The section modulus describes the relationship between the bending load and the bending stress. I've shown a somewhat arbitrary load in the figure below, but any ground or air or handling load will introduce loads to the ribs. For that matter, load will go in to the ribs from the rear spar. Also, if the load is indeed from the fuel, unless the tank is attached to the top cap of the rib, all the shear force will be on the bottom cap, because the load path round the back of the rib and then forward across the top cap appears relatively flexible compared to the more direct load on the bottom cap.
The shear is the actual force being carried and I showed that in red. The blue vertical arrows represent the reaction shear forces due to this load, while the horizontal arrows represent the reaction forces on the spar from the load. In your photo, the shear is carried into the spar by the flange on the front of the rib, and the fasteners between the rib and spar there, and that appears to be in order. But there's very little material at the forward end of the ribs to connect to those flanges, and it's not clear how that shear load is transferred.
If the stiffness at those points is considerably lower than the beefed-up rib flange assembly, the forward end of the ribs can act as a hinge. In that case, those ribs, having little shear stiffness, can't carry much load.
This stands in contrast to the significant amount of material and the generous radius of the forward lightening hole at the front of the RV-12 ribs, as the factory designed them. Also, note the formed bead around the hole, which helps prevent local buckling.
Hence my post.
Dave