As an addendum. Putting a rib
transverse, on the layup
I described, will not help much, except directly above the rib
. If using my layup, and you really want a rib, put it longitudinally, across the uni, and that'll help spread the load. Be sure your biax is 45 degrees each way to the uni.
If you don't want to use uni, and are using 0/90 glass, then I'd not bother with the biax, as it's just going to add weight. Use a total of about 2000gsm of 0/90, on each side, and install your rib, and you'll be fine.
Resin infusion gives 1mm thickness for 1000gsm of glass. Hand layup is about 1mm thick for 600-750gsm of glass, depending on how much you squeegy.
1000gsm is about 34oz (someone correct me here)
My apologies if you're not familiar with the metric system, but that's how we built our boat
. 1mm is 1/25th of an inch.
Cheers, and good luck.