You might try using deep trusses constructed in the walls and coordinate door and window openings, else a Vierendeel type truss... with the long spans, you note, I'd be looking at steel and not concrete.  I'd stay away from cable stayed framing <G>.  I've not seen anything on the net... but for lateral loading (assuming little or no seismic) you might have to construct the vertical supports as a three dimensional spaceframe.  It's high and has good exposure.  Sounds like a really neat project.

Dik

The spans don't seem too big for a steel structure - a three storey (5 + 3 + 3m) frame with 14m span should not pose great challenge, especially when there are only offices in the above storeys and no machinery. The frames will be repeated (e.g. at each 7m and then you will have 5 frames).

This way you will need no columns for the office storeys eather and you will give the architect full freedom for design.

Mike

I agree with mikesg,

Any columns in the office floors will need to be supported by the lower beam anyway and will increase its size, best to just get each floor to support its own loads.

This is assuming that the office is the same width as the workshop-is this correct?

Why the concrete plates? is this a fire rating requirement?