When we built that rear addition floor, we didn't have the engineered plans... We placed the joists 16" on center... The plans called for 19.5" on center with three braces from the rim board to the first joist...
Pretty sure I didn't need to, but I put the braces in because they were on the plans...
It wasn't easy getting them in either,
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On the other side we had about 23" from the joist to the rim board, so I am putting another joist in... This way the floor will be built above code!! Lol!! Cost a bit more than it had to though... Learning curve!!
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You know, those braces will work great to attach the walls to!! Yeah, that was the plan!!