1. Vietnam Institute for Building Science and Technology (IBST)
Building Code Requirements for Structural Concrete (ACI 318M-11) Overview of ACI 318M Design of Prestressed Concrete Evaluation of Existing Structures
David Darwin
Vietnam Institute for Building Science and Technology (IBST)
Hanoi and Ho Chi Minh City
December 12-16, 2011

2. This morning
Overview of ACI 318M-11
Design of Prestressed Concrete (Chapter 18)
Strength Evaluation of Existing Structures (Chapter 20)

3. This afternoon
Analysis and design of
Flexure
Shear
Torsion
Axial load

4. Design of slender columns Design of wall structures
Tomorrow morning
Design of slender columns
Design of wall structures
High-strength concrete 4

5. Overview of ACI 318M-11
Legal standing Scope Approach to Design Loads and Load Cases Strength Reduction Factors

6. Legal standing
Serves as the legal structural concrete building code in the U.S. because it is adopted by the general building code (IBC).

7. Scope
ACI 318M consists of 22 chapters and 6 appendices that cover all aspects of building design

8. Chapters 1. GENERAL REQUIREMENTS 2. NOTATION AND DEFINITIONS
Scope, Contract Documents, Inspection, Approval of Special Systems
2. NOTATION AND DEFINITIONS

9. Chapters
3. MATERIALS Cementitious Materials, Water, Aggregates, Admixtures, Reinforcing Materials

10. 4. DURABILITY REQUIREMENTS
Freezing and Thawing, Sulfates, Permeability, Corrosion

11. 5. CONCRETE QUALITY, MIXING, AND PLACING 6
5. CONCRETE QUALITY, MIXING, AND PLACING 6. FORMWORK, EMBEDMENTS, AND CONSTRUCTION JOINTS

12. 7. DETAILS OF REINFORCEMENT
Hooks and Bends, Surface Condition, Tolerances, Spacing, Concrete Cover, Columns, Flexural Members, Shrinkage and Temperature Steel, Structural Integrity
Ronan Point gas explosion
Precast elements not tied together
March 11, 1968
22 stories

13. 8. ANALYSIS AND DESIGN — GENERAL CONSIDERATIONS
Design Methods; Loading, including Arrangement of Load; Methods of Analysis; Redistribution of Moments; Selected Concrete Properties; Requirements for Modeling Structures (Spans, T-beams, Joists...)

14. 9. STRENGTH AND SERVICEABILITY REQUIREMENTS
Load Combinations, Strength Reduction Factors, Deflection Control
10. FLEXURE AND AXIAL LOADS
Beams and One-way Slabs, Columns, Deep Beams, Bearing

15. 11. SHEAR AND TORSION 12. DEVELOPMENT AND SPLICES OF REINFORCEMENT

16. 13. TWO-WAY SLAB SYSTEMS 14. WALLS

17. 15. FOOTINGS 16. PRECAST CONCRETE

18. 17. COMPOSITE CONCRETE FLEXURAL MEMBERS 18. PRESTRESSED CONCRETE

19. 19. SHELLS AND FOLDED PLATE MEMBERS
Kresge Auditorium Massachusetts Institute of Technology, shown from the rear, roof being replaced, designed by Eero Saarinen (1955)

20. 20. STRENGTH EVALUATION OF EXISTING STRUCTURES 21
20. STRENGTH EVALUATION OF EXISTING STRUCTURES 21. EARTHQUAKE- RESISTANT STRUCTURES 22. STRUCTURAL PLAIN CONCRETE

21. Appendices
A. STRUT-AND-TIE MODELS* B. ALTERNATIVE PROVISIONS FOR REINFORCED AND PRESTRESSED CONCRETE FLEXURAL AND COMPRESSION MEMBERS C. ALTERNATIVE LOAD AND STRENGTH REDUCTION FACTORS

22. D. ANCHORING TO CONCRETE. E. STEEL REINFORCEMENT INFORMATION F
D. ANCHORING TO CONCRETE* E. STEEL REINFORCEMENT INFORMATION F. EQUIVALENCE BETWEEN SI-METRIC, MKS-METRIC, AND U.S. CUSTOMARY UNITS OF NONHOMOGENOUS EQUATIONS IN THE CODE

