Introduction, Design Criteria for Reinforced Concrete Structures

# Introduction, Design Criteria for Reinforced Concrete Structures | RCC & Prestressed Concrete - Civil Engineering (CE) PDF Download

Introduction / Design Criteria for Reinforced Concrete Structures

• Structural design
• Definition of design:

Determination of the general shape and all specific dimensions of a particular structure so that it will perform the function for which it is created and will safely withstand the influences which will act on it throughout its useful life.

→ Principles of mechanics, structural analysis, behavioral knowledge in structures and materials.

→ Engineering experience and intuition.
→ (a) Function, (b) strength with safety requirements will vary for structures.
→ Influences and structural response:

→  Structural mechanics:
A tool that permits one to predict the response (with a required level of accuracy, and a good degree of certainty) of a structure to defined influences.

→ Role of the designer (engineer) of a structure

→ Design criteria for concrete

Two schools of thoughts
1. Base strength predictions on nonlinear theory using actual σ- ε relation

• 1897 – M.R. von Thullie (flexural theory)
• 1899 – W. Ritter (parabolic stress distribution theory]

2. Straight-line theory (elastic)

• 1900 – E. Coignet and N. de Tedesco (the straight-line (elastic) theory of concrete behavior)

→ Working Stress Design (WSD) – Elastic theory
2. Use linear elastic analysis techniques to obtain the resulting internal forces (load effects): bending, axial force, shear, torsion
At service loads: σmax  ≤ σall
e.g.   compression in bending
0.50 σ = fy flexure

o Ultimate Strength Design (USD)

• The members are designed taking inelastic strain into account to reach ultimate strength when an ultimate load is applied to the structure.
• The load effects at the ultimate load may be found by
(a) assuming a linear-elastic behavior
(b) taking into account the nonlinear redistribution of actions.
• Sectional design is based on ultimate load conditions.
• Some reasons for the trend towards USD are
(a) Efficient distribution of stresses
(b) Allows a more rational selection of the load factors
(c) Allows designer to assess the ductility of the structure in the post-elastic range

o Limit State Design

• Serviceability limit state:
Deformation, fatigue, ductility.
• Ultimate limit state:
Strength, plastic collapse, brittle fracture, instability, etc.
• It has been recognized that the design approach for reinforced concrete (RC) ideally should combine the best features of ultimate strength and working stress designs:
(c) crack widths at service load

o ACI (American Concrete Institute) Code emphasizes:

• (a) strength provisions
• (b) serviceability provisions (deflections, crack widths)
• (c) ductility provisions (stress redistribution, ductile failure)

→ Design factors
o 1956 – A.L.L. Baker (simplified method of safety factor determination)
o 1971 – ACI Code (load factors and capacity (strength, resistance) reduction factors)
o 2002 – ACI 318 Building Code
o Design loads (U) are factored to ensure the safety and reliability of structural performance.
o Structural capacities (φ) of concrete material are reduced to account for inaccuracies in construction and variations in properties.
o Semi-probabilistic design is achieved by introducing the use of load factors,γi, and capacity reduction factors, φ.
o Load factors – ACI 318 Building Code

U = 1.4(D + F)
U = 1.2(D + F + T) + 1.6(L + H) + 0.5(Lr or S or R)
U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.8W)
U = 1.2D + 1.6W + 0.5L + 1.0(Lr or S or R)
U = 1.2D + 1.0E + 1.0L + 0.2S
U = 0.9D + 1.6W + 1.6H
U = 0.9D + 1.0E + 1.6H
where D = dead load; F = lateral fluid pressure; T = self-straining force (creep, shrinkage, and temperature effects); L = live load; H = load due to the weight and lateral pressure of soil and water in soil; Lr = roof load; S = snow load; R = rain load; W = wind load; E = earthquake load.
• ACI 318-02 also provides exceptions to the values in above expressions. o Capacity reduction factors – ACI 318 Building Code ƒ Members subject to structural actions and their associated reduction factor (φ) Beam or slab in bending or flexure: 0.9 Columns with ties: 0.65 Columns with spirals: 0.70 Columns carrying very small axial loads: 0.65~0.9 for tie stirrups and 0.7~0.9 for spiral stirrups. Beam in shear and torsion: 0.75
• Relation between resistance capacity and load effects

