Took me a little time to get my head around what you are proposing.
The max wide of the building span is 20m, however due to the 4 columns providing the Atrium corners you only had to deal with a maximum unsupported span of 7.7m.
You will also have to consider how the beam will produce substantial point loading on your external walls and may required additional columns at the outer wall ends of each beams too.
This is more that some structural Calc's you need an engineer to be involved on it from day one.