Introduction to Structural Concrete Design

Introduction to Structural Concrete Design

• The course is CEPRCD30 (Principles of Reinforced Concrete)
• Instructor: Jerome Z. Tadiosa, CE, MSc
• Institution: National University – Manila

Intended Learning Outcomes

• Describe the structural design process and considerations.
• Explain the description, development, and classification of structural concrete.
• Identify and describe the design philosophies, relevant codes, and standards.
• List and describe the materials used in structural concrete construction.

Reading Guide

• Read chapters 1-3, Wight (2016)
• Read chapter 1, McCormac & Brown (2016)
• Read sections 1.1-1.2, Chapter 1, Salmon et. al. (2009)

Lecture Outline

• Introduction to structural design process
• Introduction to structural concrete design
• Materials in structural concrete design

Structural Design

• Defined as a mixture of art and science, combining experienced engineer's intuitive feeling for the behavior of structures with a solid knowledge of basic engineering principles.
• A properly designed structure must meet four criteria:
  • Appropriateness (functionality and aesthetics)
  • Economy (optimal benefit-cost ratio)
  • Structural adequacy (strength and serviceability requirements)
  • Maintainability (minimum maintenance cost and time)

General Design Process

• Major phases of general design process include:
  • Define client's needs (function, aesthetics, budget)
  • Develop project concept (schematics, preliminary framework, materials)
  • Design individual systems (structural analysis and design, utilities and other systems)
• Structural design is sequential and iterative in nature.
• It follows a series of steps, and may involve reiteration of prior steps

Structural Design Process (Detailed)

• Planning: setting and finalizing project details
• Preliminary structural configuration: initial arrangement of structural members
• Establishment of loads: applying loads to structure model, based on material, function and site conditions
• Preliminary member selection: initial sizing of structural members
• Structural analysis: modelling and analyzing structure to determine forces and deformations
• Evaluation: checking individual members against strength and serviceability requirements
• Redesign: repetition of previous steps based on results
• Final decision: determining if the latest design iteration is optimal

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.
• Concrete is a mixture of hydraulic cement, aggregates and water, with or without admixtures, fibers, or other cementitious materials.
• Plain concrete is structural concrete with no reinforcement or less reinforcement than the minimum specified for reinforced concrete in the applicable building code.
• Reinforced concrete is structural concrete reinforced with no less than the minimum amount of prestressing steel or non-prestressed reinforcement as specified in the applicable building code.
• Reinforced concrete is classified as either steel-reinforced or prestressed concrete.

Advantages of Structural Concrete

• High compressive strength
• Great resistance to fire and water
• Rigidity
• Low maintenance
• Long service life
• Economical material for substructures and floor slabs
• Moldable
• Inexpensive component costs
• Lower required labor skill requirement

Disadvantages of Structural Concrete

• Low tensile strength
• Formwork requirements
• Low strength-to-weight ratio
• Low strength-to-volume ratio
• Variations in properties due to proportioning and mixing (quality consistency issues)

Historical Background of Concrete

• Lime mortar, first used in the Minoan civilization in Crete (~2000 B.C.).
• Romans (3rd century B.C.) mixed lime mortar with volcanic ash (pozzolana) to form stronger, water-resistant mortar.
• John Smeaton (pre-1800) used a mixture of limestone and clay to create a water-resistant cement for the Eddystone Lighthouse.
• Joseph Aspdin (1824) created Portland cement by heating ground limestone and clay, named after Portland stone.
• I.C. Johnson (1845) discovered that accidentally overheating cement mixture produced clinker, which made stronger cement.
• Other personalities who were involved in the development of structural concrete design include W.B. Wilkinson (1854), Joseph Lambot (1848), Thaddeus Hyatt (1850s), Joseph Monier(1867), W. E. Ward (1875) and E.L. Ransome (1870s-1880s)

Limit States

• Defined as the conditions of a structure or structural member when it becomes unfit for its intended use.
• Strength limit states: structural failure
• Serviceability limit states: disruption of a structure's functionality
• Special limit states: involves damage or failure due to abnormal conditions (e.g. external calamities, fire, corrosion effects)
• Limit states design involves: identifying failure modes, determining acceptable safety levels, structural design

Strength Design Method

• The basic criterion is that the capacity or resistance of a member should be greater than or equal to the demand or load effects placed on the member.
• This is done using the strength design method (USD):

USD Load Combinations

• Load combinations used for strength design or load and resistance factor design
• Factored loads combinations used in designs

Service Load Combinations

• Load combinations used in designs based on allowable stress or allowable strength

Structural Safety

• Reasons for setting load factors and strength reduction factors: variability in strength, variability in loadings, consequences of failure

Codes and Standards for Structural Concrete

• 2015 National Structural Code of the Philippines (NSCP) Vol. 1, Chapter 2: Minimum Design Loads; Chapter 4: Structural Concrete
• ACI 318M-14 and ACI 318R-14.
• Other ACI codes and standards

Design for Economy

• Economy is a major goal in structural design, influenced by both construction costs and financing charges tied to 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

Design for Sustainability

• Durability and longevity are key factors for selecting reinforced concrete for construction.
• 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 construction is a compromise between economic, social and environmental factors.

Materials for Structural Concrete Construction

• Concrete is a composite material of aggregates (coarse and fine), cement, water, and admixtures.
• Aggregates make up the bulk of the weight and volume of concrete.
• Cement and water mix (cement paste) acts as a binding agent.
• Admixtures enhance properties (e.g. strength and workability).
• Concrete is strong in compression but week in tension, hence reinforcement.

