Introduction / Design Criteria for Reinforced Concrete Structures
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
2. Straight-line theory (elastic)
→ 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: σmax ≤ σall
e.g. compression in bending
0.50 σ = fy flexure
o Ultimate Strength Design (USD)
o Limit State Design
o ACI (American Concrete Institute) Code emphasizes:
→ 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
→ Making of concrete
o Aggregates – Coarse and fine
o Chemical admixtures
→ 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 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
→ Properties of steel reinforcement
o Young’s modulus, Es
o Yield strength, fy
o Ultimate strength, fu
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