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Title: Structure Codes and the Design Basis of RC Structures


1
Structure Codes and the Design Basis of RC
Structures
  • By Prof Dr. Qaisar Ali
  • Civil Engineering Department
  • UET Peshawar
  • drqaisarali.com
  • drqaisarali_at_nwfpuet.edu.pk

2
Topics Addressed
  • Building Codes and the ACI Code
  • Objectives of Design
  • Design Process
  • Limit States and the Design of Reinforced
    Concrete
  • Basic Design Relationship
  • Structural Safety

3
Topics Addressed
  • Design Procedure Specified in the ACI Code
  • Design Loads for Buildings and other Structures
  • Customary Dimensions and Construction Tolerances
  • Admixtures
  • Factors Affecting Strength of Concrete
  • High Strength Concrete

4
Topics Addressed
  • Durability of Concrete
  • Concrete Subjected to High Temperatures
  • Reinforcing Steel
  • Chapter 3 Materials
  • Chapter 4 Durability of Concrete
  • Chapter 5 Concrete Quality, Mixing and Placing

5
Building Codes and the ACI Code
  • General Building Codes
  • Cover all aspects of building design and
    construction from architecture to structural to
    mechanical and electrical---. UBC, IBC and
    Euro-code are general building codes.
  • Seismic Codes
  • Cover only seismic provisions of buildings such
    as SEAOC and NEHRP of USA, BCP-SP 07 of Pakistan.

6
Building Codes and the ACI Code
  • Material Specific Codes
  • Cover design and construction of structures using
    a specific material or type of structure such as
    ACI, AISC, AASHTO etc.
  • Others such as ASCE
  • Cover minimum design load requirement, Minimum
    Design Loads for Buildings and other Structures
    (ASCE7-02).

7
Building Codes and the ACI Code
  • General Building Codes in USA
  • The National Building Code (NBC),
  • The Standard Building Code (SBC),
  • The Uniform Building Code (UBC),

8
Building Codes and the ACI Code
  • General Building Codes in USA
  • The International Building Code IBC,
  • Published by International Code Council ICC for
    the first time in 2000, revised every three
    years.
  • The IBC has been developed to form a consensus
    single code for USA.
  • Currently IBC 2012 is available.
  • UBC 97 is the last UBC code and is still existing
    but will not be updated. Similarly NBC, SBC will
    also be not updated.
  • In future only IBC will exist.

9
Building Codes and the ACI Code
  • Seismic Codes in USA
  • NEHRP (National Earthquake Hazards Reduction
    Program) Recommended Provisions for the
    Development of Seismic Regulations for New
    Buildings developed by FEMA (Federal Emergency
    Management Agency).
  • The NBC, SBC and IBC have adopted NEHRP for
    seismic design.
  • SEAOC Blue Book Structural Engineers Association
    of California (SEAOC), has its seismic provisions
    based on the Recommended Lateral Force
    Requirements and Commentary (the SEAOC Blue
    Book) published by the Seismology Committee of
    SEAOC.
  • The UBC has adopted SEAOC for seismic design.

10
Building Codes and the ACI Code
  • Building Code of Pakistan
  • Building Code of Pakistan, Seismic Provision BCP
    SP-07 has adopted the seismic provisions of UBC
    97 for seismic design of buildings.
  • IBC 2000 could not be adopted because some basic
    input data required by IBC for seismic design
    does not exist in Pakistan.

11
Building Codes and the ACI Code
  • The ACI MCP
  • ACI MCP (American Concrete Institute Manual of
    Concrete Practice) contains 150 ACI committee
    reports revised every three years.
  • ACI 318 Building Code Requirements for
    Structural Concrete.
  • ACI 315 The ACI Detailing Manual.
  • ACI 349 Code Requirement for Nuclear Safety
    Related Concrete Structures.
  • Many others.

12
Building Codes and the ACI Code
  • The ACI 318 Code
  • The American Concrete Institute Building Code
    Requirements for Structural Concrete (ACI 318),
    referred to as the ACI code, provides minimum
    requirements for structural concrete design or
    construction.
  • The term structural concrete is used to refer
    to all plain or reinforced concrete used for
    structural purposes.
  • Prestressed concrete is included under the
    definition of reinforced concrete.

