The following diagram is a rough sketch for a conceptual idea of a sub-structure. Not drawn to scale, angles are not correct. Only for demonstration purposes.
Idea: Outside frame made of 2″x12″ boards. On two opposite sides of the frame will be a “lip” made of 2″x4″ or 2″x6″ boards. I-Beams (pre-fabbed or made on site) will rest on the “lip” and be attached. Top of the I-Beams will be flush with the top of the frame. OSB board (or other sub-floor material) will be attached to both the frame and the I-Beam tops. This will serve as the base structure which will be attached to the foundation, and also have the walls built upon it. After completion of walls, final flooring surface will be applied to the sub-structure. Pending on the span, bracing may be needed under the I-Beams. Under 20′ spans may not need bracing. I-Beams are mostly held together and attached to the frame via liquid nails. Framing nails and hangers can also be utilized. Size of boards/thickness needs to be determined for needs.