REINFORCED CONCRETE DESIGN IN COMPLIANCE WITH BNBC-2020 (ACI 318M-08) Date: 10 Jan 2022 Session 5: Concrete 1 – Material Aspects Dr. S. K. Ghosh President, S. K. Ghosh Associates LLC S-8A SEng PRP Training Program 1 Standards for Cement, Aggregate, Water, Admixtures, and Reinforcement Standards for Cement, Aggregate, Water, Admixtures, and Reinforcement ACI 318-08 CHAPTER 3 - MATERIALS BNBC-2020 VOLUME 1, Part 5 BUILDING MATERIALS, Chapter 2 BUILDING MATERIALS, Section 2.3 CEMENT AND CONCRETE VOLUME 2, Part 6 STRUCTURAL DESIGN, Chapter 5 CONCRETE MATERIAL 3.2 (5.2.1) Cementitious materials 3.2.1 (5.2.1.1) — Cementitious materials shall conform to the relevant specifications as follows: (a) Composition, Specification and Conformity Criteria for Common Cements (BDS EN 197-1:2003) (b) Portland cement: ASTM C150/C150M; (c) Blended hydraulic cements: ASTM C595/C595M[, excluding Type IS (≥70) which are not intended as principal cementing constituents of structural concrete]; [Expansive hydraulic cement: ASTM C845] 3.2 (5.2.1) Cementitious materials 3.2.1 (5.2.1.1) — Cementitious materials shall conform to the relevant specifications as follows: (d) Hydraulic cement: ASTM C1157; (5.2.4.6) Fly ash and natural pozzolan: ASTM C618; (5.2.4.7) Ground-granulated blast-furnace slag: ASTM C989; (2.3.3) Sulphate resisting Portland cement: BDS 612; (2.3.3) Portland cement: BDS 232; [Silica fume: ASTM C1240] BDS EN 197-1 – Common Cement - CEM I Portland cement - CEM II Portland-composite cement - CEM III Blast furnace cement - CEM IV Pozzolanic cement - CEM V Composite cement ASTM C150 – Portland cement Type I Normal Type IA Normal, air-entrained Type II Moderate sulfate resistant Type IIA Moderate sulfate resistant, air-entraining Type III High early strength Type IIIA High early strength, air-entraining Type IV Low heat of hydration Type V High sulfate resistance ASTM C595 – Blended hydraulic cements Type IS (X) Portland blast-furnace slag cement Type IP (X) Portland-pozzolan cement Type IS (50) contains 50% by mass of slag Type IP: (A) air-entraining (MS) moderate sulfate resistant (HS) high sulfate resistant (LH) low heat of hydration (MH) moderate heat of hydration ASTM C1157 – Hydraulic cements Type GU General use Type HE High early strength Type MS Moderate sulfate resistant Type HS High sulfate resistant Type MH Moderate heat of hydration Type LH Low heat of hydration ASTM C618 – Fly ash for concrete ASTM C618. Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete. ASTM C989 – Ground granulated blast furnace slag ASTM C989. Standard Specification for Slag Cement for Use in Concrete and Mortars. 3.2 (5.2.1) Cementitious materials 3.2.2 (5.2.1.2) — Cementitious materials used in the Work shall correspond to those used as the basis for selecting concrete mixture proportions. See 5.2 (Selection of concrete proportions). 3.3 (5.2.2) Aggregates 3.3.1 (5.2.2.1) — Concrete aggregates shall conform to one of the following specifications: Coarse and Fine Aggregates from Natural Source for Concrete: BDS 243: 1963 (a) Normal-weight: ASTM C33; (b) (2.3.2) Lightweight: ASTM C330. (2.3.2.1) Exception: Aggregates that have been shown by special test or actual service to produce concrete of adequate strength and durability and approved by the building official. 3.3 (5.2.2) Aggregates 3.3.1 (5.2.2.1) — Concrete aggregates shall conform to one of the following specifications: Aggregates for Radiation-Shielding Concrete: ASTM C637; Lightweight Aggregate for Insulating Concrete: ASTM C332; Artificial lightweight aggregates for concrete masonry units: IS 9142. 3.3 (5.2.2) Aggregates 3.3.2 (5.2.2.2) — Nominal maximum size of coarse aggregate shall be not larger than: (a) 1/5 the narrowest dimension between sides of forms, nor (b) 1/3 the depth of slabs, nor (c) 3/4 the minimum clear spacing between individual reinforcing bars or wires, bundles of bars, individual tendons, bundled tendons, or ducts. BDS 243 – Aggregates for Concrete BDS 243:1963. Coarse and Fine Aggregates from Natural Sources for Concrete. ASTM C330 – Lightweight aggregates ASTM C330/330M. Lightweight aggregates. Standard Specification for Lightweight Aggregates for Structural Concrete Mixing water for concrete [Water used in mixing concrete shall conform to ASTM C1602M] 3.4.2 (5.2.3.2) — Mixing water for prestressed concrete or for concrete that will contain aluminum embedments, including that portion of mixing water contributed in the form of free moisture on aggregates, shall not contain deleterious amounts of chloride ion. See 4.3.1 (5.5.3). Mixing water for concrete 5.2.3.3 — Non-potable water shall not be used in concrete except the following conditions: (a) Selection of concrete proportions shall be based on concrete mixes using water from the same source. (b) Non-potable water is permitted only if specified comparative mortar test cubes made with non-potable water produce at least 90 percent of the strength achieved with potable water. Reinforcement for concrete Reinforcement for concrete 3.5.2 (5.3.1.3) — Welding of reinforcing bars shall conform to AWS D1.4. Type and location of welded splices and other required welding of reinforcing bars shall be indicated on the design drawings or in the project specifications. ASTM specifications for bar reinforcement, except for ASTM A706, shall be supplemented to require a report of material properties necessary to conform to the requirements in AWS D1.4. Reinforcement for concrete 3.5.3 (5.3.2) — Deformed reinforcement 3.5.3.1 (5.3.2.1) — Deformed reinforcing bars shall conform to the requirements for deformed bars in one of the following specifications: Plain and ribbed bars: BDS ISO 6935-1, and BDS ISO 6935-2; Carbon steel: ASTM A615M; Rail steel and axle steel ASTM A996M. Bars from railsteel shall be Type R. Reinforcement for concrete 3.5.3 (5.3.2) — Deformed reinforcement Low alloy steel: ASTM A706M; Cold worked steel bars: IS 1786 or BS 4461; Rail-steel bars: ASTM A616M; Axle-steel bars: ASTM A617M; Zinc coated (galvanized) steel bars: ASTM A767M Epoxy-coated reinforcing steel: ASTM A775M [Stainless steel: ASTM A955M] Reinforcement for concrete 3.5.3.2 (5.3.2.2) — . . . for bars with fy exceeding 420 MPa, the yield strength shall be taken as the stress corresponding to a strain of 0.35 percent. 