Built the base using 10inch 16ga studs
Can you define this better?
FYI, a stud is a component of a framed wall that is positioned vertically. There is no "gauge" to a wooden stud.
A joist is a framing member that is positioned horizontally and is used to support floors and ceilings.
A 2x4 joist will not support much weight at any span over 4'. It is better to use larger joists as the price difference is not great and they will be able to handle the loads involved.
No part of a wooden frame should contact the ground. That includes AC2 treated wood and other species like Cedar and Redwood that have some resistance to moisture. If typical measures aren't indicated to support decking, bricks can be used and will elevate the frame enough for added durability. Placing wood on the ground will lead to rot but some folks may wish to buy wood, build it, and then burn it when done. YMMV