Understanding Stresses and Neutral Axis in Structural Design

[VooDoo]

Hey guys,

A picture of the question is attached, I am not seeking answers just an explanation of some theory.

Now I understand that

point A will be under compression due to the weight, tension due to the bending and shear due to torque produced by the wind

point b will be under compression due to weight, tension due to bending (but different tension to point A because of the different moment) and shear due to torque produced by the wind

point C will be under compression due to weight and compression due to bending and shear due to torque produced by the wind

Now when applying the bending stress equation (MY/I) wouldn't either point B and C together or point A on its own lie on the neutral axis? And hence the bending stress would be zero Or would Y (distance from the neutral axis) always be 50mm?

http://img23.imageshack.us/img23/5238/1233333333.th.jpg

 

[nvn]

VooDoo: If the top edge of the sign is parallel to the y axis, and the x-axis is vertical, then points B and C lie on the neutral axis for moment My, and point A lies on the neutral axis for moment Mz.

 

[Astronuc]

If the wind is blowing into the sign (into the page/screen), the A will be under tension. The force would apply a torque about the axis through BC. If the wind blows out of the scree/page, then A would be under compression.

One has to look at the distance a point is from the axis of rotation, and the torque applied with respect to that axis.

The post and sign apply a downward load on the base. Torques apply a tension or compression depending on whether they rotate away from or toward a point.

The base is a cantilever joint.

 

[PhanthomJay]

Annd don't forget the longitudinal shear from the wind load, maximum at the transverse neutral axis.