Column Buckling

[ridgeline]

When you consider the 'unbraced length' on a column when analyzing buckling potential, how would you determine if a particular 'brace' provides enough support to actually be considered a 'brace'?

For example, I have a 16' long column type memeber pinned at each end with an 8 kip compressive force acting on it in the y-direction (see attached sketch). It is fully braced in the z-direction and has connections at 4' intervals that provide 300lb of bracing each way in the x-direction. How would you determine the bracing potential of these braces?

 

[BAretired]

It depends on the code you are using. But, probably it is safe to assume that, if the brace can take 2% of the vertical load, it is adequate. In your case, that would be 0.16k at each brace point.

If you want to go into more detail, CSA S16-01 (the Canadian Standard) gives you more precise methods for calculating brace forces and deformations.

BA

 

[ridgeline]

If, for example, the braces could only handle 1% of the vertical load (and thus could not be considered a full brace), is it possible to analyze their potential contribution to buckling resistance? Could you possibly consider a reduction in your total unbraced length? How would you quantify that?

 

[BAretired]

It is not just a question of lateral force capacity. It is also a question of deformation. Consider a column, hinged at each end and hinged and laterally braced at mid height.

If the central node is eccentric from the two end nodes, it requires a lateral force to maintain equilibrium. The force can readily be determined from statics if the eccentricity is known or assumed.

If the brace is very stiff, there is little movement and the initial calculation of brace force is close. If the brace is very flexible, the central node moves and the force required to restore equilibrium increases.

The procedure originally outlined by George Winter and included in CSA S16-01 considers both aspects. There is not enough room in this forum to include the entire procedure, but you can find it in the CSA code.

Two percent is usually pretty conservative. You may be okay with one percent if the brace deformation is small.

BA

 

[Lion06]

Don't forget about brace stiffness requirements. See App.6 in AISC 360-05.