Bednar support lug top plate question

[McT178]

Bednar's method of determining the stress in the top plate of a support lug has me stumped (page 154, 2nd edition). Bednar uses Fd/h to determine the force on the plate. This would be a force on the radial plane of the vessel. This force is then transformed into a moment by multiplying it by the width of plate (can't figure out why), and the section modulus seems to imply that bending is in the plates strong axis. I would think a buckling equation similar to the gusset design would be better suited. I just don't like to use equations I don't understand. What am I missing?

 

[JStephen]

The assumption is that the whole chair assembly is trying to rotate around the bottom, which will give an outward force acting on the top plate. It is then treated as a small beam spanning between the side plates with uniform load.

 

[McT178]

Thanks for the reply. The moment equation is given as Fda/8h; where Fd/h is the outward force you described. I would understand this better if Fd/8h was divided by "a" (lug width) to give a distributed load on the top plate and then the distributed load was used to determine the moment. Is the bending vertically between the side plates or more of a buckling?

 

[JStephen]

The equation is for beam bending in a horizontal plane.

 

[McT178]

OK..so it is sort of a simplified ultra conservative method that does not account for any welding to the vessel that would resist the bending. I guess this is fine because the top plate is not as important and it wouldn't cost much to over design.