6" steel channel used as floor joist and runners underneath, not sure of spacing.total base frame welded. 3/4" plywood floor, then first layer long sides with 3" log end over hang. used landscape timbers as logs.short ends squared up, mark where short log meet s at corner, trace around on long side, then routered out notch.alternate notch placement each layer from long sides to short. we made jig from sawhorse n groved 2x4.
we had to measure each log to keep our 12'x8' frame, logs not uniform or straight. most spikes were 6" long and we used longer spikes on warped logs to help bring them square.