3.5.3.4 (2.3.6a) — Bar mats for concrete reinforcement shall conform to ASTM A184M. Reinforcing bars used in bar mats shall conform to ATM A615M or ASTM A706M. Yield Strength of Reinforcement ACI 318-14 ACI 318-11 Reinforcement for concrete 3.5.3.5 (2.3.6a) — Deformed wire for concrete reinforcement shall conform to ASTM A496M, shall not be smaller than size D4 (nominal diameter: 5.72 mm), and for wire with a specified yield strength, fy exceeding 410 MPa, fy shall be the stress corresponding to a strain of 0.35 percent. Reinforcement for concrete Wire Size - Individual wire size designations are based on the cross-sectional area of a given wire. “M” stands for metric. "W" represents a plain wire and "D" represents a deformed wire. The number following the "W" and the "D" represents the cross-sectional area of the wire in mm2. A D10 wire would indicate a deformed wire having a cross-sectional area of 10 mm2. Reinforcement for concrete 3.5.3.6 (2.3.6b) — Welded plain wire reinforcement shall conform to ASTM A185M, except that for wire with fy exceeding 420 (410) MPa, the yield strength shall be taken as the stress corresponding to a strain of 0.35 percent. Welded intersections shall not be spaced farther apart than 300 mm in direction of calculated stress, except for welded wire reinforcement used as stirrups. Reinforcement for concrete 3.5.3.7 (2.3.6a) — Welded deformed wire reinforcement shall conform to ASTM A497M, except that for wire with fy exceeding 420 (410) MPa, the yield strength shall be taken as the stress corresponding to a strain of 0.35 percent…… Reinforcement for concrete 3.5.3.7 (2.3.6a) (Contd.) — ……Welded intersections shall not be spaced farther apart than 400 mm in direction of calculated stress, except for welded deformed wire reinforcement used as stirrups. [Deformed wire larger than MD200 is permitted when used in welded wire reinforcement conforming to ASTM A497M but shall be treated as plain wire for development and splice design]. Reinforcement for concrete 3.5.3.8 (5.3.2.3) — Galvanized reinforcing bars shall comply with ASTM A767M. Epoxy-coated reinforcing bars shall comply with ASTM A775M or with ASTM A934M. Bars to be galvanized or epoxy-coated shall conform to one of the specifications listed in 3.5.3.1 (5.3.2.1). Reinforcement for concrete 3.5.3.9 — Epoxy-coated wires and welded wire reinforcement shall conform to ASTM A884M. Wires to be epoxy-coated shall conform to 3.5.3.4 and welded wire reinforcement to be epoxy coated shall conform to 3.5.3.5 or 3.5.3.6. Reinforcement for concrete 3.5.3.10 — Deformed stainless steel wire and deformed and plain stainless steel welded wire for concrete reinforcement shall conform to ASTM A1022M, except deformed wire shall not be smaller than size MD25 or larger than size MD200, and the yield strength for wire with fy exceeding 420 MPa shall be taken as the stress corresponding to a strain of 0.35 percent. Deformed wire larger than MD200 is permitted where used in welded wire reinforcement conforming to ASTM A1022M, but shall be treated as plain wire for development and splice design. Reinforcement for concrete 3.5.3.10 (Contd.) — Spacing of welded intersections shall not exceed 300 mm for plain welded wire and 400 mm for deformed welded wire in direction of calculated stress, except for welded wire reinforcement used as stirrups in accordance with 12.13.2. Reinforcement for concrete 3.5.4 (5.3.3) — Plain reinforcement 5.3.3.1 — Plain bars shall conform to one of the specifications listed in Section 5.3.2.1(a) (BDS ISO 6935-1 and BDS ISO 6935-2), (b) (ASTM A615M), (c) or (d) (ASTM A996M) 3.5.4.2 (5.3.3.2) — Plain wire for spiral reinforcement shall conform to ASTM A82M, except that for wire with fy exceeding 420 MPa, the yield strength shall be taken as the stress corresponding to a strain of 0.35 percent. Reinforcement for concrete 3.5.5 – Headed shear stud reinforcement 3.5.5.1 – Headed studs and headed stud assemblies shall conform to ASTM A1044M. Headed shear stud reinforcement Reinforcement for concrete 3.5.6 (2.3.6d) — Prestressing steel 3.5.6.1 (2.3.6d) — Steel for prestressing shall conform to one of the following specifications: (a) Wire: ASTM A421M; (b) Low-relaxation wire: ASTM A421M including Supplement “Low Relaxation Wire”; (c) Strand: ASTM A416M; (d) High-strength bar: ASTM A722M. Reinforcement for concrete 3.5.6.2 (2.3.6d) — Wire, strands, and bars not specifically listed in ASTM A421M, A416M, or A722M are allowed provided they conform to minimum requirements of these specifications and do not have properties that make them less satisfactory than those listed in ASTM A421M, A416M, or A722M. Reinforcement for concrete 3.5.7 (5.3.4) — Structural steel, steel pipe, or tubing 3.5.7.1 (5.3.4.1) — Structural steel used with reinforcing bars in composite compression members meeting requirements of 10.16.7 (6.3.10.8) or 10.16.8 (6.3.10.9) shall conform to one of the following specifications: (a) Carbon steel: ASTM A36M; (b) High-strength low-alloy steel: ASTM A242M; (c) High-strength, low-alloy, Structural Manganese Vanadium Steel: ASTM A572M; Reinforcement for concrete 3.5.7 (5.3.4) — Structural steel, steel pipe, or tubing 3.5.7.1 (5.3.4.1) (Contd.) — (d) High-strength, low-alloy, Columbium - Vanadium steel: ASTM A572M; (e) High-strength, low-alloy, 345 MPa steel: ASTM A588M; [Structural shapes: ASTM A992M] Reinforcement for concrete 3.5.7.2 (5.3.4.2) — Steel pipe or tubing for composite compression members composed of a steel encased concrete core meeting requirements of 10.16.6 (6.3.10.7) shall conform to one of the following specifications: (a) Black steel, hot-dipped, zinc-coated: Grade B of ASTM A53M; (b) Cold-formed, welded, seamless: ASTM A500M; (c) Hot-formed, welded, seamless: ASTM A501M. Reinforcement for concrete 3.5.8 — Steel discontinuous fiber reinforcement for concrete shall be deformed and conform to ASTM A820M. Steel fibers shall have a length-to-diameter ratio not smaller than 50 and not greater than 100. 