Lateral torsional buckling is a buckling phenomenon observed in unrestrained beams. When a beam subjected to loads results in both lateral displacement and twisting, then it is said to undergo lateral-torsional buckling.
More causes and features of the lateral-torsional buckling phenomenon in beams are explained below.
Causes of Lateral Deflection in Beams
Lateral torsional buckling is observed in steel beams that are unrestrained. Unrestrained steel beams are beams whose compression flange is free to move or displace in the lateral direction and also to rotate.
The figure-2 below shows the lateral movement of the compression and tension flange. Under the action of load, the compression flange tries to deflect away laterally. At the same time, the tension flange tries to maintain the beam straight.
Certain restoring forces are created due to the lateral bending of the beam. These restoring forces oppose the lateral bending of the beam and try to keep it straight. These restoring forces alone cannot keep the beam straight. So, the lateral component of tensile forces ( Tension flange) together with the restoring forces determines the resistance of the beam against buckling.
Causes of Torsional Effect in Beams
The forces within the flanges not only cause the beam to deflect but also to twist about the longitudinal axis of the beam, as shown in figure-3. The resistance to twisting of the beam section is dependent on the torsional resistance of the beam section.
The torsional stiffness is governed by the thickness of the beam flange. Hence, the beams with large flange thickness have larger bending strength compared with the beams with the same depth but lesser flange thickness.
Factors Affecting Lateral Torsional Buckling in Beams
Three main factors that influence the lateral-torsional buckling in beams are the:
- Location of the Load Applied
- The Shape of Bending Moment Applied
- End Supports
1. Location of the load applied
The susceptibility of the beam section to the effects of lateral-torsional buckling is governed by the distance between the location of the load applied and the shear center of the section.
The section is more prone to lateral-torsional buckling if the load is applied on a point above the shear center of the section. The effect is less if the load applied is along the shear center. The application of load below the shear center has very little chance to undergo lateral-torsional buckling.
Those loads applied above the shear center is called as destabilizing loads. The loads applied at or below the shear center are called as non-destabilising loads.
Also Read: What is Shear center?
2. The Shape of Bending Moment
A section with uniform bending moment along the length has less buckling resistance compared with the section subjected to different bending moment distribution.
3. End Support Conditions
When the end supports of the beam section are more restrained increases the buckling resistance. An end supports that offer less restraints have a decreasing buckling resistance.