Introduction to Reinforced Concrete

Reinforced concrete is a composite material in which concrete's relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength or ductility.

Materials

Concrete

Compressive Strength ( $f'_c$ ): The primary property, typically ranges from 20 MPa to 100+ MPa.
Tensile Strength: Very low, approximately 10% of compressive strength. Ignored in flexural design.
Modulus of Elasticity ( $E_c$ ): $E_c = 4700\sqrt{f'_c}$ (for normal weight concrete).
Creep & Shrinkage: Time-dependent deformations that must be accounted for.

Reinforcing Steel

Yield Strength ( $f_y$ ): The stress at which steel begins to deform plastically. Common grades: Grade 40 (276 MPa), Grade 60 (414 MPa).
Modulus of Elasticity ( $E_s$ ): Taken as 200,000 MPa.
Ductility: Allows the structure to deform significantly before failure, providing warning.

Design Codes

Design in the Philippines typically follows the National Structural Code of the Philippines (NSCP 2015), which is heavily based on the American Concrete Institute (ACI 318) code.

Design Philosophy: LRFD

The Load and Resistance Factor Design (LRFD) or Ultimate Strength Design (USD) method is used.

$\phi R_n \geq \sum \gamma_i Q_i$

Where:

$\phi R_n$ : Design Strength (Nominal Strength $\times$ Strength Reduction Factor)
$\sum \gamma_i Q_i$ : Required Strength (Factored Loads)

Basic Load Combinations (NSCP 2015)

$U = 1.4D$
$U = 1.2D + 1.6L$
$U = 1.2D + 1.0L + 1.0E$ (Earthquake)
$U = 1.2D + 1.0L + 1.0W$ (Wind)

Solved Problems

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