13
Building Codes and the ACI Code
  • The ACI 318 Code
  • 7 parts, 22 chapters and 6 Appendices.
  • Brief visit of the code

14
Building Codes and the ACI Code
  • Legal Status of The ACI 318 Code
  • The ACI 318 code has no legal status unless
    adopted by a state or local jurisdiction.
  • It is also recognized that when the ACI code is
    made part of a legally adopted general building
    code, that general building code may modify some
    provisions of ACI 318 to reflect local conditions
    and requirements.

15
Building Codes and the ACI Code
  • The Compatibility Issue in BCP SP-2007
  • Building Code of Pakistan, Seismic Provision BCP
    SP-07 has adopted the seismic provisions of UBC
    97 for seismic design of buildings.
  • As the UBC 97 has reproduced ACI 318-95 in
    Chapter 19 on concrete, the load combinations and
    strength reduction factors of ACI 318-02 and
    later codes are not compatible with UBC 97 and
    hence BCP SP-07. Therefore ACI 318-02 and later
    codes cannot be used directly for design of a
    system analyzed according to the seismic
    provisions of UBC 97.

16
Building Codes and the ACI Code
  • The Compatibility Issue in BCP SP-2007
  • To resolve this issue, BCP SP-2007 recommends
    using ACI 318-05 code for design except that load
    combinations and strength reduction factors are
    to be used as per UBC 97.
  • The IBC adopts the latest ACI code by reference
    whenever it is revised and hence are fully
    compatible.

17
The Design Design Team
  • General
  • The design covers all aspects of structure, not
    only the structural design.
  • The structural engineer is a member of a team
    whose members work together to design a building,
    bridge, or other structure.

18
Objectives of Design
  • Four Major Objectives of Design
  • Appropriateness This include,
  • Functionality, to suit the requirements.
  • Aesthetics, to suit the environment.
  • Economy
  • The overall cost of the structure should not
    exceed the clients budget.

19
Objectives of Design
  • Four Major Objectives of Design
  • Structural Adequacy (safety)
  • Strength.
  • Serviceability.
  • Maintainability
  • The structure should be simple so that it is
    maintained easily.

20
The Design Process
  • Three Major Phases of Design
  • The clients needs and priorities.
  • Development of project concept.
  • Design of Individual systems.

21
Basic Design Relationship
  • Limit State Design approach
  • Capacity is reduced and demand is increased based
    on scientific rationale. In LSD approach, we have
  • f Mn Mu (a Ms )
  • f Vn Vu (a Vs )
  • f Pn Pu (a Ps )
  • f Tn Tu (a Ts )
  • f strength reduction factor
  • a load amplification factor

22
Structural Safety
  • Variability in Resistance
  • Effects of simplifying assumptions
  • The fig shows Comparison of measured (Mtest) and
    computed (Mn) failure moments for 112 similar RC
    beams

23
Structural Safety
  • Variability in Loads
  • Fig shows variation of Live loads in a family of
    151sft offices.
  • The average (for 50 buildings) sustained live
    load was around 13 psf in this sample.
  • 1 of measured loads exceeded 44 psf.
  • Building code specify 50 psf for such buildings
    (ASCE 7-02)

24
Structural Safety
  • Conclusion
  • Due to the variability of resistances and load
    effects, there is definite chance that a
    weaker-than-average structure will be subjected
    to a higher- than-average load.
  • In extreme cases, failure may occur.
  • The load factors and resistance factors are
    selected to reduce the probability of failure to
    a very small level.

25
Design Procedures Specified in the ACI Code
  • The Design Philosophy of the ACI Code
  • 9.1.1- structures and structural members shall be
    designed to have design strengths at all sections
    at least equal to the required strength
    calculated for the factored loads and forces in
    such combinations as are stipulated in this code.
  • 9.1.2- members also shall meet all other
    requirements of this code to ensure adequate
    performance at service load levels.

26
Design Procedures Specified in the ACI Code
  • The Design Philosophy of the ACI Code
  • This process is called strength design in the ACI
    code.
  • In the AISC Specifications for steel design, the
    same design process is known as LRFD (Load and
    Resistance Factor Design).
  • Strength design and LRFD are methods of
    limit-state design, except that primary attention
    is always placed on the ultimate limit states,
    with the serviceability limit states being
    checked after the original design is completed.

