Introduction to Structural Concrete Design PDF

Introduction to Structural Concrete Design CEPRCD30 (Principles of Reinforced Concrete) Jerome Z. Tadiosa, CE, MSc Assistant Professor 2, Civil Engineering College of Engineering National University – Manila Intended Learning Outcomes Describe the s...

Introduction to Structural Concrete Design CEPRCD30 (Principles of Reinforced Concrete) Jerome Z. Tadiosa, CE, MSc Assistant Professor 2, Civil Engineering College of Engineering National University – Manila Intended Learning Outcomes Describe the structural design process and considerations. Explain the description, development, and classification of structural concrete. Enumerate and describe the design philosophies in structural concrete design, and the relevant codes and standards used. Enumerate and describe the materials used in structural concrete construction. Reading Guide Read the following book chapters and other references for better understanding of lecture: Chapters 1-3, Wight (2016) Chapter 1, McCormac & Brown (2016) Sections 1.1-1.2, Chapter 1, Salmon et.al. (2009) Lecture Outline 1. Introduction to structural design process 2. Introduction to structural concrete design 3. Materials in structural concrete design Introduction to Structural Design Structural Design “May be defined as a mixture of art and science, combining the experienced engineer’s intuitive feeling for the behavior of structures, with a sound knowledge of basic engineering principles, to produce a safe, economical structure that will serve its intended purpose.” (Salmon et.al., 2009) A properly-designed structure should satisfy four criteria: Appropriateness: functionality and aesthetics Economy: optimal benefit-cost ratio, preferably minimum cost Structural adequacy: strength and serviceability requirements Maintainability: minimum maintenance cost and time General Design Process Major phases of general design process include the following: Definition of client’s needs and priorities: function, aesthetic preferences, budget Development of project concept: schematics, preliminary framework, materials Design of individual systems: structural analysis and design, utilities and other systems Structural design is sequential and iterative in nature. It follows a series of steps without skipping. It involves decisions that may result to a series of repeats of previous steps. Structural Design Process 1. Planning: setting and finalizing project details 2. Preliminary structural configuration: initial arrangement of structural members 3. Establishment of loads: applying loads on structure model depending on material, function, and site conditions 4. Preliminary member selection: initial sizing of structural members 5. Structural analysis: modeling and analyzing structure to determine forces and deformations Structural Design Process 6. Evaluation: checking individual members against strength and serviceability requirements, as well as client specifications 7. Redesign: repetition of previous steps depending on the results of the evaluation 8. Final decision: determining whether the latest iteration of design is optimum Structural Concrete Defined as “plain or reinforced concrete in a member that is part of a structural system required to transfer loads along a load path to the ground” (ACI, 2013) Concrete is a “mixture of hydraulic cement, aggregates, and water, with or without admixtures, fibers, or other cementitious materials”. (ACI, 2013) Plain concrete is a “structural concrete with no reinforcement or with less reinforcement than the minimum amount specified for reinforced concrete in the applicable building code”. (ACI, 2013) Introduction to Structural Concrete Design Structural Concrete Reinforced concrete is a “structural concrete reinforced with no less than the minimum amount of prestressing steel or non- prestressed reinforcement as specified in the applicable building code”. (ACI, 2013) Reinforced concrete may be classified as either steel-reinforced concrete (using rebars) or prestressed concrete (using tendons). This course will focus mainly on steel-reinforced concrete, with a brief overview on prestressed concrete. Advantages of Structural Concrete High compressive strength Great resistance to fire and water Rigidity Low maintenance Long service life Most economical material for substructures and floor slabs Moldability Cheap cost of components Lower required labor skill requirement Disadvantages of Structural Concrete Low tensile strength Formworks requirement Low strength-to-weight ratio Low strength-to-volume ratio Variation of properties due to proportioning and mixing (quality consistency issue) Historical Background of Concrete Lime mortar was first used in the Minoan civilization (Crete, ~2000 B.C.). In the 3rd century B.C., Romans mixed lime mortar with volcanic ash (pozzolana) to create stronger, water-resistant mortar. John Smeaton (before 1800) designed the Eddystone Lighthouse using Roman cement but also experimented with a mix of limestone and clay to create water-resistant cement. Joseph Aspdin (1824) created Portland cement by heating ground limestone and clay, named after Portland stone, a high-grade limestone. Historical Background of