SM transverse frame & floor

[DUCRUY Jacques]

Hello,

I am surprised when I compare the requirements for SM of tranverse frame and floor between ABS ORY and ISO 12215-5.

For calculation of SM of a transverse frame, it seem to me that the definition of the "unsupported length" is the same in the 2 methods ... et the design pressure near the same. But ISO use a "curvature factor" ; this factor, for a ratio crown/length of 0.18, is 0.5. Result, the SM ISO is nearly 50 % inferior to SM ABS.

For the SM of floor, the formula are :
W = 183.33 b l² p/allowable stree for ABS
W = 83.33 b l² p/allowable stress for ISO.

In the two cases (frame and floor), the scantling for ISO are inferior to the scantling of ABS. At you opinion, why theses differences ?

Thank-you by advance.

Best Regards


Jacques

[fcfc]

I see your request do not attract much insterest.

Just my own vision of things :

The 183 factor for ORY is for centerline floors. For other stringers it is like ISO, 83.3.

Current ISO (2008) only has 83 , but more than strongly discourage unsupported stringer ends. On previous version of ISO, stringer supported at both end has 83, but stringers only supported at one side had 125. (see what is called end fixity).

So, for me in ISO, a centerline stringer has 83 factor , but NEEDS to be supported at both ends by longitudinal stringers.

For ABS, centerline stringer do not need to be supported at ends, but has a factor of 183. But it is bad design practice because it creates a hard point at plating / stringer end.

NB ISO is an evolution of ABS. ABS ORY is no longer used for sailboats under 80 ft.

[DUCRUY Jacques]

Thank-you very much for your answer.

May I aks you, Please, what is your opinion about the "curvature factor" of traverse frame, who is not take in account by ABS ?

Best Regards


Jacques