3.5.9 — Headed deformed bars shall conform to ASTM A970M, and obstructions or interruptions of the bar deformations, if any, shall not extend more than 2db from the bearing face of the head. Admixtures for concrete 3.6.1 (5.2.4.5) — Admixtures for water reduction and setting time modification shall conform to ASTM C494M. Admixtures for use in producing flowing concrete shall conform to ASTM C1017M. 3.6.2 (5.2.4.4) — Air-entraining admixtures shall conform to ASTM C260M. 3.6.3 — Admixtures to be used in concrete that do not conform to 3.6.1 and 3.6.2 shall be subject to prior approval by the licensed design professional. Admixtures for concrete 2.3.5 — Admixtures to be used in concrete shall be subject to prior approval by the Building Official and conform to following standards: Admixtures for concrete, mortar and grout – Parts 1 and 2: BDS EN 934-1 and BDS EN 934-2 Fly ash or other Pozzolanas: ASTM C618 Ground granulated blast-furnace slag: ASTM C989 Pigment for colored concrete: ASTM C979 Admixtures for concrete 3.6.4 (5.2.4.3) — Calcium chloride or admixtures containing chloride from sources other than impurities in admixture ingredients shall not be used in prestressed concrete, in concrete containing embedded aluminum, or in concrete cast against stayin-place galvanized steel forms. See 4.3.1 (5.5.1.2) and 6.3.2 (5.5.2.1). 3.6.5 — Admixtures used in concrete containing expansive cements conforming to ASTM C845 shall be compatible with the cement and produce no deleterious effects. ASTM C494M Type A Water-reducing admixtures Type B Retarding admixtures Type C Accelerating admixtures Type D Water-reducing and retarding admixtures Type E Water-reducing and accelerating admixtures Type F Water-reducing, high-range admixtures Type G Water-reducing, high-range, retarding admixtures Type S Specific performance admixtures EN 934 - Admixtures for concrete, mortar and grout BDS EN 934-1. Admixtures for Concrete, Mortar and Grout-Part 1: Common Requirements. BS EN 934-2. Admixtures for Concrete, Mortar and Grout-Part 2: Concrete Admixtures Definitions, Requirements, Conformity, Marking and Labelling. Storage of materials 3.7.1 (2.1.3) — Cementitious materials and aggregates shall be stored in such manner as to prevent deterioration or intrusion of foreign matter. 3.7.2 — Any material that has deteriorated or has been contaminated shall not be used for concrete. Durability Requirements for Concrete Special exposures 5.5.1.1 — For concrete intended to have low permeability when exposed to water, the water cement ratio shall not exceed 0.50. 5.5.1.2 — For corrosion protection of reinforced concrete exposed to brackish water, sea water or spray from these sources, the water cement ratio shall not exceed 0.4. If minimum concrete cover required by Sec 8.1.8 Chapter 8 is increased by 12 mm, water cement ratio may be increased to 0.45. Suggested workability Table 6.5.1: Suggested Workability of Concrete for Various Placing Conditions Placing Conditions Degree of Workability Values of Workability Concreting of thin sections with vibration Very low 20-10 seconds Vee-Bee time, or 0.75-0.80 compacting factor Concreting of lightly reinforced sections with vibration Low 10-5 seconds Vee-Bee time, or 0.80-0.85 compacting factor Medium 5-2 seconds Vee-Bee time, or 0.85- 0.92 compacting factor, or 25-75 mm slump for 20 mm aggregate* High Above 0.92 compacting factor, or 75-125 mm slump for 20 mm aggregate* Concreting of lightly reinforced sections without vibration or heavily reinforced section with vibration Concreting of heavily reinforced sections without vibration * Slump test shall be performed as per ASTM C143. For smaller aggregates, the values will be lower. Durability requirements Water Soluble Sulfate Table 6.5.2: Requirements for normal weight aggregate concrete exposed to sulphate containing solutions Sulphate Exposure Water Soluble Sulphate (SO4) in Soil, percent by Weight Sulphate (SO4) in Water, (ppm) Cement Type1 Maximum Water Cement Ratio, by Weight Negligible 0.00-0.10 0-150 - - Moderate2 0.10-0.20 150-1500 Other than CEM I and B type 0.50 Severe 0.20-2.00 1500-10,000 Other than CEM I and B type 0.45 Very Severe Over 2.00 Over 10,000 Other than CEM I and B type 0.45 Notes: Pozzolan that has been determined by test or service record to improve sulphate resistance when used in concrete containing Type V cement. 