When deciding for the beam section to have, the first consideration to be done is what purpose it need to serve i.e. what kind of loads it will deal with. In general the following cross section choices are available for a beam in market –
- Rectangular/Square Section
- Box/Hollow Rectangular Section
- Circular Section
- Hollow Circular Section
- Channel/C Section
- I Section
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A beam in general may have to deal with the following loads or a combination of them:
For simplicity we will consider separate and singular application of each load on different cross section beams.
To calculate the effect of axial loads on the beam the factors to be considered are applied Force/Load and cross section area of the beam. Considering same weight sections and same material beam means the cross section are is same for all and hence for axial load purpose all sections are equally effective.
Shear stresses developed on the beam are also dependent on Force applied and cross-sectional area.
Hence, it is quite evident that for same weight and material section the choice for structurally better section is governed by the bending stresses developed on the beam.
Bending Stress is defined as the stress induced in the beam as a result of external bending moment applied on the beam. It can easily be calculated by following Equation which is derived from the famous bending equation:
σb = (Mc/I)
Where M – Moment caused by the applied force/load
I – Second Moment of Area of the cross section of structure
c – Distance of extreme fiber from the neutral fiber
σb – Bending Stress
Therefore the lower the bending stress for same load, the better the section would be structurally. Hence I (Second Moment of Area) is very crucial as the greater its value the lower will be bending stresses developed in the beam. When calculated the value of I is maximum for the I-section, C-section and the box section. However for I-section the moment of area remains fairly constant along other axes as opposed to other sections.
Hence due to its better load bearing capabilities and also symmetric design I-section is preferred over other sections.