What is the purpose of sub floor bracing?
Bracing needs to be provided at all levels to work. Imagine a building in a wind that had bracing to resist the wind at the roof and wall levels, but not at the subfloor level. The side load of the wind could be transferred through the roof and walls, but when it gets to the subfloor the piles have no way to transfer it to the ground because they are just designed to resist gravity loads. So they will fail in strong winds. It is a bit like a skycraper with columns on every level but one. That level has nothing to resist the gravity so it will fail.
