Introduction to Laterally Unsupported Beams
In the case of a laterally unsupported beam, there is a chance of lateral-torsional buckling. So, before determining the full bending stress of the section, the lateral-torsional buckling moment must be found.
The torsional buckling happens in steel rolled sections but not in Reinforced Cement Concrete (RCC) or stocky sections.
The torsional buckling occurs due to three factors:
(1) Cross-sectional shape (2) Support conditions (3) Effective length of the beam. So, lateral buckling strength is calculated based on those factors.
To find the design bending strength of a laterally unsupported beam, the bending stress reduction factor and the bending section modulus must be known.
When a beam is laterally unsupported, its design bending strength reduces, compared to a laterally supported beam.
The basic steps in designing a laterally unsupported beam are:
Calculate the service load and factored loads on the beamCalculate the trial plastic section for the beamSelect a suitable section based on increased plastic section modulusCheck the beam for shearCheck beam for deflection as per the codal provisionsCheck the beam for web bucklingCheck the beam for web crippling
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