does anyone know how you calculate the load bearing capacity of 100mm x 75mm x 1500mm,

Jonti

I do. But I don't know what an RCJ is. Something like an RSJ? Greentram's eurobeam software is nice and easy to use (and has a demo version). Runs OK under wine. But obviously you can just do the raw beam sums. Of course the raw 'at what uniform/point load will it fail' isn't all that useful for real building design (that has to get past any sort of inspector). You need to know a load of factors and assumed loads which get added up to provide various load combinations. Then design for the largest one (which in my case was exceptional snow load + maintenaince point load).

RCJ - do you mean a concrete beam with an appropriate amount of re-bar in it?
Here such things are cast in place and the size, amount of re-bar and position of that re-bar is determined by a structural / civil engineer
Peter