A beam is a structural member which has one dimension greater than the other two and placed in a horizontal plane. It's a flexural member which can take bending moment and shear force. The cross-section of a beam can either be rectangular or T-shaped or circular etc.
To resist the bending moment and shear force, an adequate section is required which is neither too bulky nor too small.
The geometry of the section of the beam depends on section modulus and section modulus itself depends on the moment of Inertia of that section.
IS 456 has given the criteria for the selection of the section of a beam which is based on the deflection of the beam.
As per IS 456:2000 the ratio of the span and effective depth should not be greater than 20, 7 and 26 for the simply supported beam, cantilever beam and continuous beam respectively. These values are valid for span up to 10 m above which these values must be modified by multiplying them with (10/span in meter).
Effective depth of a beam is the depth of the beam from the top fibre to the CG of the reinforcement provided in the tension zone.
The difference between the overall depth and effective depth is called effective cover.
And, the Nominal cover is provided so that the steel bars are fully embedded, and it is not exposed to exterior conditions like rain. It also helps in maintaining the required connection between concrete and steel bars.
IS 456:2000 has provided the values of Nominal Cover on the bases of exposure condition. Click here
There are 3 types of sections on the bases of stress condition of concrete and that of steel.
To resist the bending moment and shear force, an adequate section is required which is neither too bulky nor too small.
The geometry of the section of the beam depends on section modulus and section modulus itself depends on the moment of Inertia of that section.
IS 456 has given the criteria for the selection of the section of a beam which is based on the deflection of the beam.
As per IS 456:2000 the ratio of the span and effective depth should not be greater than 20, 7 and 26 for the simply supported beam, cantilever beam and continuous beam respectively. These values are valid for span up to 10 m above which these values must be modified by multiplying them with (10/span in meter).
Effective depth of a beam is the depth of the beam from the top fibre to the CG of the reinforcement provided in the tension zone.
The difference between the overall depth and effective depth is called effective cover.
Effective cover = nominal cover + diameter of stirrups + half the diameter of main reinforcement steel bar.
And, the Nominal cover is provided so that the steel bars are fully embedded, and it is not exposed to exterior conditions like rain. It also helps in maintaining the required connection between concrete and steel bars.
IS 456:2000 has provided the values of Nominal Cover on the bases of exposure condition. Click here
Sections of beams
There are 3 types of sections on the bases of stress condition of concrete and that of steel.
- Under Reinforced Section: When the permissible stress in steel is reached in a beam prior to that of concrete.
- Balanced Section: When the permissible stress in steel and that of concrete reaches at the same time.
- Over Reinforced Section: When the permissible stress in concrete reaches prior to that of steel.
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