Critical Imperfections for Columns bracing

[D0be]

Hi all,

Initial imperfection (Do)is one of the most important steps to determine the bracing strength and stiffness requirements.

In the AISC 2005, and the number of Yura and Hewig papers show that Do=0.002Lb (Lb: unbraced length. However, this is based on the “full-bracing”- Winter’s model. Actually I do not understand fully about why factor 0.002 was used.

Regarding to nodal bracing system (equal bracing distance), when the axial load reach to the buckling load at each unbraced segments then full-bracing occur. Therefore, we can use Do=0.002Lb to determine the brace stiffness and forces.

My question is if the axial load is smaller than buckling load at each unbraced segment how we can determine Do? Any Ideal about relation between buckle shapes to Initial imperfection (Do)?


Any Ideal is much appreciated.

Thanks

D0be

 

[271828]

Are you asking what brace strength is required if the axial load is less than the buckling load?

If so, then use the Lq concept that's in the last sentence of AISC 2005 Appendix 6.2.

Your post reads like you're confusing initial imperfection with required brace strength.

 

[271828]

Oops, sorry, I meant required brace stiffness.

 

[D0be]

Thanks 271828,

I understand Lq concept (in Appendix 6 -AISC2005) that counts for column flexural rigidity.

My key problem is how to determine the initial imperfection (Do) for the nodal bracing system with many brace points ? say 3 braced points.

If the applied load (P) > buckling load between the braced points [ Pe=(pi)^2*EI/(Lb)^2], how to determine the Do?

If 0 <P < Pe=(pi)^2*EI/(Lb)^2, how to determine the Do?


Do=0.002Lb, right? Can we base on the buckle shapes to determine initial imperfection?

(0.002 =1/500 is based on erection tolerances ASCI Code of standard Practice 2000)

thanks,
Do

 

[271828]

Does the first paragraph of the Commentary to Appendix 6.2 help? It talks about the number of braced points along the column.