1 For types of cement see BDS EN 197:2003 or ASTM C150 and C595 2 Sea water Durability requirements Corrosion of Reinforcement Corrosion of reinforcement 5.5.3.1 — For corrosion protection, maximum watersoluble chloride ion concentrations in hardened concrete at ages from 28 to 42 days contributed from the ingredients including water, aggregates, cementitious materials, and admixtures, shall not exceed the limits of Table 6.5.3. When testing is performed to determine water soluble chloride ion content, test procedure shall conform to AASHTO T260, "Methods of Sampling and Testing for Total Chloride Ion in Concrete and Concrete Raw Materials". Corrosion of reinforcement 5.5.3.2 — When reinforced concrete will be exposed to brackish water, sea water, or spray from these sources, requirements of Sections 5.5.1.1 and 5.5.1.2 for water cement ratio, or concrete strength and minimum cover requirements of Sec 8.1.8 Chapter 8 of this Part shall be satisfied. Table 6.5.3: Maximum chloride-ion content for corrosion protection Type of Member Maximum Water-Soluble Chloride Ion (Cl) in Concrete, Percent by Weight of Cement Prestressed concrete 0.06 Reinforced concrete exposed to chloride in service 0.15 Reinforced concrete that will be dry or protected from moisture in service 1.00 Other reinforced concrete construction 0.30 Minimum concrete strength 5.5.4 — Minimum concrete strength for structural use of reinforced concrete shall be 20 N/mm2. However, for buildings up to 4 story, the minimum concrete strength may be relaxed to 17 N/mm2. Corrosive environments 8.1.7.8 — If a thickness of cover for corrosive environment or other severe exposure conditions greater than the concrete covers specified in Sections 8.1.7.1 to 8.1.7.6 is required, such greater thicknesses shall be specified. For corrosion protection, a specified concrete cover for reinforcement not less than 50 mm for walls and slabs and not less than 65 mm for other members may be used. For precast concrete members a specified concrete cover not less than 40 mm for walls and slabs and not less than 50 mm for other members may be used. Corrosive environments 8.1.7.8 (contd.) — Minimum compressive strength of concrete for the corrosive environment or other severe exposure conditions shall be 25 MPa with minimum cement of 400 kg per cubic meter. Coarse aggregate shall be 20 mm down well-graded stone chips and fine aggregate shall be coarse sand of minimum FM 2.20. For any non-structural member like drop wall, railing, fins etc., 12 mm down well graded stone chips may be used as coarse aggregate. Corrosive environments 8.1.7.8 (contd.) — Use of brick chips (khoa) as coarse aggregate is strictly prohibited for the corrosive environment or other severe exposure conditions. Water cement ratio shall be between 0.4-0.45. Potable water shall be used for all concreting. Concrete Quality, Mixing, and Placing Concrete quality, mixing, and placing 5.1.1 (5.12.1.1) — Concrete shall be proportioned to provide an average compressive strength, fcr' , as prescribed in 5.3.2 (5.6.2.2) and shall satisfy the durability criteria of Chapter 4 (Section 5.5). Concrete shall be produced to minimize the frequency of strength tests below fc' , as prescribed in 5.6.3.3 (5.12.3.3). (5.5.4) For concrete designed and constructed in accordance with the code, fc' shall not be less than 17 MPa. Concrete quality, mixing, and placing 5.1.2 (5.12.1.2) — Requirements for fc' shall be based on tests of cylinders made and tested as prescribed in 5.6.3 (5.12.3). 5.1.3 (5.12.1.3) — Unless otherwise specified, fc' shall be based on 28-day tests. If other than 28 days, test age for fc' shall be as indicated in design drawings or specifications. Concrete quality, mixing, and placing 5.1.4 — Where design criteria in 8.6.1, 12.2.4(d), and 22.5.6.1 provide for use of a splitting tensile strength value of concrete, laboratory tests shall be made in accordance with ASTM C330 to establish a value of fct' corresponding to fc' . 5.1.5 (5.12.1.4) — Splitting tensile strength tests shall not be used as a basis for field acceptance of concrete. 5.1.6 — Steel fiber-reinforced concrete shall conform to ASTM C1116. The minimum fc′ for steel fiber reinforced concrete shall conform to 5.1.1. Concrete quality, mixing, and placing 5.2.1 (5.6.1.1) — Proportions of materials for concrete shall be established to: (a) 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 bleeding; (b) Meet requirements for applicable exposure categories of Chapter 4 (Section 5.5); (c) Conform to strength test requirements of 5.6 (5.12). Concrete quality, mixing, and placing 5.2.2 (5.6.1.2) — Where different materials are to be used for different portions of proposed Work, each combination shall be evaluated. 5.2.3 (5.6.1.3) — Concrete proportions shall be established in accordance with 5.3 (5.6.2) (Proportioning on the basis of field experience or trial mixtures, or both) or, alternatively, 5.4 (5.6.3) (Proportioning without field experience or trial mixtures) and shall meet applicable requirements of Chapter 4 (Section 5.5). Proportioning on the basis of field experience or trial mixtures, or both 5.3 (5.6.2) — Proportioning Concrete Mix on the Basis of Field Expérience and/or Trial Mixtures 5.4 (5.6.3) — Proportioning by Water Cement Ratio 5.5 (5.6.4) — Average Strength Reduction Evaluation and acceptance of concrete 5.6.1 (5.12.1) — Concrete shall be tested in accordance with the requirements of 5.6.2 (5.12.2) through 5.6.5 (5.12.5). … . 5.6.2 (5.12.2) — Frequency of Testing 5.6.3 (5.12.3) — Laboratory Cured Specimens 5.6.4 (5.12.4) — Field Cured Specimens 5.6.5 (5.12.5) — Investigation of Low Strength Test Results Steel fiber-reinforced concrete 5.6.6.1 — Acceptance of steel fiber-reinforced concrete used in beams in accordance with 11.4.6.1(f) shall be determined by testing in accordance with ASTM C1609. In addition, strength testing shall be in accordance with 5.6.1 Steel fiber-reinforced concrete 5.6.6.2 — Steel fiber-reinforced concrete shall be considered acceptable for shear resistance if conditions (a), (b), and (c) are satisfied: a) The weight of deformed steel fibers per cubic meter of concrete is . . .. b) The residual strength obtained from flexural testing . . . at a midspan deflection of . . .. c) The residual strength obtained from flexural testing . . . at a midspan deflection of . . .. Construction aspects 5.7 (5.7) — Preparation of equipment and place of deposit 5.8 (5.8) — Mixing 5.9 (5.9) — Conveying 5.10 (5.10) — Depositing 5.11 (5.11) — Curing 5.11.3 (5.11.3) — Accelerated curing 5.12 — Cold weather requirements 5.13 — Hot weather requirements Formwork, Embedments, and Construction Joints Formwork, embedments, and construction joints 6.1 (5.16.1) — Design of formwork 6.2 (5.16.2) — Removal of forms, shores, and reshoring 6.3 (5.16.3) — Embedments in concrete 6.4 (5.16.4) — Construction joints REINFORCED CONCRETE DESIGN IN COMPLIANCE WITH BNBC-2020 (ACI 318M-08) Date: 10 Jan 2022 Session 5: Concrete 1 – Strength Design Method Dr. S. K. Ghosh President, S. K. Ghosh Associates LLC S-8A SEng PRP Training Program Allowable Stress Design and Strength Design Two design philosophies have long been prevalent: • Allowable Stress Design • Strength Design Allowable Stress Design and Strength Design • Allowable stress design (ASD) was the principal method used from the early 1900s until the early 1960s. • ACI 318-56 was the first standard to officially recognize and permit the use of strength design (SD). • Starting from the 1963 edition of ACI 318, there was a rapid transition to strength design, largely because of its rational approach and provision of better safety and reliability at the strength limit state. Allowable Stress Design and Strength Design • ACI 318-63 treated both ASD and SD on an equal basis, but large portions of ASD (e.g., bond, shear and diagonal tension, combined axial load and bending moment, etc.) were modified to reflect behavior at nominal strength. • ACI 318-77 - ASD moved to Appendix B • ACI 318-02 – ASD deleted from the standard Strength Design Philosophy Strength Design: Basics ACI 318M-08 Section 9.1.1 (6.1.3.3) The strength design method requires the design strength of a member at a section to be equal to or greater than the required strength at that section, calculated from the code-specified load combinations as are stipulated in Chapter 2, Loads. Required Strength (U) ≤ Design Strength Strength Design: Required Strength Required Strength = Combination of Service Load Effects × Respective Load Factors Service Load = The load specified by the building code, which a structure is expected to support under normal usage Service Load Effects = Internal forces (axial force, shear force, bending moment, etc.) generated in a member by the code-specified service loads, and determined using structural analysis Load Factor = Factors to account for probable variations in service load (≥1.0) Strength Design: Required Strength Strength-Level Load = The maximum load that any structure is designed to support Load factors scale the effects of the service loads on a structural member up to the effects of strength-level loads. Strength Design: Design Strength Design Strength = φ × Nominal Strength φ = Strength reduction factor (≤1.0) Nominal Strength = Strength of a member or a crosssection calculated using the assumptions and strength design provisions of ACI 318M (BNBC 2020). Strength Design: Design Strength Design Strength = φ × Nominal Strength Both nominal strengths and φ-factors for reinforced concrete members are to be determined in accordance with ACI 318M-08 (BNBC 2020) Strength Design: Notation Required Strength: Pu = Factored axial force at section Vu = Factored shear force at section Mu = Factored bending moment at section Tu = Factored torsional moment at section Strength Design: Notation Nominal Strength: Pn = Nominal axial strength at section Vn = Nominal shear strength at section Mn = Nominal flexural strength at section Tn = Nominal torsional moment strength at section Strength Design: Notation Design Strength: φPn = Design axial strength at section φVn = Design shear strength at section φMn = Design flexural strength at section φTn = Design torsional moment strength at section Strength Design: Load Factors Account for: 1. Probable variation in specified service loads because of Variation in member sizes Variation in material densities Structural and nonstructural alterations Degree of accuracy in load determination Strength Design: Load Factors Account for: 2. Uncertainties in calculation of load effects on a member because of Assumptions of stiffness, span length, etc. Other assumptions and simplifications in the structural model used for analysis Strength Design: Strength Reduction Factors Account for: 1. Probable variation in material strength because of Quality control Effect of testing speed Difference between in-situ strength and specimen strength Stresses due to creep and shrinkage Strength Design: Strength Reduction Factors Account for: 2. Probable variation in member sizes because of Tolerances in formwork Rolling and fabrication tolerances allowed for reinforcing bars Tolerances in reinforcement placement Strength Design: Strength Reduction Factors Account for: 3. Various design assumptions and simplifications that cause the design behavior of structural members to deviate from their actual behavior. 4. Consequences of failure. Strength Design: Strength Reduction Factors Unified Design Provisions: A modified version of the strength design method first introduced in ACI 318-95 Consistent design provisions for reinforced and prestressed concrete beams and columns Strength Design: Strength Reduction Factors Unified Design Provisions: Strength Reduction Factor, φ, Under Flexure and/or Axial Load Depends on Net Tensile Strain, εt Strength Design: Strength Reduction Factors Unified Design Provisions: Net tensile strain — The tensile strain at nominal strength exclusive of strains due to effective prestress, creep, shrinkage, and temperature effects. Strength Design: Strength Reduction Factors Unified Design Provisions: Extreme tension steel — The reinforcement (prestressed or nonprestressed) that is the farthest from the extreme compression fiber. Strength Design: Strength Reduction Factors Unified Design