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

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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.

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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).

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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),

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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.

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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.

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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.

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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.

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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.

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Building Codes and the ACI Code

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

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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.

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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.

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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.

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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.

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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.

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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.

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The Design Process

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

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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

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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

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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)

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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.

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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.

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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.

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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

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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

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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

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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.

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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.

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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.

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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)

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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.

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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

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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
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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.

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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.

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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.

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Deformed Bar Reinforcement

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

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Deformed Bar Reinforcement

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

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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.

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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.

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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.

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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.

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