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**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

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