Mistake in Horizontal Saddles procedure (Moss 4-11) ?

Hi, PvElite implements procedure 4-11 by Moss.
For horizontal loads in longitudinal direction, outside and inside ribs are verified by determining a unit force per length "fu", and then using this unit force to check Outside and Inside ribs as if they where beams.

Now, Moss and consequently PvElite define fu as reported in attached image.

I wonder, why defining fu=FL / 2*A ?
Wouldn't it be correct to use FL / A ?

It seems that Moss and PvElite use half the required force per unit length to do the calculation...

Hello fedeghi

If you could make a Service Request using the customer support system in PV Elite, I can best answer your question. Just mark the request for my attention.

I can say here that the method by Moss is very convoluted, and I suspect there is an easier way of doing the analysis.

fedeghi,

I think your "2" is because there are 2 saddles equally loaded by the longitudinally force FL (which is a global on vessel horizontal force loading the saddles).
As a confirmation of Mr. Delaforce opinion about the convoluted development in Moss book, in fig. 4-56 the original intention was to consider FL/2n instead of FL/(2A)*e=fu*e, but this is just a detail.

I would add that in "M" formula the factor 0.5=1/2 is likely to be due to the guided cantilever behavior assumed for ribs, here it is not related to saddles.

In the end, trying to conclude my understanding of Moss model: the horizontal vessel is loaded longitudinally with force FL and connected to base by- lets say about- n guided cantilevers (ribs in left saddle) + n guided cantilevers (ribs in right saddle) and each guided cantilever on the left side has an identical correspondent in the right side. The transverse rigidity of the assembly of guided cantilevers in left side is identical with those of the assembly on the right side. We can conclude that both assemblies (ribs in saddles) are in bending due to FL/2 and after that we try to calculate the load and stress in each rib- guided cantilever. Make sense for you?

Best regards.