All masts, whether free-standing or stayed, are designed to the boat's transverse maximum righting moment because the engineering determination of that load is so easy to calculate and it results in the predominant load on the mast and rig. It is immaterial what the actual wind speed, wind pressure distribution and boat's heel angle are. A boat is a free-floating body, and the heeling moment is always equal and opposite to the righting moment, so we go with the righting moment as the load to greatly simplify the problem.
In the case of carrying a spinnaker downwind on a free-standing rig, the potential exists for forward tripping of the boat stern over bow, but this condition is so unstable and transitory that it quickly reduces to a bad broach as the boat suddenly rounds up into the wind. The bending stress in the fore/aft direction rarely, if ever, has time to increase to a suddenly high and momentary level. It reduces quickly to a high transverse load as the boat broaches. Therefore, the fore/ast situation is usually ignored in the engineering of the mast. As I stated earlier, it is simply prudent to set a running backstay when flying a spinnaker to keep the mast stable and from pumping too much which could increase cyclic loading beyond normal levels. If I know a spinnaker is going to be carried, I will usually check that situation just in case.
Tom's note above about a Freedom losing a mast on flying a spinnaker is interesting as I would not have expected that since the Freedom masts were usually built pretty heavy. There was a situation back in the late 80s or early 90s were a batch of bad masts when through the TPI factory, and a number of them broke in relatively benign conditions. I can't remember which model of Freedom it was, perhaps the Freedom 35, and I wonder if that was the situation there.
Eric
Shock loads from a spinnaker can be much larger off the wind than a main and jib upwind. The boat can heel further, the mast can bend more, both relieving shock loads upwind. Longitudinal hull stability is much greater than righting moment and the mast has to take almost the whole shock load. Also, the spinnaker usually exerts the highest load on the mast on a reach. I saw a Freedom loose its mast while gaining on me in a race. They were late in setting the new running back after a jibe.