Concrete Accidental overheating of the cement mixture produced clinker, later found to make stronger cement (discovered by I.C. Johnson in 1845). Some personalities involved in the development of structural concrete design include: W. B. Wilkinson (1854): Patented a reinforced concrete floor system using hollow plaster domes and steel mine-hoist ropes as reinforcement. Joseph Lambot (France, 1848): Built a reinforced concrete rowboat and patented his concept in 1855, showing reinforced beams and columns with iron bars. Historical Background of Concrete Some personalities involved in the development of structural concrete design include: Thaddeus Hyatt (U.S., 1850’s): Experimented with reinforced concrete beams using longitudinal bars and stirrups for shear, though his work remained unknown until 1877. Joseph Monier (France, 1867): Patented reinforced concrete for tubs, followed by patents for pipes, tanks, plates, bridges, and stairs between 1868 and 1875. W. E. Ward (U.S., 1875): Built the first reinforced concrete house in the United States on Long Island. E. L. Ransome (California, 1870’s-1880’s): Patented a twisted steel reinforcing bar in 1884, developed his own design procedures, and built significant structures using reinforced concrete. Historical Background of Concrete Some personalities involved in the development of structural concrete design include: Coignet and de Tedeskko (1894): Extended Koenen's theories to develop the working-stress design method for reinforced concrete flexure, widely used from 1900 to 1950. E. Freyssinet (1928): Pioneered the use of high-strength steel wire for prestressing, solving the problem of concrete creep, and developed anchorages and designs for bridges and structures. A series of patents shaped the science of reinforced concrete, with an English textbook in 1904 listing 43 patented systems across countries between 1875 and 1900. Limit States Limit states are the conditions of a structure or a structural member when it becomes unfit for its intended use. Limit states are classified into three basic groups: Strength limit states: involve structural failure or collapse of a part or most of a structure (e.g. loss of equilibrium, failure, progressive collapse, formation of plastic mechanism, instability, fatigue) Serviceability limit states: involve disruption of functional use of a structure; does not necessarily be followed by collapse (e.g. excessive deformations, excessive cracking, undesirable vibrations) Special limit states: involve damage or failure due to abnormal conditions (e.g. extreme calamities, fire, corrosion effects) Limit States Limit states design involves the following: Identification of all possible failure modes or limit states Determination of acceptable levels of safety for each limit state Structural design considering significant limit states For structural concrete, limit states design is done by using the (ultimate) strength design method (USD). It is a “design method that requires service loads to be multiplied by load factors and computed nominal strengths to be multiplied by strength reduction factors”. (ACI, 2013) Strength Design Method The basic criterion for strength requirement for structural design is that the capacity (or resistance) of a member should be greater than or equal to the demand (or load effects) placed on the said member, i.e. capacity ≥ demand. For structural concrete design using USD: 𝜙𝑅𝑛 ≥ 𝑄𝑢 = ෍ 𝛾𝑖 𝑄𝑖 Φ: strength reduction factor Rn: nominal member strength Qu: total factored load γi, Qi: load factor γi for ith type of load Qi USD Load Combinations (Art. 203.3, Section 203, Chapter 2, 2015 NSCP Vol. 1) 22 Service Load Combinations (Art. 203.4, Section 203, Chapter 2, 2015 NSCP Vol. 1) 23 Structural Safety Reasons for setting load factors and strength reduction factors: Variability in strength: differences in material properties and section dimensions; simplified design assumptions Variability in loadings: differences in material densities and actual load intensities while the structure is in use Consequences of failure: taking account some limit states that may cause higher potential losses in life and property, depending on how they happen in structural members Codes and Standards for Structural Concrete Some of the codes and standards used in structural concrete design include: 2015 National Structural Code of the Philippines (NSCP) Vol. 1 Chapter 2: Minimum Design Loads Chapter 4: Structural Concrete ACI 318M-14 (Building Code Requirements for Structural Concrete) ACI 318R-14 (Commentary on Building Code Requirements for Structural Concrete) Other ACI codes and standards (some will be introduced throughout the course) Design for Economy Economy is a major goal in structural design, influenced by both construction costs and financing charges tied to the speed of construction. In cast-in-place buildings, floor and roof systems make up about 90% of the total structural cost. Material costs increase with larger column spacing, but formwork reuse can reduce costs. Beam, slab, and column sizes should be chosen to maximize form reuse. Overcomplicating design to save on materials can increase forming costs and complexity, leading to higher overall costs. Design