Any wood, metal, or even concrete stretched out too far without support will become springy. Although a springy floor may not be in danger of breaking through, there are construction regulations designed to avoid the trampoline effect. Span tables are developed for the building code by the Canadian Wood Council to give acceptable distances between supports for each type of floor assembly. If the supports are too far apart for the size of your floor joists and you cannot simply support them underneath with another beam and posts, you can remove some of the spring from a floor by making it more rigid. Adding or re-nailing cross bracing in-between the joists will help, as will nailing 1×3 strapping across the joists. In new construction, gluing the subfloor to the joists or using a thicker subfloor in addition to bracing will stiffen the floor even more.