Provisions: εt = Net tensile strain dt = Depth to extreme tension reinforcement Strength Design: Strength Reduction Factors Unified Design Provisions: Excessive Cracking Excessive Deflection Net Tensile Strain Is a Measure of Both Strength Design: Strength Reduction Factors Unified Design Provisions: Compression-controlled strain limit — The net tensile strain at balanced strain conditions. Strength Design: Strength Reduction Factors Unified Design Provisions: Compression-controlled section: Sections are compression-controlled if the net tensile strain, εt, is equal to or less than the compression-controlled strain limit. For Grade 420 reinforcement, and for all prestressed reinforcement, it shall be permitted to set the compression-controlled strain limit equal to 0.002. (ACI 318M-08 Section 10.3.3 (6.3.3.3)) Strength Design: Strength Reduction Factors Unified Design Provisions: Tension-controlled section: Sections are tensioncontrolled if the net tensile strain, εt, is equal to or greater than 0.005. (ACI 318M-08 Section 10.3.4 (6.3.3.4)) Strength Design: Strength Reduction Factors Unified Design Provisions: Transition sections: Sections with εt between the compression-controlled strain limit and 0.005 constitute a transition region between compressioncontrolled and tension-controlled sections. (ACI 318M-08 Section 10.3.4 (6.3.3.4)) Strength Design: Strength Reduction Factors Unified Design Provisions εt ≤ 0.002: compressioncontrolled εt ≥ 0.005: tensioncontrolled 0.002 < εt < 0.005: transition region ACI 318M-08 Fig. R10.3.3 (6.6.3) Strength Design: Strength Reduction Factors 9.3.2 (6.2.3.2) — Strength reduction factor shall be as given in 9.3.2.1 through 9.3.2.7 (Sections 6.2.3.2.1 to 6.2.3.2.6): 9.3.2.1 (6.2.3.2.1) — Tension-controlled sections as defined in 10.3.4 (6.3.3.4)………………...0.90 9.3.2.2 (6.2.3.2.2) — Compression-controlled sections, as defined in 10.3.3 (6.3.3.3): (a) Members with spiral reinforcement conforming to 10.9.3 (6.3.9.3)………………. 0.75 (b) Other reinforced members… 0.65 Strength Design: Strength Reduction Factors 9.3.2.2 (6.2.3.2.2) (contd.) — For sections in which the net tensile strain, εt, is between the limits for compression-controlled and tension-controlled sections, φ shall be permitted to be linearly increased from that for compression-controlled sections to 0.90 as εt increases from the compression-controlled strain limit to 0.005 (Also see Figure 6.6.1). While interpolating, it shall be permitted to round φ to second digit after decimal. Strength Design: Strength Reduction Factors ACI 318M-08 Fig. R9.3.2 (6.6.1) Strength Design: Strength Reduction Factors (6.2.3.2.3) — It shall be permitted for compressioncontrolled sections, as defined in Sec 6.3.3.3, the following optional, more conservative alternative values of strength reduction factor φ, where less controlled construction environment justifies such selection according to engineering judgment of the designer: For members with spiral reinforcement conforming to Sec 6.3.9.3………………………… 0.70 Other reinforced members………… 0.60 Strength Design: Strength Reduction Factors Compression-controlled sections (e.g. columns) are assigned lower φ-factors than tension-controlled sections (e.g. beams) because: compression-controlled sections generally have less ductility compression-controlled sections are more sensitive to variation in concrete strength Generally, the consequences of a column failure are more severe than those of a beam failure Strength Design: Strength Reduction Factors Compression-controlled sections in members with spiral reinforcement are assigned a higher φ-factor than those in members with tie reinforcement because: Members with spiral reinforcement have greater toughness and ductility, as demonstrated by performance in the Pentagon (U.S. DoD) building after it was hit by one of the 9/11 planes Strength Design: Strength Reduction Factors 9.3.2.3 (6.2.3.2.4) — Shear and torsion……… 0.75 9.3.2.4 (6.2.3.2.5) — Bearing on concrete (except for post-tensioned anchorage zones and strut-and-tie models)………………………………………….. 0.65 9.3.2.5 (9.9.2d) — Post-tensioned anchorage zones ………………………………………………….... 0.85 9.3.2.6 (6.2.3.2.6) — Strut-and-tie models (Appendix A(I)), and struts, ties, nodal zones, and bearing areas in such models…………………………………. 0.75 Strength Design: Strength Reduction Factors 9.3.2.7 (9.9.2f) — Flexural sections in pretensioned members where strand embedment is less than the development length as provided in 12.9.1.1: (a) From the end of the member to the end of the transfer length……………………… 0.75 (b) From the end of the transfer length to the end of the development length shall be permitted to be linearly increased from……… 0.75 to 0.9 Where bonding of a strand does not extend to the end of the member, strand embedment shall be assumed to begin at the end of the debonded length. Strength Design: Strength Reduction Factors ACI 318M-08 Section 9.3.2.7 Strength Design: Safety Margin Safety Margin is provided by: 1. Reduction in member strength by φ-factor 2. Increase in the imposed load by load factor In essence, Load Factor * Safety Margin = Strength Reduction Factor ( φ ) *Direct calculation like this is not always possible Strength Design: Safety Margin Design of a beam for flexural moment from dead load Design Eq.: Mu ≤ φMn From 2009 IBC Eq. 16-1 (2.7.3.1 Eq. 1): Mu = 1.4 × Bending moment from code specified dead load φ = 0.9 for beam flexure (ACI 318M-08 Section 9.3.2 (6.2.3.2)) Safety Margin = 1.4/0.9 = 1.56 Strength Design: Safety Margin Load factors and strength reduction factors are provided after considering: Probability of actual imposed loads exceeding and member strengths falling short of the respective design values, AND Severity of failure if the above does happen Strength Design: Safety Margin Various