></ul></ul><ul><ul><li>For URM, tensile stress in masonry is less than allowable limits and for RM, tensile stress neglected </li></ul></ul><ul><ul><li>ACI follows this for URM and RM </li></ul></ul><ul><ul><li>In IS code applies only to URM </li></ul></ul><ul><ul><li>Does not find place in Eurocode, NZS 4230 </li></ul></ul>
  14. 14. Design Philosophies . . . <ul><li>Limit State Design </li></ul><ul><ul><li>Adopted by ACI, IBC 2000 and New Zealand codes </li></ul></ul><ul><ul><li>Proportion masonry members such that </li></ul></ul><ul><ul><li>Design strength ≥ Required strength </li></ul></ul><ul><ul><li>where, Design strength = Nominal strength  φ </li></ul></ul><ul><ul><li> φ is the strength reduction factor </li></ul></ul><ul><ul><li>Required strength is computed from design load combinations of building code </li></ul></ul>
  15. 15. <ul><ul><li>Eurocode - 6 </li></ul></ul><ul><ul><ul><li>Limit state design for Collapse and Serviceability </li></ul></ul></ul><ul><ul><ul><li>Partial safety factors for loads and materials are specified separately instead of strength reduction factor </li></ul></ul></ul><ul><ul><ul><li>Partial safety factor for loads depends on the load combinations </li></ul></ul></ul><ul><ul><ul><li>Partial safety factor for materials depends on the type of masonry unit and the failure mode </li></ul></ul></ul>
  16. 16. <ul><li>Assumptions in LSD and LRFD </li></ul><ul><ul><li>Strain continuity between reinforcement, grout and masonry </li></ul></ul><ul><ul><li>Max Compressive strain ε mu ≈ 0.0035 for clay material </li></ul></ul><ul><ul><ul><ul><ul><li> ≈ 0.0025 for concrete (CMU) </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li> ≈ 0.008 confined masonry ( NZ) </li></ul></ul></ul></ul></ul><ul><ul><li>Stress in steel, f s = E s ε ≤ f y </li></ul></ul><ul><ul><li> = f y for ε ≥ ε y ( = f y / E s ) </li></ul></ul>
  17. 17. <ul><ul><li>Flexural strength is assessed by neglecting tensile strength of masonry </li></ul></ul><ul><ul><li>But deflections assessed by including tensile strength </li></ul></ul><ul><ul><li>Masonry stress uniform over hatched block </li></ul></ul><ul><ul><li>0.8 f m (ACI) 0.85f m (IBC) </li></ul></ul><ul><li>Assumptions in LSD and LRFD </li></ul>Equivalent rectangular masonry stress distribution Equivalent rectangular masonry stress distribution for confined masonry according to NZS
  18. 18. <ul><li>ACI </li></ul><ul><li>(f a / F a )  1 </li></ul><ul><ul><li>f a = Calculated compressive stress </li></ul></ul><ul><ul><li>F a = Allowable compressive strength </li></ul></ul><ul><ul><li> = 0.25 f m R </li></ul></ul>Capacity reduction factor for slenderness Accounts for material uncertainties Axial Compression - ASD
  19. 19. <ul><ul><li>R = 1 – (h/40t) 2 for h/t ≤ 29 </li></ul></ul><ul><ul><li>R = (20t/h) 2 for h/t > 29 </li></ul></ul><ul><ul><li>P ≤ ¼ P e where </li></ul></ul>Slenderness can affect capacity either as a result of inelastic buckling or because of additional bending moment Axial Compression - ASD
  20. 20. IS 1905 :1987: a stress reduction factor k s which depends on slenderness ratio and eccentricity of load (Table 9 of Code) Slenderness effects on axial compression Axial Compression - ASD
  21. 21. Image is not clear The previous fig is the same….
