Lateral Torsional Buckling
Mandeep Singh Kohli, PMP?
Energy | EPC | Project Management | Oil & Gas | Green Hydrogen | SAF | Civil & Structural | Modularization
Lateral Torsional Buckling or LTB is a concept which many young engineers find confusing.
Is it Buckling ?
Is it Torsion ?
Is it Minor axis bending ?
A very common definition isLateral Torsional Buckling is :
"LTB is a buckling mode which occurs because the compression flange of a cross-section buckles under action of flexural compression"
Then how can one explain the phenomena of Lateral Torsional Buckling of a Cantilever beam, where it is usually the 'Top Flange' or the 'Tension Flange' which buckles and restraining the 'Bottom flange' which is under 'compression' is not of much help.
And why would a section which is otherwise stable suddenly buckle, why would something want to buckle or change its shape ?
Energy Considerations
Every object has a tendency to achieve a state of lowest energy, objects which are at a height have higher energy, and if they come down their potential energy reduces. Similarly buckling can be understood as a cross section's tendency to buckle so that it attains a state of lower energy.
When an I section undergoes Lateral Torsional Buckling two things happen.
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One it moves laterally?and second, it rotates about a point below its bottom flange. The load by virtue of the combined lateral movement and rotation moves downward, to a state of lower energy.
Because such a lower energy state is possible, Lateral Torsional Buckling takes Place.
This also helps to explain the fact, why a load applied to the bottom flange will lead to a higher LTB capacity.
Since a load applied at the Bottom flange will have a tendency to restore the section back to its original position, it requires more energy to cause LTB and hence results in a Higher LTB Capacity of the cross-section.
Whereas if it the same load is applied at the TOP flange it will tend to further destabilise it, and result in a lower LTB Capacity.
In the end it's all about the amount of energy required to make a member undergo LTB. Buckling will happen as long as the resultant state is of lower energy.
Building on this, don't assume that it's always the compression flange which will buckle, for cantilever members it could be the tension flange which we need to brace to prevent buckling.
Hope you find the above discussion useful. Please share your comments & feedback.
Structural Lead, H2 Green Steel, Fluor New Delhi
1 年Nice Article Mandeep. Later torsional buckling effect for columns governing in bending is also critical especially when column depth is too deep say 1100 or 1500 mm and longitudinal beams depth is small say 300 or 400 mm. Share your thoughts on that. Can we break column bucking lengths at such locations? Does providing stiffeners and connecting top and bottom flanges help us to break lateral torsional buckling lengths of columns?
Senior Civil Structural Engineer | Team Leadership | Structural Design | Aff.M.ASCE | C.Eng (I)
2 年"Building on this, don't assume that its always the compression flange which will buckle, for cantilever members it could be the tension flange also." Mandeep Singh Could you please elaborate on this.
Senior Civil Structural Engineer | Team Leadership | Structural Design | Aff.M.ASCE | C.Eng (I)
2 年This is a good read and clarifies the concept well. I am just adding my two cents here.
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2 年For cantilever when load applied at Top, bottom flange is under compression & applied load at Top makes the Top flange to undergo LTB. If the same load applied at bottom flange it gives higher LTB capacity to balance the potential energy.? Simpler terms load should be applied at lowest post of cross section to achieve minimum Potential energy for stability.? Great explanation ??