27
Design Loads for Buildings and Other Structures
  • ACI 318-02, Section 8.2-LOADING
  • 8.2.2 Service loads shall be in accordance with
    the general building code of which this code
    forms a part, with such live load reductions as
    are permitted in the general building code.
  • Section R8.2 The provisions in the code are for
    live, wind, and earthquake loads such as those
    recommended in Minimum Design Loads for
    Buildings and Other Structures,(ASCE 7).
  • If the service loads specified by the general
    building code (of which ACI 318 forms a part)
    differ from those of ASCE 7, the general building
    code governs. However, if the nature of the loads
    contained in a general building code differs
    considerably from ASCE 7 loads, some provisions
    of this code may need modification to reflect the
    difference.
  • A

28
Design Loads for Buildings and Other Structures
  • ASCE Recommendations on Loads
  • ASCE 7-02 sections 1 to 10 are related to design
    loads for buildings and other structures.
  • The sections are named as general, load
    combinations, dead, live, soil, wind, snow, rain,
    earthquake and ice loads.
  • Brief visit of ASCE 7-02, Section 1 to 10

29
Design Loads for Buildings and Other Structures
  • Loads on Structure During Construction
  • During the construction of concrete buildings,
    the weight of the fresh concrete is supported by
    formwork, which frequently rests on floors lower
    down in the structure.
  • ACI section 6.2.2 states the following
  • No construction loads exceeding the combination
    of superimposed dead load plus specified live
    load (un-factored) shall be supported on any
    un-shored portion of the structure under
    construction, unless analysis indicates adequate
    strength to support such additional loads

30
Customary Dimensions and Construction Tolerance
  • Difference in Working and As-Built Drawings
    Dimensions
  • The actual as-built dimensions will differ
    slightly from those shown on the drawings, due to
    construction inaccuracies.
  • ACI Committee 117 has published a comprehensive
    list of tolerance for concrete construction and
    materials.
  • As an example, tolerances for footings are 2
    inches and ½ inch on plan dimensions and 5
    percent of the specified thickness.

31
Admixtures
  • A material (usually in liquid form) other than
    cement, water and aggregates, that is used as an
    ingredient of concrete and is added to the batch
    immediately before or during mixing to change
    properties of fresh or hardened concrete.

32
Admixtures
  • Uses
  • Admixtures are used to
  • achieve certain properties in concrete more
    effectively than by other means.
  • maintain the quality of concrete during the
    stages of mixing, transporting, placing, and
    curing in adverse weather conditions.
  • reduce the cost of concrete construction.

33
Admixtures
  • Types
  • As per ACI Committee 212, admixtures have been
    classified into following groups
  • Air-entraining Admixtures causes the development
    of a system of microscopic air bubbles in
    concrete, mortar, or cement paste during mixing.
    Air-entrained concrete should be used wherever
    water saturated concrete may be exposed to
    freezing and thawing. Air entrainment also
    improves the workability of concrete.

34
Admixtures
  • Types
  • Accelerating Admixtures causes an increase in
    the rate of hydration of the hydraulic cement and
    thus shortens the time of setting, increases the
    rate of strength development, or both.
  • Water Reducing and Set-Controlling Admixtures
    Reduce the water requirements of a concrete
    mixture for a given slump, modify the time of
    setting, or both.

35
Admixtures
  • Types
  • Admixtures for Flowing Concrete Flowing Concrete
    is concrete that is characterized as having a
    slump greater than 190 mm (7-1/2 in.) while
    maintaining a cohesive nature.
  • Miscellaneous
  • Freeze Resistant, Pigments, Bonding, Grouting
    etc. (Refer ACI 212 for details and more types of
    miscellaneous admixtures)

36
Properties of Concrete
  • Factors Affecting Concrete Strength
  • In addition to mixing, conveying, placing and
    compaction, the strength of concrete primarily
    depends on
  • Water Cement Ratio Decrease in water cement
    ratio increases the strength.
  • Aggregate Cement Ratio Decrease in aggregate
    cement ratio increases the strength up to a value
    of around 2.0. Further decrease may cause
    decrease in strength.

37
Properties of Concrete
  • Factors Affecting Concrete Strength
  • Aggregate The concrete strength is affected by
    the aggregate strength, its surface texture, its
    grading and maximum size of the aggregate.
  • Curing Prolonged moist curing leads to the
    highest concrete strength

38
Properties of Concrete
  • Rate of Strength Gain
  • ACI Committee 209 3-21 has proposed the
    following equation to represent the rate of
    strength gain for concrete made from Type 1
    cement and moist-cured at 70F.
  • f c(t) f c(28) t/(4 0.85t)
  • Where f c(t) is the compressive strength at
    age t in days.

