Limit States Design in Structural Engineering

Introduction to Structural Concrete Design

• The course is titled CEPRCD30 (Principles of Reinforced Concrete)
• The instructor is Jerome Z. Tadiosa, CE, MSc
• The instructor works as an Assistant Professor 2 in Civil Engineering at the College of Engineering, National University – Manila.

Intended Learning Outcomes

• Students will learn about the structural design process and considerations.
• Students will understand the description, development, and classification of structural concrete.
• Students will learn about the design philosophies in structural concrete design and relevant codes and standards.
• Students will learn about the materials used in structural concrete construction.

Reading Guide

• Students should study chapters 1-3 from Wight (2016)
• Students should read chapter 1 from McCormac & Brown (2016)
• Students should review sections 1.1-1.2 from Salmon et.al. (2009)

Lecture Outline

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

Structural Design

• Structural design is a blend of art and science.
• It combines an experienced engineer's intuitive understanding of structure behavior with a sound knowledge of basic engineering principles.
• A properly designed structure must satisfy four criteria:
  • Appropriateness (functionality & aesthetics)
  • Economy (optimal benefit-cost ratio, ideally minimum cost)
  • Structural adequacy (strength & serviceability requirements)
  • Maintainability (minimum maintenance cost & time).

General Design Process

• The general design process includes several major phases:

  • Defining client needs and priorities (function, aesthetics, budget)
  • Developing project concept (schematics, preliminary framework, materials)
  • Designing individual systems (structural analysis & design, utilities, other systems)

• Structural design is sequential and iterative

• It involves a series of steps that may require repetition of earlier steps.

Structural Design Process (Detailed Steps)

• Planning: defining and finalizing project details.
• Preliminary structural configuration: initial arrangement of structural members.
• Establishing loads: applying loads to structure model, dependent on material, function, and site conditions.
• Preliminary member selection: initial sizing of structural members.
• Structural analysis: modeling & analyzing structure to determine forces & deformations.
• Evaluation: checking individual members against strength and serviceability requirements (client specifications).
• Redesign: repeating previous steps based on evaluation results.
• Final decision: determining if the latest design iteration is optimal

Structural Concrete

• Structural concrete is either plain or reinforced concrete that's part of a structural system, transferring loads through a load path to the ground.
• Concrete is a mixture of hydraulic cement, aggregates, water, and optionally admixtures, fibers, or other cementitious materials.
• Plain concrete has no reinforcement or less than the minimum specified in building codes.
• Reinforced concrete uses reinforcement equal to or more than the minimum amount as per applicable codes. Reinforced concrete can be further classified as either steel-reinforced concrete (using rebars) or prestressed concrete (using tendons). This course primarily focuses on the former.

Advantages of Structural Concrete

• High compressive strength
• Resistance to fire and water
• Rigidity
• Low maintenance
• Long service life
• Economical material for substructures and floor slabs
• Moldable
• Low labor skill requirement for most components

Disadvantages of Structural Concrete

• Low tensile strength
• Needs formwork
• Low strength-to-weight ratio
• Low strength-to-volume ratio
• Quality/consistency issues due to mixing variations

Historical Background of Concrete

• Lime mortar was the earliest form of concrete, used by Minoans (Crete, ~2000 B.C.).
• Romans improved lime mortar by adding volcanic ash (pozzolana) for strength and water resistance in the 3rd century B.C.
• John Smeaton experimented with mixes like limestone and clay for water resistance in the 18th century, as part of lighthouse design.
• Joseph Aspdin invented Portland cement in 1824 by heating limestone/clay, named after Portland stone (a high-grade limestone).
• Other key figures in concrete development include: W. B. Wilkinson, Joseph Lambot, Thaddeus Hyatt, Joseph Monier, W. E. Ward, and E. L. Ransome.

Limit States

• Limit states are conditions where a structure or member becomes unsuitable for its intended function.
• Strength limit states involve collapse or failure (e.g., collapse, instability, fatigue).
• Serviceability limit states relate to disruption of the structure's function without collapse (e.g., excessive deformations, cracking, vibrations).
• Special limit states refer to failures from abnormal conditions (e.g., fire, corrosion, extreme weather events).

