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According to the timber framing manual I have for pine (F5) single span bearers (supported at each end with no intermediate support) supporting floor joists only:
140x70 at a bearer spacing of 1200 is good for 2400 span**
140x70 at a bearer spacing of 1500 is good for 2200 span.
The sizes increase for bearers supporting load bearing walls however I doubt your shed is going to be heavy enough to worry about that.
For the floor joists (single spans):
120x35 joists at 600 spacing can do 1800 span
120x45 joists at 600 spacing can do 2000 span
120x35 joists at 450 spacing can do 1900 span
120x45 joists at 450 spacing can do 2100 span
for continuous spans:
120x35 joists at 600 spacing can do 2100 span
120x45 joists at 600 spacing can do 2300 span
120x35 joists at 450 spacing can do 2200 span
120x45 joists at 450 spacing can do 2400 span
90x45 joists at 600 spacing can do 1600 span**
** this should work for your shed dimensions and footing layout.
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