Assuming a 1.5kN load is required, C16 joists and rebated 100mm posts the following applies for a decking structure 5.5m x 3.5m
The beams are set out 1.375m spans (between posts) with the beams at 1.7m spans.
The joists are then sat on top of the beams at 400 centres.
The method of orientation will cost more, more posts, more labour. The reason for the dimensions, well this does apply the above principal of joist, and beam spans and as you need the additional components one may as well space them equidistant.
