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
1. Assess loads (service loads) (Building Code Requirements)
2. Use linear elastic analysis techniques to obtain the resulting internal forces (load effects): bending, axial force, shear, torsion
At service loads: σ_max  ≤ σ_all
e.g.   compression in bending
 0.50 σ = f_y 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:
  (a) strength at ultimate load
  (b) deflections at service load
  (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,γ_i, and capacity reduction factors, φ.
o Load factors – ACI 318 Building Code

• Load combinations
  U = 1.4(D + F)
  U = 1.2(D + F + T) + 1.6(L + H) + 0.5(L_r 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
   resistance ≥sum of load effects
  For a structure loaded by dead and live loads the overall safety factor is
   

→ Making of concrete
       o Cements 

• Portland cements
• Non-portland cements

      o Aggregates – Coarse and fine
      o Water

      o Chemical admixtures

• Accelerating admixtures
• Air-entraining admixtures
• Water-reducing and set-controlling admixtures
• Finely divided admixtures
• 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

→ Advantages and disadvantages of concrete
    o Advantages

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

    o Disadvantages

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

→  Properties of steel reinforcement
    o Young’s modulus, E_s
    o Yield strength, f_y
    o Ultimate strength, f_u
    o Steel grade
    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