for Economy Simplified designs reduce the chances of errors, save time, and result in more economical structures. Avoid haunched beams and deep spandrel beams, which complicate form movement. It’s more cost-effective to use consistent beam depths, even for varying spans. Standard column sizes should be used for three or four stories (or the whole building) to simplify formwork. Reinforcement amounts and concrete strength can vary with load. Complex reinforcement designs increase labor costs, so it is more economical to design columns with 1.5-2% reinforcement and beams with 50-66% of the maximum allowable reinforcement. Design for Economy Grade-60 reinforcement is widely used for columns and beams. For slabs, Grade-40 reinforcement may be advantageous when controlled by minimum reinforcement ratios, though availability should be checked. Concrete strength has little effect on flexural strength of floors, so using high-strength concrete offers little advantage except in flat- plate systems where shear capacity is critical. High-strength concrete is more economical for columns since their strength is directly related to the concrete strength. Design for Sustainability Durability and longevity are key factors for selecting reinforced concrete in construction, contributing to sustainability. Reinforced concrete is valued for its aesthetic qualities, versatility, and both initial and life-cycle economic benefits, including thermal properties that reduce energy costs. Sustainable/green construction is seen as a compromise between economic, social, and environmental factors. Reinforced concrete fits within this framework. Design for Sustainability Aesthetics and occupant comfort: Concrete allows for innovative architectural designs and improves comfort through thermal mass, natural lighting, and reduced need for hazardous finishes. Its durability contributes to long-term sustainability by reducing renovations and maximizing service life. Long service life: Reinforced concrete structures are durable, with a service life typically exceeding 50 years and often over 100 years, reducing long-term costs and resource use. Reducing carbon footprint: Concrete’s energy-saving properties during its service life help reduce CO2 emissions, but concerns remain over cement production. Materials for Structural Concrete Construction Concrete Concrete is a composite material composed of aggregates (coarse and fine), cement, water, and admixtures (in some cases). The aggregates make up the bulk of the weight and the volume of the concrete. The mixture of cement and water (cement paste) act as a binding agent to hold the aggregates together as it hardens. Admixtures are used to improve some properties of concrete, such as strength and workability. Concrete is a brittle material, and in addition, it is strong in compression but weak in tension. This behavior leads to the use of reinforcement in concrete. Concrete The stress-strain relationship of concrete is nonlinear and appears to be somewhat ductile as well. This is due to the development of microcracking within the concrete as it is subjected to loads. Microcracks are internal cracks with lengths ranging between 1/8” and 1/2”. Microcracks are classified as either bond cracks or mortar cracks. Concrete mix design for general structural purposes are usually performed using traditional proportions, DPWH-modified proportions or ACI 211.1-91. Mechanism of Concrete Failure in Compression There are four stages in the microcrack development of concrete under uniaxial compression loading: Development of no-load bond cracks due to shrinkage of cement paste during the hydration process Development of bond cracks due to stresses in the aggregates exceeding their strengths as the load stress reaches ~30-40% of the concrete compressive strength Development of localized mortar cracks between bond cracks as the load stress reaches ~50-60% of the concrete compressive strength Increase in mortar cracks as the load stress goes up to ~75-80% of the concrete compressive strength Compressive Strength of Concrete Concrete sample preparation and testing are performed in accordance with ASTM C31 (Standard Practice for Making and Curing Concrete Test Specimens in the Field) and ASTM C39 (Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens) standards. Test cylinders are prepared with two possible sizes (H/D = 2.0): 6-in (150 mm) diameter by 12-in (300 mm) height 4-in (100 mm) diameter by 8-in (200 mm) height Standard concrete age considered as concrete compressive strength for design purposes is 28 days. Factors Affecting Concrete Compressive Strength Water-cement (w/c) ratio Lower w/c generally leads to higher compressive strength Type of cement Type I (normal): used in ordinary construction Type II (modified): lower heat of hydration than Type I; used for sites with moderate sulfate exposure Type III (high early strength): has higher heat of hydration that Type I Type IV (low heat): used for mass concrete applications (dams, large walls etc.); replaced with combination of Types I and II with fly ash Type V (sulfate resisting): used for underground structural elements exposed to soils with sulfates Factors Affecting Concrete Compressive