Downbearing / crown support question.

[Ric Brekne]

I suppose most of the rebuilders are already on the road to Rochester... 
but I thought I'd ask anyways....

Assuming for the moment that one could provide adequate and total 
support for crown and downbearing completely independant of ribs in a 
standard single ply panel.... what would the primary design 
considerations for ribs be ?

Cheers
RicB