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Structure Codes and the Design Basis of RC Structures

Structure Codes and the Design Basis of RC Structures. By: Prof Dr. Qaisar Ali Civil Engineering Department UET Peshawar drqaisarali.com drqaisarali@nwfpuet.edu.pk. Topics Addressed. Building Codes and the ACI Code Objectives of Design Design Process

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Structure Codes and the Design Basis of RC Structures

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  1. Structure Codes and the Design Basis of RC Structures By: Prof Dr. Qaisar Ali Civil Engineering Department UET Peshawar drqaisarali.com drqaisarali@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 client’s 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 client’s needs and priorities. • Developmentof 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 (α Ms ) • f Vn ≥ Vu (α Vs ) • f Pn ≥ Pu (α Ps ) • f Tn ≥ Tu (α Ts ) • f = strength reduction factor • α = 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. 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. Design Procedures Specified in the ACI Code

  27. 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 Design Loads for Buildings and Other Structures

  28. 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 Design Loads for Buildings and Other Structures

  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 70°F. 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

  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.

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