fenderman,
There is not straight answer to this question.
If you were to use graded timber (C14) and it was 150x50 at 400mm centres the beams could take a UDL of 75kg/sqm over a 3m span. Your suggestion to double up the timbers and double the centres would make no gain.
end
It is traditional to increase the thickness of a joist where it will carry the weight of an upstairs fireplace or to double the joists when carrying a upstairs block wall.
What are you trying to do?
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