Chapter 4: Introduction to Design of Concrete Structures

4.1 Properties of Reinforced Concrete

This section would cover the fundamental characteristics of the two primary components of
reinforced concrete and how they interact:

a. Concrete:
• Compressive Strength: Concrete is strong in compression but weak in tension.

• Tensile Strength: Significantly lower than compressive strength, often ignored in design.

• Modulus of Elasticity: Relates stress and strain, varies with concrete mix and strength.

• Shrinkage and Creep: Time-dependent deformations that can affect long-term structural
performance.

• Durability: Resistance to deterioration from environmental factors (e.g., water, chemicals,

freeze-thaw).

• Mix Design: Factors influencing concrete properties, including water-cement ratio, aggregate
type, and admixtures.

b. Reinforcing Steel:
• Tensile Strength: High tensile strength to carry tensile forces in the concrete.

• Yield Strength: Stress at which steel begins to deform permanently.

• Modulus of Elasticity: Important for determining deformation behavior.

• Ductility: Ability to deform significantly before fracturing.

• Corrosion: Vulnerability to rust, which can compromise structural integrity.

• Compatibility: The need for similar thermal expansion rates to prevent cracking.

4.2 Reinforced Concrete

This is a broader topic that elaborates on how reinforced concrete works as a structural material:

• Why Reinforce Concrete: Explains the purpose of steel reinforcement to overcome concrete's
weakness in tension.

• Basic Principles: Introduces the fundamental concepts behind reinforced concrete design:
 • Tensile Zones: Where reinforcement is primarily placed.

• Compressive Zones: Primarily carried by concrete.

• Neutral Axis: The boundary between tensile and compressive zones.

• Types of Reinforcement:

• Rebar: Deformed steel bars, usually used in beams, slabs, and columns.

• Welded Wire Mesh: Used in slabs and pavements.

• Prestressing Steel: Used to induce compressive stresses in concrete, increasing its load-
carrying capacity (more advanced, could be a separate topic).

• Concrete Cover: The protective layer of concrete surrounding the reinforcement, which serves
to protect against corrosion and provide adequate bond.

• Reinforcement Detailing: Guidelines on spacing, lapping, and end details of reinforcement for
efficient load transfer.

4.3 Building Design Codes Provisions

This part discusses how building codes specify requirements for concrete structures:

• Purpose of Building Codes: To ensure safety, durability, and serviceability of structures.

• Load Factors: Factors of safety applied to design loads to account for uncertainties in load
estimates and material strengths.

• Strength Reduction Factors: Factors of safety applied to material strengths to account for
variations in construction quality and material properties.

• Serviceability Requirements: Code provisions for deflection limits, cracking limits, and
minimum concrete cover requirements.

• Reinforcement Requirements: Code stipulations on minimum and maximum reinforcement

ratios, spacing, and detailing.

• Specific Code Requirements: (e.g., ACI 318 in the US, Eurocode 2 in Europe). This section
would mention the specific sections of the chosen code that are related to the design of
reinforced concrete structures.

• Design Methods: (e.g., Strength Design or Working Stress Design depending on the chosen
code)
4.4 Behavior of RC Beams Under Loading
This section delves into how reinforced concrete beams behave under different loading
scenarios:

• Flexural Behavior: How beams bend under loading, creating tensile and compressive stresses.

• Cracking: Initial cracking of concrete in the tensile zone at relatively low loads.

• Elastic Stage: Linear relationship between load and deformation.

• Ultimate Strength: The load at which the beam fails (either through steel yielding or concrete
crushing).

• Moment-Curvature Relationship: Demonstrates how the bending moment and curvature of a

beam relate to each other.

• Shear Behavior: The tendency of a beam to slide along its length under a shear force.

• Diagonal Cracking: Formation of diagonal cracks due to combined shear and tensile stresses.

• Shear Reinforcement: Stirrups or bent-up bars to resist shear forces.

• Deflection: The amount a beam bends under a given load.

• Serviceability Limits: Acceptable levels of deflection to prevent functional or aesthetic

issues.

Failure Modes:
• Tension Failure: Steel yielding followed by excessive deformation and eventual concrete
crushing.

• Compression Failure: Concrete crushing before the steel yields.