39
Properties of Concrete
  • Rate of Strength Gain and Cement Types
  • Figure shows the effect of type of cement on
    strength gain of concrete (moist cured w/c
    0.49).

I Normal II Modified III High early
strength IV Low heat V Sulfate resisting
40
High Strength Concrete
  • Concretes with strengths in excess of 6000 psi
    are referred to as high strength concrete.
  • The resulting concrete has a low void ratio.
  • Only the amount of water needed to hydrate the
    cement in the mix is provided.

41
High Strength Concrete
  • UET Lab Results for Producing High Strength
    Concrete
  • Mix design results for 6000 and 8000 psi
    concrete.
  • Admixture used Sikament 520BA

Table-A Table-A Table-A Table-A Table-A Table-A Table-A
Trial Test Proportion No. of cylinders Date of preparation Date of Testing Slump (in) Avg. Strength (psi)
6000 psi (112) w/c (0.36) 6 25/6/2010 22/7/2010 2.5 6100
8000 psi (10.81.5) w/c (0.31) 6 28/6/2010 25/7/2010 2 8000
42
Durability of Concrete
  • Three most common durability problems in concrete
    are
  • Corrosion of steel in concrete.
  • Breakdown of the structure of concrete due to
    freezing and thawing.
  • Breakdown of the structure of concrete due to
    chemical action.

43
Concrete Subjected to High Temperatures
  • Compressive Strength of Concrete at High
    Temperatures

44
Deformed Bar Reinforcement
  • ASTM A 615, Specification for Deformed and Plain
    Carbon-Steel Bars for Concrete Reinforcement.
  • ASTM A 706, Specification for Low-Alloy Steel
    Deformed and Plain Bars for Concrete
    Reinforcement.
  • ASTM A 996, Specification for Rail-Steel and
    Axle-Steel Deformed Bars for Concrete
    Reinforcement.

45
Deformed Bar Reinforcement
  • Variation in Yield Strength
  • Distribution of mill test yield strength for
    grade 60 steel.

46
Deformed Bar Reinforcement
  • Strength of Reinforcing Steel at High
    temperatures
  • Deformed reinforcement subjected to high
    temperatures in fires tends to lose its strength.

47
ACI Chapter 3 Materials
  • Tests of Materials
  • A complete record of tests of materials and of
    concrete shall be retained by the inspector for 2
    years after completion of the project, and made
    available for inspection during the progress of
    the work.
  • Water
  • Water used in mixing concrete shall be clean and
    free from injurious amounts of oils, acids,
    alkalis, salts, organic materials, or other
    substances deleterious to concrete or
    reinforcement.

48
ACI Chapter 3 Materials
  • Steel Reinforcement
  • Reinforcement shall be deformed reinforcement,
    except that plain reinforcement shall be
    permitted for spirals or pre-stressing steel and
    reinforcement consisting of structural steel,
    steel pipe, or steel tubing shall be permitted as
    specified in this code.

49
ACI Chapter 4 Durability of Concrete
  • Sulfate exposures
  • Concrete to be exposed to sulfate-containing
    solutions or soils shall conform to requirements
    of Table 4.3.1 or shall be concrete made with a
    cement that provides sulfate resistance and that
    has a maximum water-cementitious materials ratio
    and minimum compressive strength from Table
    4.3.1.

50
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Average Strength of Concrete Produced in the
    Field
  • It is emphasized in this chapter that the average
    strength of concrete produced in the filed should
    always exceed the specified value of fc' used in
    the structural design calculations.
  • This is based on probabilistic concepts, and is
    intended to ensure that adequate concrete
    strength will be developed in the structure.

51
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Average Strength of Concrete Produced in the
    Field
  • Variation in Strength
  • Variations in the properties or proportions of
    constituents of concrete, as well as variations
    in transporting, placing, and compaction of the
    concrete, lead to variations in the strength of
    the finished concrete. In addition, discrepancies
    in the tests will lead to apparent differences in
    strength.