Limit States Design

• Limit states design identifies all potential failure modes.
• Acceptable safety levels are determined for each limit state.
• Structural design considers significant limit states

Strength Design Method

• Capacity (resistance) of a structural member should exceed demand (load effects)
• For structural concrete design using ultimate strength design (USD)
  • ФR_n ≥ ∑Q_i
  • Ф = strength reduction factor
  • R_n = nominal member strength
  • Q_i = total factored load
  • Y_i, Q_i = load factor y_i for ith type of load

USD Load Combinations

• Specific load combinations are used for strength design as per 2015 NSCP standards.

Service Load Combinations

• Specific load combinations are used for allowable strength design as per 2015 NSCP standards.

Structural Safety

• Load factors and strength reduction factors account for variability in member strength and loading.
• Higher losses from failure (e.g., life, property) affect design by considering potential losses.
• Consequences of failure need to be considered in the design.

Codes and Standards for Structural Concrete

• Relevant codes & standards include the 2015 National Structural Code of the Philippines (NSCP) Volume 1, Chapter 2 (Minimum Design Loads) and Chapter 4 (Structural Concrete).
• ACI 318M-14 (Building Code Requirements for Structural Concrete) and ACI 318R-14 (Commentary on Building Code Requirements for Structural Concrete) are also important references.
• Other ACI codes may also be referenced.

Design for Economy

• Economy (cost optimization) is a key goal in structural design.
• Speed of construction and costs related to financing are part of this design consideration.
• Costs of materials (especially cast-in-place floor and roof systems) and construction materials like formwork selection should be considered.

Design for Sustainability

• Durability and longevity are key aspects of sustainable design.
• Aesthetic value, adaptability, and life-cycle economic benefits including cost savings from thermal properties are factored into design.
• Economic factors, social impacts, and environmental factors need to be balanced in sustainable design.

Materials for Structural Concrete Construction

• Concrete is a composite of aggregates, cement paste, water, and sometimes admixtures.
• Aggregates bulk the concrete.
• Cement paste acts as a binding agent.
• Admixtures improve properties like strength and workability.
• Concrete is strong in compression but weak in tension, which necessitates reinforcement.

Concrete

• The stress-strain relationship of concrete is non-linear and appears somewhat ductile.
• Microcracking within concrete develops due to sustained loads.
• Microcrack lengths range between 1/8 inch and 1/2 inch.

Mechanism of Concrete Failure in Compression

• There are four stages of microcrack development in uniaxial compression.
• Bond cracks (due to shrinkage during hydration) initially develop.
• Crack development increases with the loading stresses, reaching ~30 to ~40% of the compressive strength.
• Localized mortar cracks develop between bond cracks, around ~50 to ~60% of the compressive strength.
• As loading stress approaches ~75 to ~80% of compressive strength, more mortar cracks develop.

Compressive Strength of Concrete

• Sample preparation and testing follow ASTM standards (C31 & C39)
• Test cylinders can be 6 inches (150 mm) in diameter by 12 inches (300 mm) in height, or 4 inches (100mm) in diameter by 8 inches (200mm) in height.
• Standard concrete age considered is 28 days.

Factors Affecting Concrete Compressive Strength

• Water-cement (w/c) ratio: lower w/c typically leads to higher compressive strength.
• Type of cement (e.g., Type 1, Type II, Type III, Type IV, Type V): different types of cement will result in varying concrete properties.

Factors Affecting Concrete Compressive Strength (Supplementary Cementitious Materials, Aggregates, Mixing Water)

• Supplementary cementitious materials (e.g., pozzolans): can reduce heat of hydration, improve workability.
• Aggregates: Quality, type, strength, grading, and workability are critical to compressive strength.
• Mixing water: Potable water is typically used for mixing, and saltwater should be avoided.

Factors Affecting Concrete Compressive Strength (Curing & Loading Conditions)

• Curing conditions: Humidity and temperature, especially early days after curing.
• Optimal curing conditions are crucial for young-aged concrete.
• Age: Strength increases with age (especially within the first 7 days), provided the curing conditions are optimal.
• Concrete strength increases until 28 days.
• Maturity of concrete: Strength gains relate to temperature.
• Rate of loading: Lower strain rates result in lower recorded compressive strength.

Tensile Strength of Concrete (Modulus of Rupture)

• Tensile strength can be determined by ASTM C78 (flexural strength test) or ASTM C496 (splitting tensile strength).

Modulus of Rupture

• ACI 419 provides concrete modulus of rupture (fr) calculations, specifically for lightweight concrete.
• Modification factors for lightweight concrete are included (Table 419.2.4.2).

Factors Affecting Tensile Strength of Concrete

• Same factors affect compressive and tensile strength.
• Concrete with crushed rock tends to have higher tensile strength than concrete with rounded gravel.

