Eccentric Bending Moment on WT

[humanengr]

When calculating the moment acting on a WT due to eccentric axial load applied to the flange of the WT:
Can I use eccentricity = Yp (distance to Plastic Neutral Axis, PNA),
or is it required to use Y (distance to geometric neutral axis ?

 

[canwesteng]

Good question... I would say no. The eccentric moment occurs because of the distance between the location of the axial force in the member and the location of the applied force. The centroid of the axial stress in the member will still occur at the geometric centroid of the member, even if you can develop the plastic moment capacity of the section.

 

[KootK]

I think that the trick here is to recognize that moments are location independent. As long as you've properly accounted for everything, you can take your moments about the elastic centroid, the plastic neutral axis, or Sweden. The only reason to choose one location over another is computational efficiency.

In summary, there's no "right" place about which to take the moments and also no wrong place. You merely need to choose a location for computational purposes and then ensure that the internal stresses on your WT cross section produce a moment that balances the moment produced by the external load. With the internal stresses in hand, you're off to the races as far as member design goes and where you choose to take you moments from becomes of little consequence.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

If a member carries an axial load P and a moment M, the eccentricity is M/P and is normally understood to be measured from the geometric neutral axis.

BA