Originally Posted by aaronled
I would like to position each joist laying on its side where it's going to go (in the centers - not sister joist)
If your current deflection is L/360, what deflection would you like to have? L/480?
From the deflection design goal, the joist size [this gives I, the moment] and material [this gives E, the modulus of elasticity] can be figured.
But since I = b(h^3)/12, increasing h [height] by 10% gives a 33% increase in moment [(1.1)^3 = 1.33], but increasing b [width] by 10% gives only a 10% increase in moment.
That's why joists are on edge. It's the best use of the material for the money spent.
In your case you might do better running small steel I beams down the middle between the joists. I have a list of moments and E values for various types of steel I beams.
Or, if you can find a way to fasten a 2x under each existing joist to increase its height and therefore its I, you can dial up however much stiffness you want, at a loss of some headroom.