→ Making of concrete
o Cements

• Portland cements
• Non-portland cements

o Aggregates – Coarse and fine
o Water

• Polymers (for polymer-modified concrete)
• Superplasticizers
• Silica-fume admixture (for high-strength concrete)
• Corrosion inhibitors

→  Raw material components of cement
o Lime (CaO)
o Silica (SiO2)
o Alumina (Al2O3)

→  Properties of portland cement components

→ Types of portland cements
o Type I: All-purpose cement
o Type II: Comparatively low heat liberation; used in large structures
o Type III: High strength in 3 days
o Type IV: Used in mass concrete dams
o Type V: Used in sewers and structure exposed to sulfates

→ Mixture design methods of concrete
o ACI method of mixture design for normal strength concrete
o Portland Cement Association (PCA) method of mixture design

→ Quality tests on concrete
o Workability
o Air content
o Compressive strength of hardened concrete
o Flexural strength of plain concrete beams
o Tensile strength from splitting tests

• Ability to be cast
• Economical
• Durable
• Fire resistant
• Energy efficient
• On-site fabrication
• Aesthetic properties

• Low tensile strength
• Low ductility
• Volume instability
• Low strength-to-weight ratio

→  Properties of steel reinforcement
o Young’s modulus, Es
o Yield strength, fy
o Ultimate strength, fu
o Geometrical properties (diameter, surface treatment)

→ Types of reinforced concrete structural systems
o Beam-column systems
o Slab and shell systems
o Wall systems
o Foundation systems

The document Introduction, Design Criteria for Reinforced Concrete Structures | RCC & Prestressed Concrete - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course RCC & Prestressed Concrete.
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## FAQs on Introduction, Design Criteria for Reinforced Concrete Structures - RCC & Prestressed Concrete - Civil Engineering (CE)

 1. What are the design criteria for reinforced concrete structures?
Ans. The design criteria for reinforced concrete structures include factors such as load capacity, durability, serviceability, safety, and constructability. These criteria ensure that the structure can withstand the applied loads, resist environmental factors such as corrosion, maintain its functionality, provide a safe environment for occupants, and be constructed efficiently.
 2. How do load capacity and safety factor affect the design of reinforced concrete structures?
Ans. Load capacity refers to the maximum load that a reinforced concrete structure can withstand without failure. The safety factor is a measure of the margin of safety included in the design to account for uncertainties and variations in loads and material properties. A higher safety factor increases the structural reliability but may result in a larger and more expensive structure.
 3. What is the role of durability in the design of reinforced concrete structures?
Ans. Durability is a crucial aspect of the design of reinforced concrete structures as it ensures that the structure can withstand the effects of environmental factors, such as corrosion, chemical attack, and weathering. Proper selection of materials, design details, and construction techniques are essential to enhance the durability and longevity of the structure.
 4. How does serviceability influence the design of reinforced concrete structures?
Ans. Serviceability refers to the ability of a reinforced concrete structure to perform its intended functions without excessive deflections, vibrations, or cracking. The design must consider factors such as occupant comfort, functionality, and aesthetic aspects. Adequate stiffness, control of deflections, and appropriate reinforcement detailing are important considerations for achieving desired serviceability.
 5. What are the key considerations for constructability in the design of reinforced concrete structures?
Ans. Constructability refers to the ease and efficiency of constructing a reinforced concrete structure. Key considerations include the feasibility of construction methods, availability of materials and equipment, ease of formwork, constructability of reinforcement detailing, and coordination between different construction activities. Designing with constructability in mind helps in reducing construction time, costs, and potential issues during the construction phase.

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