23. Approach to design
Qd = design loads Sn = nominal strength Sd = design strength M = safety margin

24. Design Strength  Required Strength Sd = Sn  Qd Sd = design strength = Sn  = strength reduction factor  = load factors Qd = design loads  and  in Chapter 9 of ACI 318M

25. Loads Qd
specified in ASCE 7, Minimum Design Loads for Buildings and Other Structures American Society of Civil Engineers (ASCE) Reston, Virginia, USA

26. Loads
Dead loads (D)* Live loads (L)* Roof live loads (Lr)* Wind loads (W)  full load Earthquake loads (E)  full load Rain loads (R)* Snow loads (S)* * Service-level loads

27. Loads
Impact – include in L Self-straining effects (temperature, creep, shrinkage, differential settlement, and shrinkage compensating concrete) (T) Fluid loads (F) Lateral soil pressure (H) Factored Load = U = Qd

28. Load cases and load factors by ASCE 7 and ACI 318M
U = 1.4D U = 1.2D + 1.6L (Lr or S or R) U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.5W) U = 1.2D + 1.0W + 1.0L + 0.5(Lr or S or R) U = 1.2D + 1.0E + 1.0L + 0.2S

29. Load cases and load factors by ASCE 7 and ACI 318M
U = 0.9D + 1.0W U = 0.9D + 1.0E

30. Load factors by ACI 318M
If W based on service-level forces, use 1.6W place of 1.0W If E based on service-level forces, use 1.4E in place of 1.0E Details of other cases covered in the Code

31. Strength reduction () factors
Tension-controlled sections 0.90 Compression-controlled sections Members with spiral reinforcement 0.75 Other members 0.65 Shear and torsion 0.75 Bearing 0.65 Post-tensioning anchorages 0.85 Other cases 0.60 – 0.90

32. Tension-controlled and compression- controlled sections

33. T-beam d h b hf bw As dt

34. Strain through depth of beam

36. Design Strength ( x nominal strength) must exceed the Required Strength (factored load)
Bending Mn  Mu Axial load Pn  Pu Shear Vn  Vu Torsion Tn  Tu

37. Load distributions and modeling requirements

38. Chicago place

40. Structure may be analyzed as elastic
using properties of gross sections Ig = moment of inertia of gross (uncracked) cross section Beams: Ib = ½ Ig  Iweb = Columns: Ic = Ig =

41. Analysis by subframes
1. The live load applied only to the floor or roof under consideration, and the far ends of columns built integrally with the structure considered fixed

42. 2. The arrangement of load may be limited to combinations of
factored dead load on all spans with full factored live load on alternate spans, and
factored dead load on all spans with full factored live load on two adjacent spans

43. (a) (b) (c)

44. Moment and shear envelopes

45. Columns designed to resist
axial forces from factored loads on all floors or roof and maximum moment from factored live loads on a single adjacent span of the floor or roof under consideration
loading condition giving maximum ratio of moment to axial load
More on columns

46. For frames or continuous construction, consider effect of unbalanced floor or roof loads on both exterior and interior columns and of eccentric loading due to other causes For gravity load, far ends of columns built integrally with the structure may be considered fixed At any floor or roof level, distribute the moment between columns immediately above and below that floor in proportion to the relative column stiffness

47. Simplified loading criteria

48. ln
Beams, two
or more spans
Beams, two
spans only
Slabs,
spans ≤ 3 m
Beams,  col stiffnesses ≥ 8  beam stiffnesses

49. Max –ve left
Max +ve
Composite
Max –ve right

50. Allowable adjustment in maximum moments for t  0.0075

51. Design of prestressed concrete (Chapter 18)

52. Behavior of reinforced concrete

53. Reinforced concrete under service loads

54. Theory of prestressed concrete
Stresses

57. Methods of prestressing concrete members
Pretensioning
Post-Tensioning
Pretensioned concrete is typically cast in a stressing bed where the tendons are pulled prior to placing the concrete. Post-tensioned concrete is stressed after the concrete has hardened.

