Bending deflection factor 0.017

Does anyone know how this number is derived? Its in the ISO rules for calculation the required minimum outside skin inertia. I just can figure out where its come from.

Thanks for any help in advance.

 

I'll bet it is a function of the material yeild stress and the effective shell beam span to depth ratio. Simply, it is the limit of comfort of deflection were the center eccentricty still allows for simple beam theory given the Euler end fixitivity conditions. If you design a shell so the maximum deflection is 1.7% of the span between bulkheads, you don't have to worry about secondary load effects due to loss of geometry.

 

Hi, Thanks for this. Yes you are right. I also presume this stays within reasonable limits for small deflection theory too...

 

I have over 35 years experience as an engineer, and that sounds about right. But the terminology is kind of obscure, rules terminology is always problematic.

I suspect the idea is with more skin support, you can have a thinner skin between supporting frames/stringers etc. So a limit of deflection of 1.7 percent is reasonable. the deflection is a function of the loads, the material properties, and the geometry. The problem is when you have a buckling condition, like on a slender column or a hollow structure, it become difficult to predict when failure will occur, if the geometry is maintained it will carry relatively high loads, but if it buckles, it will hold almost no loads. Try standing on an empty aluminum beverage can, and than tapping lightly on the side, it will hold your weight until you deflect the side wall just a tiny bit.

 

That's the analogy I was going to use, pointing out that, depending on beverage selection, removing the hydrostatic support could be the most fun of the demonstration.

 

Yes, it is, If you have access to a copy of the AISC Steel Construction Manual there is a rule for camber and deflection for beams and girders relating maximum permitted stress to depth:span ratio.

 

BTW, there is also a bending deflection factor for FRP stringers. But this time, the value has been fixed to 5% . (K1s).

The maximum deflection is only used for sandwich plating and fiberglass stringers, and not for other materials (wood , metals).

It is also not used when you compute FRP plating or stringers with annex H.

 

"Make it thicker used to be the old adage" if it looks right it usually is.
Worked for thousands of ships in the past.