Strength Supplementary cementitious materials Use of other cementitious materials (pozzolans like silica fume and fly ash, ground granulated blast-furnace slag) reduce heat of reduction and, in some cases, improve workability. Aggregate Concrete strength also depends on the strength, grading, quality, and toughness of aggregates. Mixing water Potable water is required for concrete mixing. Salt water is generally avoided due to presence of salts that may destroy the microstructure of the concrete. Factors Affecting Concrete Compressive Strength Curing conditions Moisture and temperature conditions, as well as curing duration, also affect the development of concrete strength. Age of concrete Concrete strength increases with age, especially within the first seven (7) days of curing, provided that optimal curing conditions are in place. Maturity of concrete Young-age concrete gains strength as long as the concrete maintains a threshold temperature of ~ -10°C to -12°C. Rate of loading Testing at low strain rates results to lower recorded strength; higher strain rates result to higher recorded strength Tensile Strength of Concrete (Modulus of Rupture) Tensile strength may be determined by performing materials testing in accordance with ASTM C78 or ASTM C496 standard. ASTM C78: Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading) ASTM C496: Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens Modulus of Rupture Arts. 419.2.3 & 419.2.4, Sec. 419, Chapter 4, 2015 NSCP Vol. 1 Factors Affecting Tensile Strength of Concrete Same factors affect both compressive and tensile strength of concrete. Concrete made with crushed rock has higher tensile strength (~20% higher) than concrete made with rounded gravel. Tensile strength of concrete develops more quickly than compressive strength. Stress-Strain Curve of Concrete in Compression Modulus of Elasticity and Poisson’s Ratio of Concrete Modulus of Elasticity Art. 419.2.2, Sec. 419, Chapter 4, 2015 NSCP Vol. 1 Poisson’s Ratio Varies between 0.11 and 0.21; may also fall within 0.15 and 0.20 Recommended values based on various studies are: 0.20 (compression) 0.18 (tension) 0.18-0.20 (tension + compression) Time-Dependent Volume Changes Shrinkage Decrease in the volume of concrete during hardening and drying under constant temperature. Types of shrinkage include: Drying shrinkage: Loss of adsorbed water from gel particles, primarily affected by relative humidity (highest below 40% RH). Drying shrinkage causes tensile stresses in the outer concrete surface and compressive stresses in the interior. Autogenous shrinkage: Occurs without moisture loss, associated with hydration reactions, more significant in high-performance concretes. Carbonation shrinkage: Happens in carbon dioxide-rich environments, contributing significantly to total shrinkage at certain humidity levels (e.g., 50% RH). Shrinkage has lesser effects in larger members due to higher volume-to- surface-area ratio. Time-Dependent Volume Changes Creep Permanent deformation of a material due to a combination of sustained loads and/or elevated temperatures In concrete, creep strains develop over time if the load remains, due to the thinning of adsorbed water layers between gel particles. Creep slows down over time as bonds form between gel particles in their new positions. Creep strains develop over two to five years and can be one to three times the magnitude of the initial elastic strain. Increased creep leads to greater deflections over time, stress redistribution, and reduced prestressing forces. Time-Dependent Volume Changes Creep It is influenced by several factors like sustained stress ratio (actual stress divided by strength), concrete age, humidity, member size, concrete composition or proportion, temperature, cement type, and water-cement ratio. Time-Dependent Volume Changes Thermal Expansion Thermal expansion in concrete depends on the composition, moisture content, and age. The coefficient of thermal expansion varies based on aggregate type, as follows: Siliceous aggregates: 5-7 (10-6)/°F Limestone or calcareous aggregates: 3.5-5.0 (10-6)/°F Lightweight concrete: 3.6-6.2 (10-6)/°F General value: 5.5 (10-6)/°F The coefficient of thermal expansion may also increase with temperature, especially at high temperature situations. Durability Issues in Concrete Structures Corrosion of steel in the concrete Involves oxidation process and requires a source of oxygen and moisture Fresh concrete has a pH of around 13 (alkaline), which helps prevent corrosion. It starts as soon as the concrete pH drops below 11-12. Surface rust on steel reinforcement used before concrete pouring helps improve the bond between concrete and steel. However, as the rust expands within the reinforcement section, it may cause spalling and cracking on the concrete member. Corrosion control measures involve minimum concrete strength and w/c ratio, minimum clear concrete cover, and limiting chloride content of the concrete mix. Epoxy-coated reinforcements may also be used for corrosion control. Durability Issues in Concrete Structures Breakdown of structure due to freezing and thawing When concrete freezes, pressures develop in the water-filled pores, potentially breaking down