52
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Average Strength of Concrete Produced in the
    Field
  • Variation in Strength
  • Figure shows the distribution of strengths in a
    sample of 176 concrete cylinder tests for the
    concrete having nominal strength of 3000 psi
  • Strength less than nominal 9
  • Strength more than nominal 167

53
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • ACI recommendations for achieving specified
    strength in the field
  • The ACI code recommends that selection of
    concrete proportions for achieving a specified
    concrete strength in the field shall be based on
    the required average compressive strength of
    concrete f cr' and not on the specified
    strength.
  • ACI table 5.2.2.

54
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Selection of Concrete Proportions
  • Proportions of materials for concrete shall be
    established to provide
  • Workability and consistency to permit concrete to
    be worked readily into forms and around
    reinforcement under conditions of placement to be
    employed, without segregation or excessive
    bleeding
  • Resistance to special exposures as required by
    Chapter 4 of ACI
  • Conformance with strength test requirements of
    ACI 5.6.

55
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Selection of Concrete Proportions
  • Recommendations for selecting proportions for
    concrete are given in detail in Standard
    Practice for Selecting Proportions for Normal,
    Heavyweight, and Mass Concrete (ACI 211.1).

56
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Sampling Frequency for Strength Tests
  • ACI R5.6.2.1 As a measure of quality control,
    the code recommends following criteria for
    collecting samples of concrete cylinders from a
    given class of concrete
  • Once each day a given class is placed, nor less
    than
  • Once for each 150 yd3 of each class placed each
    day, nor less than
  • Once for each 5000 ft2 of slab or wall surface
    area placed each day.
  • In calculating surface area, only one side of the
    slab or wall should be considered.

57
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Strength Test
  • A strength test shall be the average of the
    strengths of two cylinders made from the same
    sample of concrete and tested at 28 days or at
    test age designated for determination of fc'.

58
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Criterion for Satisfactory Concrete Strength
  • ACI 5.6.3.3 Strength level of an individual
    class of concrete shall be considered
    satisfactory if both of the following
    requirements are met
  • (a) Every arithmetic average of any three
    consecutive strength tests equals or exceeds fc'
  • (b) No individual strength test (average of two
    cylinders) falls below fc' by more than 500 psi
    when fc' is 5000 psi or less or by more than
    0.10fc' when fc' is more than 5000 psi.

59
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • The steps taken to increase the average level of
    test results
  • It will depend on the particular circumstances,
    but could include one or more of the following
  • An increase in cementitious materials content
  • Changes in mixture proportions
  • Reductions in or better control of levels of
    slump supplied
  • Closer control of air content
  • An improvement in the quality of the testing,
    including strict compliance with standard test
    procedures

60
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Investigation of Low Strength Test Results
  • If any strength test of laboratory-cured
    cylinders falls below specified value of fc' by
    more than the values given in 5.6.3.3(b), steps
    shall be taken to assure that load-carrying
    capacity of the structure is not jeopardized.

61
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Investigation of Low Strength Test Results
  • If the likelihood of low-strength concrete is
    confirmed and calculations indicate that
    load-carrying capacity is significantly reduced,
    tests of cores drilled from the area in question
    in accordance with Method of Obtaining and
    Testing Drilled Cores and Sawed Beams of
    Concrete (ASTM C 42) shall be permitted.
  • In such cases, three cores shall be taken for
    each strength test that falls below the values
    given in 5.6.3.3(b).

62
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Investigation of Low Strength Test Results
  • According to ACI 5.6.5.4, concrete in an area
    represented by core tests shall be considered
    structurally adequate if the average of three
    cores is equal to at least 85 percent of fc' and
    if no single core is less than 75 percent of fc'.

63
ACI Chapter 5 Concrete Quality, Mixing and
Placing
  • Investigation of Low Strength Test Results
  • If criteria of ACI 5.6.5.4 are not met and if the
    structural adequacy remains in doubt, the
    responsible authority shall be permitted to order
    a strength evaluation in accordance with Chapter
    20 for the questionable portion of the structure,
    or take other appropriate action.

64
Some Humble Suggestions
  • Regular refresher courses for field staff at
    least twice a year
  • Code implementation in full letter and spirit
  • Provision for structural design cost in PC-1
  • Field execution check lists to be developed
  • Certification courses for contractors,
    fabricators and masons
  • Dissemination of FPM and the like material
  • Retrofitting of vulnerable structures
  • Strong link with Universities.

65
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