  22. 22. Slenderness ratio for different Eccentricities
  23. 23. Reinforced Masonry Axial Compression - ASD ACI 530-02 IBC 2000, NZS:4230: Part 1 , Eurocode 6, IS: 1905-1987 No Provisions have been given
  24. 24. Maximum h/t Ratio There is no directly specified limit to the h/t ratio. It is indirectly specified through the check against Euler’s buckling formula ACI 530-02 IS: 1905-1987 Maximum h/t ratio depends on the storey height and type of mortar used. IBC 2000, NZS:4230: Part 1 , Eurocode 6 No Provisions have been given
  25. 25. <ul><li>Flexural stress due to </li></ul><ul><ul><li>eccentricity of axial loading </li></ul></ul><ul><ul><li>application of horizontal loads such as wind and earthquake loads </li></ul></ul><ul><li>ACI: A member subjected to pure flexure only </li></ul><ul><li>Allowable bending compressive stress, F b = 0.33 f m </li></ul><ul><li>Calculated f b  F b </li></ul>Axial compression with flexure: ASD
  26. 26. Interaction formula Very conservative Axial compression with flexure: ASD
  27. 27. Axial compression with flexure: ASD Significance of M/Vd v factor
  28. 28. <ul><li>IS 1905:1987 </li></ul><ul><li>Bending compressive and tensile stresses </li></ul><ul><li>Permissible value for bending compressive stress is increased by 25% and then reducing it for eccentric loading causing flexure. </li></ul><ul><li>Permissible loads for 3 eccentric values </li></ul><ul><ul><li>e < t/24 </li></ul></ul><ul><ul><li>t/24 < e < t/6 </li></ul></ul><ul><ul><li>e > t/6 </li></ul></ul>Applied moment converted into equivalent eccentricity Axial compression with flexure: ASD
  29. 30. e < t/24 <ul><ul><li>t/24 < e < t/6 </li></ul></ul>e > t/6 Axial compression with flexure: ASD
  30. 31. Axial compression with flexure Flexure and Axial Wall Loading Interaction Diagram Reinforced Masonry
  31. 32. Design for Shear – ASD ACI 530-02 Shear stress shall not exceed either of 0.125 , 0.83MPa or v+0.45N v /A n IBC 2000, NZS:4230: Part 1 , Eurocode 6 No Provisions have been given IS: 1905-1987 Permissible shear stress is given by: F v = 0.1 + /6 < 0.5MPa Un Reinforced Masonry
  32. 33. If shear reinforcement is not provided ACI 530-02 For flexural members, For shear walls, a. M/Vd v < 1, F v = 0.028[4-M/Vd v ] < (0.55-0.31M/Vd v )MPa b. M/Vd v > 1, F v =0.083 < 0.24MPa F v = 0.083 < 0.35MPa, IBC 2000, NZS:4230: Part 1 , Eurocode 6, IS: 1905-1987 No Provisions have been given Design for Shear – ASD Reinforced Masonry
  33. 34. ACI 530-02 For flexural members, Fv = 0.25 < 1.03MPa For shear walls, a. M/Vd v < 1, Fv = 0.042[4-M/Vdv] < (0.82-0.031M/Vdv)MPa b. M/Vd v > 1, Fv = 0.125 < 0.52MPa If shear reinforcement is provided Design for Shear – ASD Reinforced Masonry
  34. 35. IS: 1905-1987 Permissible shear stress is given by: Fv = 0.1 + /6 < 0.5MPa IBC 2000, NZS:4230: Part 1 , Eurocode 6 No Provisions have been given ASD format If shear reinforcement is provided Design for Shear – ASD Reinforced Masonry
  35. 36. <ul><li>IS 1905:1987: Shear </li></ul><ul><li>Masonry load bearing walls also act as shear walls to resist in plane lateral loads. </li></ul><ul><li>Shear failure in URM are: (3 modes) </li></ul><ul><ul><li>Diagonal tension cracks from through mortar and masonry units </li></ul></ul>Design for Shear – ASD