• Shear Failure: Diagonal cracking and failure due to inadequate shear reinforcement.

4.5 Behavior of RC Slabs Under Loading :

This explores the response of concrete slabs under load:

• One-Way Slabs: Primarily supported by two opposite sides and bending in one direction (e.g.,
simply supported slabs between beams).

• Two-Way Slabs: Supported on all four sides and bending in both directions (e.g., slabs
supported by columns).
• Flexural Behavior: How slabs bend under load, forming tensile and compressive stresses.

• Reinforcement Detailing: Placement of reinforcement to resist tensile stresses and control

cracking.

• Shear Behavior: Consideration of shear stresses near supports.

• Deflection: Controlling slab deflection to meet serviceability requirements.

4.6 Behavior of RC Columns Under Loading

This discusses the behavior of reinforced concrete columns under compression:

• Axial Compression: Columns primarily carrying vertical compressive forces.

• Slenderness Effects: Influence of column length on buckling potential.

• Short Columns: Fail primarily due to material crushing.

• Slender Columns: Fail due to buckling (lateral deflection) before material crushing.

• Combined Axial Load and Bending: Columns subjected to eccentric loads, resulting in
bending moments.

• Reinforcement Detailing: Arrangement of longitudinal reinforcement and lateral ties or spirals.

• Interaction Diagrams: Tools for designing columns under combined axial load and bending.

Key Points to Remember:

• Interconnectedness: These topics are highly interrelated. Understanding concrete and steel
properties is essential to understand how reinforced concrete behaves.

• Code Reliance: The design of reinforced concrete structures heavily relies on building codes.

Simplification: The analysis of complex structures is often simplified using various

assumptions.

• Practical Considerations: Design considerations must balance theoretical calculations with

practical construction requirements.

• Strength Reduction Factors: Reduce material strengths to account for construction variability.

• Serviceability Limits: Deflection and cracking limits are specified.

• Reinforcement Details: Code prescribes detailing requirements (spacing, cover, etc).

• Design Methods: (e.g., Limit State Design (USD/LRFD) or Allowable Stress Design (ASD)
methods)

• Axial Compression: Columns primarily designed to carry compressive loads.

• Slenderness Effects: Long columns can buckle; shorter columns fail by crushing.

• Combined Axial Load and Bending: Eccentric loads lead to bending, combined load cases
considered.

• Reinforcement Details: Longitudinal bars resist compression; lateral ties prevent buckling.

• Interaction Diagrams: Used to design columns under combined load and bending.

4.6 Behavior of RC Columns Under Loading

This discusses the behavior of reinforced concrete columns under compression:

• Axial Compression: Columns primarily carrying vertical compressive forces.

• Slenderness Effects: Influence of column length on buckling potential.

• Short Columns: Fail primarily due to material crushing.

• Slender Columns: Fail due to buckling (lateral deflection) before material crushing.

• Combined Axial Load and Bending: Columns subjected to eccentric loads, resulting in
bending moments.

• Reinforcement Detailing: Arrangement of longitudinal reinforcement and lateral ties or spirals.

• Interaction Diagrams: Tools for designing columns under combined axial load and bending

Chapter -5 : Design of Timber Structures

5.1 Introduction to Design of Timber Structures
• Historical Use: Timber has been a primary construction material for centuries due to its
availability, renewability, and ease of use.

• Renewable Resource: Unlike concrete and steel, timber is a renewable resource, making it a
sustainable option when managed properly.

• Versatility: Timber can be used for various structural applications, including framing, decking,
roofing, and even large-span structures.
• Aesthetics: Timber provides a warm, natural, and visually appealing finish, often a key factor
in design choices.

• Modern Techniques: Advances in timber processing and preservation have expanded its use in
modern construction, allowing for more complex and efficient designs.

• Design Codes: Timber design is typically governed by national or international building codes
and standards (e.g., NDS in the US, Eurocode 5 in Europe).

• Challenges: Timber design must account for natural variability, susceptibility to decay,
moisture effects, and connections.

• Sustainability Considerations: Responsible sourcing, proper treatment, and end-of-life

planning are critical to ensuring the sustainability of timber construction.

• Why Timber?