Concrete

• The stress-strain relationship is nonlinear and appears somewhat ductile due to microcracking.
• Microcracks internal cracks/bond cracks.
• Mix design typically uses traditional proportions, DPWH-modified proportions or ACI 211.1-91 for general purposes.

Mechanism of Concrete Failure in Compression

• Four stages in microcrack development 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 exceeding aggregate strength to 30%-40% of the concrete compressive strength
• Development of localized mortar cracks between bond cracks at 50%-60% of the compressive strength
• Increase in mortar cracks up to 75-80% of compressive strength

Compressive Strength of Concrete

• Sample preparation/testing is in accordance with ASTM C31 and C39.
• Test cylinders are prepared.
• Common sizes (cylinders) are 6-inches by 12-inches diameter by 12-inches in height or 4-inches by 8-inches (diameter by height).
• The standard age for compressive strength considered in design purposes is 28 days.

Factors Affecting Concrete Compressive Strength

• Water-cement (w/c) ratio: lower ratio generally leads to higher compressive strength
• Type of cement:
  • Type I (normal): common 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): high heat of hydration
  • Type IV (low heat): mass concrete applications
  • Type V (sulfate resisting): used for underground elements exposed to sulfates

Factors Affecting Concrete Compressive Strength cont.

• Use of supplementary cementitious materials (e.g., pozzolans)
• Aggregate characteristics (strength, grading, quality, and toughness)
• Mixing water (potable water is preferred)
• Curing conditions (moisture and temperature conditions, and curing duration)
• Age of concrete
• Maturity of concrete
• Rate of loading

Tensile Strength of Concrete (Modulus of Rupture)

• Determined by performing material 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

• Modulus of rupture, fr, can be calculated by: fr = 0.622√f'c.

Factors Affecting Tensile Strength of Concrete

• Similar factors affect both compressive and tensile strength
• Concrete made with crushed rock has higher tensile strength (approx. 20% higher) than concrete made with rounded gravel.
• Tensile strength develops faster than compressive strength

Stress-Strain Curve of Concrete in Compression

• Stress-strain relationship is nonlinear and appears to behave ductility.
• This is because of microcracking during loading.

Modulus of Elasticity and Poisson's Ratio of Concrete

• Modulus of elasticity (Ec) calculated by: Ec = wc^1.50.043f'c or Ec = 4700√f'c (where wc and f'c are specific parameters).
• Poisson's ratio varies between 0.11 and 0.21, and may also fall within 0.15 and 0.20.
• Standard values from studies are 0.20 for compression, 0.18 for tension, and 0.18-0.20 for tension and compression together.

Time-Dependent Volume Changes

• Shrinkage: Decrease in concrete volume during hardening and drying under constant temperature.

  • Types of Shrinkage
    • Drying shrinkage
    • Autogenous shrinkage
    • Carbonation shrinkage

• Creep: Permanent deformation due to sustained loads and/or elevated temperatures.

• Creep strain develops over time if load remains, due to the thinning of adsorbed water layers between gel particles and slows down over time as bonds form.

• Creep strain is up to three times the magnitude of the initial elastic strain, leading to greater deflection.

• This leads to stress redistribution and reduced prestressing forces.

• Factors influencing Creep

  • Sustained stress ratio
  • Concrete age
  • Humidity
  • Member size
  • Concrete composition
  • Proportion
  • Temperature
  • Cement type
  • Water-cement ratio

• Thermal expansion

  • Depends on composition, moisture content, and age
  • Coefficient of thermal expansion varies based on aggregate type (siliceous, limestone or calcareous, lightweight concrete).
  • Coefficient may also increase with temperature.

Durability Issues in Concrete Structures

• Corrosion of steel
• Breakdown due to freezing & thawing
• Breakdown due to chemical attacks (e.g., sulfate attack, alkali-silica reaction).

Extreme Temperature Behavior of Concrete

• High temperature & fire: Concrete can perform well within a specific timeframe during fires, however, temperature gradients cause surface cracks and spalling.
• Very cold temps: Concrete strength increases with decrease in temp; subfreezing temps significantly increase compressive strength, tensile strength, and modulus of elasticity of moist concrete.
• Dry concrete is less impacted by low temps.

Steel Reinforcement

• Defined as bars, wires, strands, fibers, or other slender elements embedded in a matrix to resist forces.
• Common non-prestressed reinforcement types include hot-rolled deformed bars and welded wire fabric; prestressed concrete utilizes steel tendons.
• Recent developments include use of fiber reinforcements.

Hot-Rolled Deformed Bars

• Steel bars with lugs or deformations rolled into the surface to improve bond and anchorage.
• Classified by their producing mill based on governing ASTM specification.
• Specific ASTM standards for different steel types exist (A615, A706, A996)
• Different grades and sizes exist

Hot-Rolled Deformed Bars (cont.)

• Fatigue strength
• Strength at high temperatures: both yield and ultimate strength decrease as the temperature decreases, starting at ~850°F.

Compatibility of Concrete and Steel

• Concrete and steel reinforcement work together because concrete withstands compressive stresses, and steel reinforcement withstands tensile stresses.
• Concrete protects steel reinforcement from corrosion and fire, and both respond similarly to thermal expansion.

References (Authors & Titles)

• Various authors and titles of related publications are listed

This quiz covers key concepts from the course CEPRCD30, focusing on the principles of reinforced concrete. Learners will explore the structural design process, the classification of structural concrete, and the materials involved in its construction. It emphasizes the importance of relevant codes and standards in designing safe and effective structures.