  36. 38. Design for Shear – ASD <ul><ul><li>Sliding occurs along a straight crack of horizontal bed joints </li></ul></ul><ul><ul><ul><li>While specifying Mohr coulomb type failure criterion </li></ul></ul></ul><ul><ul><li>Stepped cracks form, alternating from head joint to bed joint depends on bond pattern of masonry </li></ul></ul>
  37. 39. IS 1905:1987: takes care of sliding failure by specifying permissible shear stress URM Average axial stress not more than 2.4 MPa
  38. 40. Allowable shear for reinforced walls
  39. 43. Capacity Design for strength of flanged wall
  40. 44. Effective Shear Areas
  41. 45. LSD
  42. 46. AXIAL COMPRESSION - LSD Un Reinforced Masonry: ACI 530-02 IS: 1905-1987, NZS:4230: Part 1 IBC 2000 Eurocode 6 Charactersistic Compressive strength of Masonry is Maximum compressive strain is limited to 0.002 No Provisions are given
  43. 47. Reinforced Masonry: ACI 530-02 Eurocode 6, IS: 1905-1987 No Provisions are given AXIAL COMPRESSION - LSD IBC 2000 NZS:4230: Part 1
  44. 48. AXIAL COMPRESSION WITH FLEXURE - LSD Un Reinforced Masonry: ACI 530-02, IBC 2000, NZS:4230: Part 1, IS 1905- 1987 No Provisions are given Eurocode 6 1. Design Md equals 2. In case of vertical load, increases to 3. Lateral resistance
  45. 49. AXIAL COMPRESSION WITH FLEXURE - LSD Reinforced Masonry: ACI 530-02 2. For walls with factored axial stress < 0.2f m , 1. For walls with factored axial stress < 0.05f m , and SR > 30 shall be designed as above with walls having t min =150mm. 3. at extreme fiber is 0.0035 for clay masonry and 0.002 for concrete masonry.
  46. 50. AXIAL COMPRESSION WITH FLEXURE - LSD Reinforced Masonry: IBC 2000 1. For wall design against out-of-plane loads, all values are same as of the ACI code, except 2. is the same as in the ACI code. NZS:4230: Part 1 1. is 0.0025 for unconfined concrete masonry and 0.008 for confined concrete masonry Eurocode 6 For singly reinforced rectangular c/s subjected to bending only IS 1905- 1987 No provisions
  47. 51. DESIGN FOR SHEAR - LSD Un Reinforced Masonry: <ul><li>ACI 530-02 </li></ul>Nominal shear strength can be obtained from code section 3.3.4 <ul><li>IBC 2000 </li></ul>Nominal shear strength can be obtained from code section 2108.10.4.1 <ul><li>Eurocode 6 </li></ul>1. F v = 0.1 +0.4 2. V n = F v tl c / <ul><li>IS: 1905-1987, NZS:4230: Part 1 </li></ul>No Provisions
  48. 52. DESIGN FOR SHEAR - LSD Reinforced Masonry: ACI 530-02 1. V n = V m + V s 2. When M/Vd v < 0.25, V n < (0.083) 6A n When M/Vd v > 1.00, V n < (0.083) 4A n For M/Vd v value between 0.25 and 1.00, V n may be interpolated IBC 2000 <ul><li>Nominal shear strength is same as in the ACI code. </li></ul><ul><li>2. If shear wall failure mode is in flexure, M n shall be at least 1.5M cr </li></ul><ul><li>for fully grouted wall or 3M cr for partially grouted wall. </li></ul>
  49. 53. DESIGN FOR SHEAR - LSD Reinforced Masonry: NZS:4230: Part 1 1. Shear strength is given in section 7.3.2.1 2. Minimum value of A v = 0.15 b w s/f y 3. 4. Required area of shear friction reinforcement is Eurocode 6 1. Ignoring shear reinforcement: 2. Taking shear reinforcement into account: IS: 1905-1987 No Provisions

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