items are considered: The type of failure, warning of failure, and existence of alternative load path(s) Importance of structural element within the structure Potential loss of life Costs to society in lost time, lost revenue, or indirect loss of life or property due to failure Cost of replacing the structure Strength Design: Safety Margin MacGregor, J.G., “Safety and Limit States Design for Reinforced Concrete,” Canadian Journal of Civil Engineering, Vol. 3, No. 4, December 1976. “The ACI…design requirements…are based on an underlying assumption that if the probability of understrength members is roughly 1 in 100 and the probability of overload is roughly 1 in 1000, the probability of overload on an understrength structure is 1 in 100,000. Load factors were derived to achieve this probability of overload. … Strength Design: Safety Margin …… Based on values of concrete and steel strength corresponding to a probability of understrength of 1 in 100, the strengths of a number of typical sections were computed. The ratio of the strength based on these values to the strength based on nominal strengths of a number of typical sections were arbitrarily adjusted to allow for the consequences of failure and the mode of failure of a particular type of member, and for a number of other sources of variation in strength.” Strength Design: Load Combinations A structural member is required to be designed for a number of combinations of the effects of various factored loads Strength design load combinations are provided in 2009 IBC Section 1605.2 (BNBC 2020 Section 2.7.3) Codes and standards 2009 IBC 102.4 Referenced codes and standards. The codes and standards referenced in this code shall be considered part of the requirements of this code to the prescribed extent of each such reference. Where differences occur between provisions of this code and referenced codes and standards, the provisions of this code shall apply. 1605.2.1 (2.7.3.1) Strength Design or LRFD Load Combinations (2009 IBC) 1. 1.4(D + F) 2. 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or R) 3. 1.2D +1.6(Lr or R) + (L or 0.8W) 4. 1.2D + 1.6W + L + 0.5(Lr or R) 5. 1.2D + 1.0E + 1.0L 6. 0.9D + 1.6W + 1.6H 7. 0.9D + 1.0E + 1.6H 2.7.3 Combinations of Load Effects for Strength Design Method Exceptions: 1. The load factor on live load L in combinations (3), (4), and (5) is permitted to be reduced to 0.5 for all occupancies in which minimum specified uniformly distributed live load is less than or equal to 5.0 kN/m2, with the exception of garages or areas occupied as places of public assembly (corresponding to f1 = 0.5 in 2009 IBC load combinations) 2.7.3 Combinations of Load Effects for Strength Design Method Exceptions: 2. The load factor on H shall be set equal to zero in combinations (6) and (7) if the structural action due to H counteracts that due to W or E. Where lateral earth pressure provides resistance to structural actions from other forces, it shall not be included in H but shall be included in the design resistance. 2.7.3 Combinations of Load Effects for Strength Design Method Exceptions: 3. For structures designed in accordance with the provisions of Chapter 6, Part 6 of this Code (reinforced concrete structures), where wind load W has not been reduced by a directionality factor, it shall be permitted to use 1.3W in place of 1.6W in (4) and (6) above. 1605.2.1 (2.1.4) Strength Design or LRFD Load Combinations (2009 IBC) D = Dead load effect F = Fluid load effect T = Effect of self-straining forces (creep, shrinkage, etc.) L = Floor live load effect (including live load reduction) H = Effect of lateral earth pressure, ground water pressure, etc. Lr = Roof live load effect (including live load reduction) R = Rain load effect W = Wind load effect E = Earthquake load effect 1605.2.1 Strength Design or LRFD Load Combinations (2009 IBC) Earthquake loads computed in accordance with the 2009 IBC are already at strength-level. This is why the load factor on the earthquake load effect, E, is 1.0. 2.7.3 Combinations of Load Effects for Strength Design Method When a structure is located in a flood zone or in tidal surge zone, the following load combinations shall be considered: 1. In Coastal Zones vulnerable to tidal surges, 1.6W shall be replaced by 1.6W+2.0Fa in combinations (4) and (6). 2. In Non-coastal Zones, 1.6W shall be replaced by 0.8W+1.0Fa in combinations (4) and (6). Fa = Effect of flood load 1605.2 Strength Design or LRFD Load Combinations (2009 IBC) For many members, the only loads considered are dead, live, wind and earthquake, and F, H, R, S, T load effects are not required to be included. Consequently, the load combinations can be simplified. 1605.2.1 (2.7.3.1) Strength Design or LRFD Load Combinations (2009 IBC) 1.4D 1.2D + 1.6L + 0.5Lr 1.2D + 1.6Lr + f1L 1.2D +1.6Lr + 0.8W 1.2D + 1.6W + f1L + 0.5Lr 1.2D + 1.0E + f1L 0.9D + 1.6W 0.9D + 1.0E f1 = 0.5 except in special circumstances 1605.2.1 (2.7.3.1) Strength Design or LRFD Load Combinations (2009 IBC) In addition to the possible variability of the specified loads, values of the load factors are also assigned based on the probability of simultaneous occurrence of different load types. For example: the load factor on dead load (D) alone (=1.4) is higher than the load factor on dead load (=1.2) when other loads such as live load (L), wind load (W) or earthquake load (E) are also present 1605.2.1 (2.7.3.1) Strength Design or LRFD Load Combinations (2009 IBC) Load factor values can also be based on other considerations, such as directionality effect for wind load (W) ASCE 7-05 6.5 (2.4.3) Analytical Procedure Directionality factor Kd introduced in 1998 • Added to equation for velocity pressure: • qz = 0.00256 Kz Kzt Kd V2 I (Eq. 6.2.17) • Separate out effect of wind load factor • Requires