Stress-Strain Curve of Concrete in Compression

• The relationship between stress and strain for concrete in compression is non-linear.

Modulus of Elasticity and Poisson's Ratio of Concrete

• ACI 419 specifies how to calculate modulus of elasticity (E_c) for concrete, including considering lightweight concrete materials.
• Recommended values for Poisson's ratio (varying between 0.11 to 0.21 or 0.15 to 0.20) based on studies.

Time-Dependent Volume Changes (Shrinkage)

• Shrinkage is a decrease in concrete volume during hardening and drying under constant temperature.
• Types of shrinkage include drying shrinkage and autogenous shrinkage. Important factors relate to relative humidity, hydration extent, and temperature during curing, impacting on the stresses in the concrete.
• Hydration of cement results not only in shrinkage but also in autogenous reactions, contributing to overall shrinkage.
• Carbonation shrinkage results from exposure to carbon dioxide.
• Shrinkage typically has smaller effect on larger members due to higher volume to surface area ratio.

Time-Dependent Volume Changes (Creep)

• Creep is permanent deformation of a material under sustained loads & elevated temperatures.
• Creep is influenced by factors such as sustained stress ratio, concrete age, humidity, member size, concrete composition, temperature, and water-cement ratio

Time-Dependent Volume Changes (Thermal Expansion)

• Thermal expansion in concrete is affected by composition, moisture content, temperature, and age.
• Coefficient of thermal expansion differs based on aggregate type (e.g., siliceous, limestone/calcareous, lightweight).

Durability Issues in Concrete Structures

• Corrosion of steel in concrete.
• Oxidation from oxygen and moisture.
• Fresh concrete has a high pH (alkaline), resisting corrosion initially.
• Freeze-thaw damage, with water in pores expanding and damaging concrete during freezing.
• Air entrainment can reduce freeze-thaw pressures

Durability Issues in Concrete Structures (Chemical Attack, Extreme Temperatures)

• Chemical attacks (e.g., sulfate attack, alkali-silica reaction) may weaken concrete
• Structures vulnerable to chemical attacks include pavements, bridge decks, parking garages, water tanks, and foundations.
• Adequate protection measures and material selection are essential to concrete durability.

Extreme Temperature Behavior of Concrete (High Temperature & Fire)

• Concrete can perform reasonably well at a moderate fire level, but temperature gradients cause cracking and spalling, potentially getting worse when the surface experiences rapid cooling.
• Changes in temperature impact elastic modulus and strength.
• Aggregate types and early-age concrete are more susceptible to severe fire impacts.
• Damage or failure due to extreme temperatures require visual inspection and removal.

Extreme Temperature Behavior of Concrete (Very Cold Temperatures)

• Concrete strength, tensile strength, and modulus of elasticity tend to increase with decreasing temperatures in moist condition.
• Dry concrete less affected.

Steel Reinforcement

• Steel reinforcement is defined as embedded bars, wires, strands, or fibers within a matrix, acting in concert with the matrix to resist forces.
• Common reinforcement types include hot-rolled deformed bars and welded wire fabric, with steel tendons used for prestressed concrete.
• Fiber reinforcement and other types are used increasingly in contemporary designs.

Hot-Rolled Deformed Bars

• Steel bars with rolled deformations (lugs, embossments) used to improve bond & anchorage into concrete.
• Different grades (based on strength) according to ASTM specifications.
• Standards include ASTM A615, A706, & A996.

Hot-Rolled Deformed Bars (Strength at High Temperatures, Fatigue Strength)

• Yield and ultimate strength of hot-rolled deformed bars decrease with increasing temperatures (&850°F).
• Fatigue strength of hot-rolled deformed bars relate to the stress range and cycles to failure.
  • Standards (specifying load ranges/cycles/tolerance limits) apply for fatigue strength.

Compatibility of Concrete and Steel

• Concrete and steel compatibility is based on compatibility.
• Concrete resists compression, while steel resists tension, and the bond must be adequate for resisting forces between these dissimilar materials.

References

• Several relevant books & publications on structural design, reinforced concrete, and construction standards (including the 2015 NSCP and ACI documents) are listed.

Explore the crucial concepts of limit states design in structural engineering through this quiz. It covers strength limit states, serviceability limit states, and the criteria for capacity and demand assessment. Test your knowledge on the fundamental aspects of structural integrity and load assessment.