Steel beams are prone to buckle along its length. In the case of a simply supported beam, this is prevented by restaining its compression flange, which prevents it from rotating along its axis. This is known as lateral-torsional buckling. First, we should look at the restrain, which dictates the effective length of the beam. This in turns governs the size of the member. The general rule to classify a restraint that prevents the compressive flange from rotating is to resist a force equivalent to 2.5% of the ultimate compression load in the top flange of the beam it is restaining. For intermediate restraint, simply tying to an adjacent beam is not stiff enough, laterally load must direct into stiff elements such as bracing and walls must exist.
If the beam does not have compressive flange restraint, end condition must be established. Beam end can be laterally, torsionally and rotationally restrained. EC3 establishes a reduction factor that is applied to the bending moment resistance of the beam.
Steel material properties: Density of steel is 7850kg/m3 and the Young Modulus (E) is 210,000N/mm2. Steel with a nominal yield strength of 275N/mm2 and 355N/mm2 are typically used. Element yield strength reduces as section max thickness increases.
Classification of beam sections: Clause 5.5.2 in BS EN 1993-1-1 groups steel beams into four classifications. These classifications are based on a steel beam section’s resistance to suffering from a local failure due to buckling. It is a subject of the section web and flange thickness.
Shear capacity: The majority of the shear force is applied to its web. If shear is failing, stiffeners can be installed to support the high shear load. Steel shear strength is 1/sq(3) yield strength.
Bending moment capacity: Governed by section modulus and yield strength. Bending moment resistance is reduced if the applied force is more than half of the plastic shear resistance of the beam. When this happens, Clause 6.2.8 of BS EN 1993-1-1 applies. This places a modification factor against the yield strength. This only applies regionally to the part of the beam where high shear occurs. Class 4 section will need further guidance from BE EN 1993-1-5.
Lateral torsional buckling: In the case of an unrestrained portion of a beam, a factor is applied to the bending moment resistance to account for the risk of LTB.This reduction is related to the slenderness of the beam. This relates to the distance between restraints at the compressive flange of the element. This can be calculated using the red book method. However, for destabilising load, a more detailed method must be used.
Temperature load: For beams that are subject to temperature difference, the element is prone to expand and shrink. When deflection is restrained by stiff elements, large axial forces will result from temperature load.
Deflection: For long-span beam, serviceability becomes an issue. The vertical deflection limits for steel beams can be found in the clause NA.2.23 of the UK NA to BS EN 1993-1-1. Deflection for total load and live load are the two typically checked item. Global deflection should also be considered, which is very often missed.