58. Prestressing steels

59. Strength of prestressing steels available in U.S.
Seven-wire strand: fpu  1725, 1860 MPa fpy (stress at 1% extension)  85% (for stress-relieved strand) or 90% (for low-relaxation strand) of fpu fpu = ultimate strength fpy = yield strength

60. Strength of prestressing steels available in U.S.
Prestressing wire: fpu  1620 to 1725 MPa (function of size) fpy (at 1% extension)  85% of fpu

61. Strength of prestressing steels available in U.S.
High-strength steel bars: fpu  1035 MPa fpy  85% (for plain bars) and 80% (for deformed bars) of fpu fpy based on either 0.2% offset or 0.7% strain

62. Maximum permissible stresses in prestressing steel
Due to prestressing steel jacking force: 0.94fpy 0.80fpu manufacturers recommendation Post-tensioning tendons, at anchorage devices and couplers, immediately after force transfer: 0.70fpu

63. Prestressed concrete members are designed based on both
Elastic flexural analysis Strength

64. Elastic flexural analysis
Considers stresses under both the Initial prestress force Pi and the Effective prestress force Pe Note: = concrete compressive strength = initial concrete compressive strength (value at prestress transfer)

65. Classes of members
U – uncracked – calculated tensile stress in precompressed tensile zone at service loads = ft  T – transition between uncracked and cracked < ft  C – cracked ft >

66. Concrete section properties
e = tendon eccentricity
k1= upper kern point
k2= lower kern point
Ic = moment of inertia
Ac = area
radius of gyration:
r2 = Ic/Ac
section moduli:
S1 = Ic/c1
S2 = Ic/c2

67. Bending moments
Mo = self-weight moment Md = superimposed dead load moment Ml = live load moment

68. Concrete stresses under Pi

69. Concrete stresses under Pi + Mo

70. Concrete stresses under Pe + Mo + Md + Ml

71. Maximum permissible stresses in concrete at transfer
Extreme fiber stress in compression, except as in (b),
Extreme fiber stress in compression at ends of simply supported members
Extreme fiber stress in tension at ends of simply supported members *
Extreme fiber stress in tension at other locations *
* Add tensile reinforcement if exceeded

72. Maximum permissible compressive stresses in concrete at service loads Class U and T members
Extreme fiber stress in compression due to prestress plus sustained load
Extreme fiber stress in compression due to prestress plus total load

73. Flexural strength
Aps
T = Apsfps

74. Stress-block parameter 1

75. Stress in prestressing steel at ultimate
Members with bonded tendons:
p = Aps/bdp = reinforcement ratio
b = width of compression face
dp = d (effective depth) of prestressing steel

76. Members with bonded tendons and non-prestressed bars:

77. Members with unbonded tendons with span/depth ratios  35: but not greater than fpy or greater than fpe MPa fpe = stress in Aps at Pe =

78. Members with unbonded tendons with span/depth ratios > 35: but not greater than fpy or greater than fpe MPa

79. Loss of prestress Prestessing steel seating at transfer
Elastic shortening of concrete
Creep of concrete
Shrinkage of concrete
Relaxation of prestressing steel
Friction loss due to intended or unintended curvature of post-tensioning tendons

80. Limits on reinforcement in flexural members
Classify as tension-controlled, transition, or compression-controlled to determine  Total amount of prestressed and nonprestressed reinforcement in members with bonded reinforcement must be able to carry 1.2  cracking load

81. Minimum bonded reinforcement As in members with unbonded tendons
Except in two-way slabs, As = 0.004Act Act = area of that part of cross section between the flexural tension face and center of gravity of gross section Distribute As uniformly over precompressed tension zone as close as possible to extreme tensile fiber

82. Two-way slabs: Positive moment regions: Bonded reinforcement not required where tensile stress ft  Otherwise, use As = Nc = resultant tensile force acting on portion of concrete cross section in tension under effective prestress and service loads Distribute As uniformly over precompressed tension zone as close as possible to extreme tensile fiber

83. Two-way slabs: Negative moment areas at column supports: As = 0
Two-way slabs: Negative moment areas at column supports: As = Acf Acf = larger gross cross-sectional area of slab-beam strips in two orthogonal equivalent frames intersecting at the columns Distribute As between lines 1.5h on outside opposite edges of the column support Code includes spacing and length requirements

84. Two-way slabs
Use Equivalent Frame Design Method (Section 13.7)

85. Banded tendon distribution
Use efficient placement of tendons
Place banded tendons above uniform tendons at column locations (common error)
Common construction sequence of layout
Banded tendons
Uniform tendons
Photo courtesy of Portland Cement Association

86. Development of prestressing strand
development length = transfer length

87. Shear for prestressed concrete members is similar to that for reinforced concrete members, but it takes advantage of presence of prestressing force