adjustment to wind load factor ( 1.3 → 1.6 ) (2.7.3 Exception 3) • Table 6-4 (6.2.12) Reason: Explicitly identify directionality effect in future editions. ASCE 7-05 2.3.2 (2.7.3.1) Strength Design Load Combinations Wind load factor: Old (1995): LF = 1.3 → included directionality effect 0.85 (directionality) x 1.53 (LF w/o directionality) = 1.3 Now: Identify directionality effect explicitly in Kd. Round load factor from 1.53 to 1.6. Seismic Strength Design Load Combinations (2009 IBC, ASCE 7-05, BNBC 2020) 1.2D + 1.0E + 1.0L (Load Combination 5) 0.9D + 1.0E (Load Combination 7) (2.5.13.4) E = Em = Emh + Ev (in Load Combination 5) E = Em = Emh - Ev (in Load Combination 7) Seismic Strength Design Load Combinations (BNBC 2020) Em = total seismic load effect including overstrength factor Emh = effect of horizontal seismic forces as defined in Sections 2.5.7 or 2.5.9 including structural overstrength Ev = effect of vertical seismic forces as defined in Section 2.5.13.2 The horizontal seismic load effect with overstrength factor, Emh , shall be determined in accordance with the following equation: Emh = Ω0Eh Vertical Earthquake Loading 2.5.13.2 The maximum vertical ground acceleration shall be taken as 50 percent of the expected horizontal peak ground acceleration (PGA): Ev = 0.50(ah)D (Eq. 6.2.56) ah = expected horizontal peak ground acceleration (in g) for design = (2/3)ZS D = effect of dead load, S = site dependent soil factor (see Table 6.2.16). Seismic Strength Design Load Combinations (2009 IBC, ASCE 7-05) Effect of Vertical Earthquake Ground Motion (2009 IBC, ASCE 7-05) Gravity and Earthquake Effects Additive U = 1.2D + 1.0E + 0.5L = 1.2D + (ρQE + 0.2SDSD) + 0.5L = (1.2 + 0.2SDS)D + ρQE + 0.5L Effect of Vertical Earthquake Ground Motion (2009 IBC, ASCE 7-05) Gravity and Earthquake Effects Counteractive U = 0.9D - 1.0E = 0.9D - (ρQE + 0.2SDSD) = (0.9 - 0.2SDS)D - ρQE ASCE 7-05 12.4.3.2 Load Combinations with Overstrength Factor Basic Combinations for Strength Design with Overstrength Factor (1.2 + 0.2SDS)D + Ω0QE + 0.5L (0.9 − 0.2SDS)D + Ω0QE Unified Design Provisions Unified Design Provisions Unified Design Provisions were first introduced in ACI 318-95 Appendix B Moved to the main body of ACI 318 in the 2002 edition Unified Design Provisions Unified Design Provisions do not alter the nominal strength calculations The provisions unify the determination of appropriate strength reduction factors for prestressed and reinforced concrete beams and columns with rectangular or nonrectangular sections, and with one or multiple layers of steel The provisions unify the treatment of moment redistribution in continuous beams of reinforced and prestressed concrete. Why Unified Provisions Needed? 1999 CODE Redistribution of Negative Moments in Continuous Members R/C P/S 8.4 (6.1.6) 18.10.4 (9.10.4) Limits for Reinforcement of Flexural 10.3.3 Members (6.3.3.3) 18.8.1 (9.9.1) 8.4 – Redistribution of Negative Moments In Continuous Nonprestressed Flexural Members (’99) 18.10.4 – Redistribution of Negative Moments in Continuous Prestressed Flexural Members (’99) 20 1 – d ωp – ω – ω′ dp 0.36β1 percent 10.3.3 – Reinforcement Limit (’99) If φ Pn < Smaller of • 0.10 f’c Ag , or • φ Pb Then 18.8.1 – Limits For Reinforcement Of Flexural Members (’99) The following ratios used to compute Mn must not to exceed 0.36β1 • ωp • [ωp + (d/dp)(ω - ω’)] • [ωpw + (d/dp)(ωw - ωw’)] 18.8.1 – Limits For Reinforcement Of Flexural Members (’99) ω is ρfy /fc′ ρ = As/bd ω′ is ρ′fy /fc′ ρ' = A's/bd ωp is ρpfps /fc′ ρp = Aps /bdp ωw is ρwfy /fc′ ρw = As/bwd ωw′ is ρw′fy /fc′ ρw' = A's/bwd ωpw is ρpwfps /fc′ ρpw = Aps /bwdp fps = stress in prestressing steel at nominal flexural strength Why Unified Provisions Needed? R/C P/S 18.0 – Notation (’99) Aps = area of prestressed reinforcement in tension zone dp = distance from extreme compression fiber to centroid of prestressed reinforcement Mn = Apsfps(dp-a/2) Aps and dp Not Compatible Unified Design Provisions Background Article Robert F. Mast, “Unified Design Provision for Reinforced and Prestressed Concrete Flexural and Compression Members,” ACI Structural Journal, March-April 1992 Unified Provisions Applicability Flexural & compression members R/C, P/S, and combinations Steel at various depths Sections of any shape Composite sections Tension-controlled columns 8.4 (6.1.5) – Redistribution of Moments in Continuous Flexural Members (’08) Applies equally to both nonprestressed & prestressed sections Based on net tensile strain, εt Percent redistribution ∆M = 1000εt Minimum εt = 0.0075 Maximum redistribution = 20% 8.4 (6.1.5) – Redistribution of Moments in Continuous Flexural Members (’08) 8.4 (6.1.5) – Redistribution of Moments in Continuous Flexural Members (’08) 8.4 (6.1.5) – Redistribution of Moments in Continuous Flexural Members (’08) Moment redistribution may not be used for slab systems designed by the Direct Design Method (see 13.6.1.7 (6.5.6.1.7)). R8.4 – Moment Redistribution R/C Net Tensile Strain, εt R8.4 – Moment Redistribution P/S Net Tensile Strain, εt 10.3.5 (6.3.3.5) – Maximum Reinforcement for Nonprestressed Flexural Members Flexural members or members with Pu < 0.10fc' Ag Net tensile strain ≥ 0.004 Replaces ρmax ≤ 0.75ρb requirement of ’99 Code New limit slightly more restrictive For rectangular sections, new limit corresponds to 0.714ρb for Grade 420 MPa steel 18.8.1 (9.9.1) – Reinforcement Limits Limits are based on net tensile strain, εt R18.8.1 (9.9.1) – Reinforcement Limits For prestressed concrete sections, net tensile strain limits for tension-controlled sections may also be stated in terms of ωp as defined in the 1999 and earlier editions of the code The net tensile strain limit of 0.005 corresponds to ωp = 0.32β1 for prestressed rectangular sections Questions? Thank you