• Lightweight: Easier to handle and transport, lower foundation costs.

• Good Strength-to-Weight Ratio: Can handle substantial loads.

• Natural Insulation: Provides better thermal insulation compared to steel or concrete.

• Carbon Sequestration: Timber stores carbon, helping to reduce greenhouse gases when used
in construction.

• Scope of design - includes design of beams, column, truss, roofs and other structural
components

5.2 Timber Properties

Understanding timber's material properties is crucial for proper design:

• Anisotropic Nature: Timber is anisotropic, meaning its properties differ depending on the
direction of the grain (longitudinal, radial, and tangential).

• Longitudinal (Parallel-to-Grain): Strongest in tension and compression.

• Radial and Tangential (Perpendicular-to-Grain): Weaker in tension and compression.

• Tensile Strength: High along the grain, low perpendicular to the grain.

• Compressive Strength: Relatively high along the grain, significantly lower perpendicular to
the grain.

• Flexural Strength: Resistance to bending.

 • Shear Strength: Resistance to sliding forces.

• Bearing Strength: Resistance to crushing at supports or connections.

• Density and Specific Gravity: Influences strength and stiffness. Higher density generally
means higher strength.

• Stiffness (Modulus of Elasticity): Measures how much a material deforms under stress.

• Higher values are more stiff (less bending, deflection).

• Varies with wood species and grain direction.

• Moisture Content:

• Affects strength, stiffness, and dimensions.

• Wood is more susceptible to decay when damp.

• Timber is usually dried to a specified moisture content before use.

• Durability and Decay:

• Susceptibility to decay from moisture, insects, and fungi.

• Preservative treatments enhance durability.

• Species varies in natural durability.

• Grading: Timber is graded based on quality, defects (knots, splits, wane), and strength.

• Grades like "Select," "No. 1," "No. 2," etc. are used in design.

• Species: Different species have distinct properties and characteristics, suitability for different
applications varies.

5.3 Design of Timber Structures

Here are key notes regarding the practical design of timber structures:

• Load Analysis: Determine the loads acting on the structure (dead loads, live loads, wind, snow,
seismic).

• Member Design:

Beams: Designed to resist bending and shear.

* Check bending stresses and shear stresses

 * Check deflection

Columns: Designed to resist compression and buckling.

• Tension Members: Designed to resist tensile forces.

• Bearing: check the stress at the supports

• Connections:

• Critical aspect of timber design, often governs structural capacity.

• Types of connections: nailed, screwed, bolted, glued, and specialty connectors.

• Connection design must consider shear, tension, and bearing.

• Use of metal connectors, steel plates and gussets.

• Load Combinations: Use appropriate load combinations specified by the building code for
factored loads.

• Material Properties: Use the design properties of the specific timber species and grade and

adjust for moisture content, load duration, treatment

• Lateral Stability: Ensure structures are stable against lateral loads (wind, seismic).

• Bracing, shear walls, and diaphragms are used.

• Serviceability: Check for excessive deflections and vibrations.

• Fire Protection: Consider fire-resistant construction methods if needed.

• Proper sizing and material selection contribute to fire resistance

• Preservative Treatment: Select the type of preservation to protect against decay and insects.

This would depend on the service and exposure of the timber

• Durability: Choose the appropriate species for the intended environment.

• Construction Considerations: Design for practicality and constructability, with specific

requirements for timber elements.

• Sustainability: Ensure responsible sourcing and consider the life cycle of timber products.

• Design Process - Involves calculation of stress and deflection, comparing results with allowable
values, and making changes as needed.
• Software - Use of specialized design software for timber structure analysis and design.

Key Design Considerations:

• Moisture: Account for the effects of moisture content changes.

• Connections: Connections are often the weakest part of a timber structure; meticulous design is
essential.

• Deflection: Be aware of deflection limits to ensure proper serviceability of structures.

• Buckling: Long slender members are susceptible to buckling and must be designed to avoid it.

• Code Compliance: Always ensure compliance with local building codes and standards.

In summary, designing timber structures requires a good understanding of timber's unique

properties, careful planning for connections, and adherence to building codes. It's a versatile and
sustainable approach to building that can create strong and aesthetically pleasing structures.