88. Post-tensioned tendon anchorage zone design
Load factor = 1.2  Ppu = 1.2Pj Pj = maximum jacking force  = 0.85
Strut and tie model

89. Strength evaluation of existing structures (Chapter 20)

90. Strength evaluation of existing structures (Chapter 20)
When it is required When we use analysis and when perform a load test When core testing is sufficient Load testing

91. A strength evaluation is required
when there is a doubt if a part or all of a structure meets safety requirements of the Code If the effect of the strength deficiency is well understood and if it is feasible to measure the dimensions and material properties required for analysis, analytical evaluations of strength based on those measurements can be used

92. If the effect of the strength deficiency is not well understood or if it is not feasible to establish the required dimensions and material properties by measurement, a load test is required if the structure is to remain in service

93. Establishing dimensions and material properties
Dimensions established at critical sections
Reinforcement locations established by measurement (can use drawings if spot checks confirm information in drawings)
Use cylinder and core tests to estimate

94. Core testing

96. If the deficiency involves only the compressive strength of the concrete based on cylinder tests
Strength is considered satisfactory if:
Three cores are taken for each low-strength test
The average of the three cores 
No individual core has a strength <

97. Steel
Reinforcing and prestressing steel may be evaluated based on representative material

98. If analysis is used, values of  may be increased
Tension-controlled 0.90  1.0 Compression controlled 0.75 and 0.65  0.90 and 0.80 Shear and torsion 0.75  0.80 Bearing 0.65  0.80

99. Load test procedure Load arrangement:
Select number and arrangement of spans or panels loaded to maximize the deflection and stresses in the critical regions
Use more than one arrangement if needed (deflection, rotation, stress)

100. Load intensity
Total test load = larger of (a) 1.15D + 1.5L + 0.4(Lr or S or R) (b) 1.15D + 0.9L + 1.5(Lr or S or R) (c) 1.3D In (b), load factor for L may be reduced to 0.45, except for garages, places of assembly, and where L > 4.8 kN/m2 L may be reduced as permitted by general building code

101. Age at time of loading  56 days

102. Loading criteria
Obtain initial measurements (deflection, rotation, strain, slip, crack widths) not more than 1 hour before application of the first load increment Take readings where maximum response is expected Use at least four load increments Ensure uniform load is uniform – no arching

103. Take measurements after each load increment and after the total load has been applied for at least 24 hours Remove total test load immediately after all response measurements are made Take a set of final measurements 24 hours after the test load is removed

104. Acceptance criteria
No signs of failure – no crushing or spalling of concrete No cracks indicating a shear failure is imminent In regions without transverse reinforcement, evaluate any inclined cracks with horizontal projection > depth of member Evaluate cracks along the line of reinforcement in regions of anchorage and lap splices

105. Acceptance criteria
Measured deflections At maximum load: 24 hours after load removed:

106. Acceptance criteria
If deflection criteria not met, may repeat the test (at least 72 hours after first test) Satisfactory if:

107. Provision for lower loading
If the structure does not satisfy conditions or criteria based on analysis, deflection, or shear, it may be permitted for use at a lower load rating based on the results of the load test or analysis, if approved by the building official

108. Case study
1905 building Chicago, Illinois USA Cinder concrete floors Load capacity OK for use as an office building?
Cinder = ashes of coal

109. Safety shoring

110. Deflection measurement devices
CET=cable extension transducers – sometimes also called “string potentiometers”

111. Load through window

113. Moving lead ingots through the window

115. Load stage 14

116. Findings
Floor could carry uniform load of 2.4 kN/m2 Building satisfactory for both apartments (1.9 kN/m2) and offices (2.4 kN/m2)

117. Summary
Overview Prestressed concrete Strength evaluation of existing structures

119. Figures copyright  2010 by McGraw-Hill Companies, Inc
Figures copyright  2010 by McGraw-Hill Companies, Inc Avenue of the America New York, NY USA Figures copyright  2011 by American Concrete Institute Country Club Drive Farmington Hills, MI USA Duplication authorized or use with this presentation only.

120. The University of Kansas
David Darwin, Ph.D., P.E.
Deane E. Ackers Distinguished Professor
Director, Structural Engineering & Materials Laboratory
Dept. of Civil, Environmental & Architectural Engineering
2142 Learned Hall
Lawrence, Kansas,
(785) Fax: (785)