Moment Magnification Question 2

[jayson34]

Hi everyone,

Say there's a slender column in biaxial bending with non-sway moment Mx and My. Using the moment magnification method, we need to calculate the Euler buckling load Pex using Kxlx & Ix, and Pey using Kyly & Iy, then find B1x and B1y to magnify Mx and My respectively. My question is: Since the least buckling load will control the design, why not use it (and the corresponding maximum B1) for both directions?

Thank you,
Jay

 

[271828]

B1 is trying to quantify the P-little delta amplification. There is a little delta in each direction. They cause both Mux and Muy to have P-little delta amplifications.

Edit: sorry I kinda misread your question!

 

[BridgeSmith]

It would be easier to do it that way, but it could be overly conservative if the unbraced length is shorter in the direction subjected to the larger moment.

 

[Datpt]

For bi-axial loaded columns, you should not check the strength in two directions separately.
You have to combine the forces together and check the column strengths based on the 3D interaction diagram.

The checking must repeated for all load combinations separately.

In the case where it is difficult to check all load combinations separately, the following cases must be checked:
- Maximum magnified Mx with accompanying N and magnified My.
- Maximum magnified My with accompanying N and magnified Mx.
- Maximum N with accompanying magnified My and magnified Mx.
- Minimum N with accompanying magnified My and magnified Mx.
- A case where magnified My and magnified Mx are both big.