the concrete structure. Air entrainment creates microscopic voids that relieve these pressures, helping the concrete resist freeze-thaw damage. Apart from air entrainment, setting minimum w/c ratio, setting minimum concrete strength, and providing proper drainage may also prevent adverse effects of freezing and thawing on concrete. Durability Issues in Concrete Structures Breakdown of structure due to chemical attack Some chemical presence and reactions in the site environment may also affect concrete durability such as sulfate attack, alkali-silica reaction (ASR), and other chemical attacks. These may be mitigated by using appropriate cement types and checking source of aggregates to be used. Concrete structures that are vulnerable to chemical attack include pavements, bridge decks, parking garages, water tanks, and foundations. Extreme Temperature Behavior of Concrete High Temperature and Fire Concrete can perform well within a certain time frame during a fire, but temperature gradients cause surface cracks and spalling. Spalling may get worse when the surface is suddenly cooled. Modulus of elasticity and strength of concrete decrease with increase in temperature. Aggregate type used in concrete mix may also affect its temperature- dependent behavior. Early-age concrete is more susceptible to adverse impacts of fire, particularly with relation to its tensile strength. Concrete changes color when heated; pink means damaged, gray means severely damaged, and damaged concrete beyond gray must be removed and replaced. Extreme Temperature Behavior of Concrete Very Cold Temperatures Concrete strength increases with decrease in temperature, esp. in moist concrete without frozen water. Subfreezing temperatures significantly increase compressive strength, tensile strength, and modulus of elasticity of moist concrete. Dry concrete is less affected by low temperatures. Steel Reinforcement Defined as “bars, wires, strands, fibers, or other slender elements that are embedded in a matrix such that they act together to resist forces” (ACI, 2013) Commonly used non-prestressed reinforcement types for concrete include hot-rolled deformed bars and welded wire fabric. For prestressed concrete, steel tendons are used. Recent developments include use of other types, especially fiber reinforcements. Hot-Rolled Deformed Bars Steel bars with lugs or deformations rolled into the surface to improve bond and anchorage into concrete May be classified depending on the governing ASTM specification at which they are produced ASTM A615 (Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement) ASTM A706 (Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement) ASTM A996 (Standard Specification for Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement) Hot-Rolled Deformed Bars Hot-Rolled Deformed Bars Bars are available in different strength grades (Grade 33/230, Grade 40/300, Grade 60/420, Grade 75/520). The grade number signifies the rated yield strength of the bar in ksi/MPa. Grade 40 & 60 are the most commonly used rebar grades in structures. Grade 75 may be used for large structural members. Grade 33 is commonly available in the provinces and only recommended for small or non-critical structures. Bars are also available in different diameters or sizes. Hot-Rolled Deformed Bars (Appendix A, Chapter 4, 2015 NSCP Vol. 1) Hot-Rolled Deformed Bars Stress-Strain Relationship Modulus of Elasticity: 200000 MPa (29000 ksi) Yield Strength: varies with rebar grade ASTM specification require that the ratio of ultimate tensile strength to yield strength for weldable bars is at least 1.25. In structural design, the relationship is assumed to be an idealized elastoplastic. Hot-Rolled Deformed Bars Fatigue Strength Hot-Rolled Deformed Bars Strength at High Temperatures Both yield and ultimate strength decrease as temperature decreases, starting at ~850°F. Compatibility of Concrete and Steel Concrete and steel reinforcement work together since concrete can withstand compressive stresses while steel reinforcement can deal with tensile stresses, provided that there is adequate bond between them. Moreover, concrete provides protection of steel reinforcement against corrosion and fire, and they also respond to thermal expansion similarly. References American Concrete Institute. (2013). ACI CT-13: ACI Concrete Terminology. USA: American Concrete Institute Association of Structural Engineers of the Philippines. (2016). National Structural Code of the Philippines 2015 Volume 1: Buildings, Towers, and Other Vertical Structures (7th ed.). Quezon City, Philippines: Association of Structural Engineers of the Philippines McCormac, J. C. & Brown, R. H. (2016). Design of Reinforced Concrete (ACI 318-14 Code Edition, 10th ed.). USA: John Wiley & Sons, Inc. Salmon, C. et.al. (2009). Steel Structures Design and Behavior: Emphasizing Load and Resistance Factor Design (5th ed.). USA: Pearson Prentice Hall Wight, J. K. (2016). Reinforced Concrete: Mechanics and Design (7th ed.). USA: Pearson Education, Inc. References Sivakugan, N. et.al. (2018). Civil Engineering Materials. USA: Cengage Learning Wight, J. K. (2016). Reinforced Concrete: Mechanics and Design (7th ed.). USA: Pearson Education, Inc.