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Principles and Methods for Reinforced Concrete

 

singly reinforced concrete beam 

Reinforced concrete is produced when steel reinforcement bars ("rebars"), plates or fibers are  incorporated into concrete. Plain Concrete itself is very weak in tension. Reinforcement gives extra tensile strength to concrete. The advent of reinforced concrete brought into existence a lot of high rise and cost-effective buildings with reduced construction duration. Reinforced concrete can be classified as (i) precast concrete and (ii) cast in-situ concrete.

Reinforced concrete can be used for  many types of structures and components, including beams, slabs, walls, columns, foundations and more.

Principles of Reinforced concrete

The principles and methods for reinforced concrete are being constantly revised as a result of theoretical and experimental result.

Two philosophies of design have been established;

1. Working Stress Method with a focus on conditions of service load. (It has become obsolete now).

2. Strength Design Method with a focus on conditions at loads greater than service loads when failure may happen.

  • Strength Design Method has adopted two load factors; U (overload factor) and φ (strength reduction factor) for safety considerations.

  • Strength design method requires that  Design Strength (axial force, shear force,bending moment) should be greater than or equal to Factored Load (required strength).

  • Design Strength is obtained by multiplying the nominal strength by strength reduction factor ( φ).Nominal strength of an element is calculated by static equilibrium and compatibility of stress and strain.

  • Factored loads are obtained by multiplying the service load by overload factor U.

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Analysis of Reinforced Concrete Beams

The nominal strength of a reinforced concrete beam is expected to be achieved when the strain in the extreme compression fiber is equal to the crushing strain of concrete (0.003).

There are two basic modes of failure of flexural reinforced concrete section;

(i) Concrete controlled (brittle mode) also known as over-reinforced: The stress in steel is less than the yield stress. In this mode of failure a sudden collapse occurs. It is not desirable.

(ii) Steel controlled (ductile mode) also known as under reinforced: yielding takes place in steel i.e., The stress in steel reaches the yield stress and the strain in steel is ≥ yield strain. The maximum compressive strain in concrete is less than the failure strain.

Balanced condition is an intermediate point between brittle and ductile failure modes. The amount of reinforcement required for a balanced section is known as a balanced reinforcement. The designer must select the amount of reinforcement less than the balanced reinforcement in order to ensure that brittle failure does not take place.

Reinforced concrete beam can be singly reinforced (rebar only in tension zone of the section) or doubly reinforced (rebar in tension and compression zones of the section).

 To learn more about strength of reinforced concrete beam please visit solved example 9-1 , example 9-2, example 9-3

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Last updated on Monday March 25, 2013