changing the angle of roof beams

[newengland]

I'm working on designing a Barrel roof, and I am using STAAD PRo to do so. when I have my girders vertical i need bigger beams then I do if I rotate my girders to the slope of the roof, I was expecting the opposite to be true, any insight on why this might be

 

[Lutfi]

can you elaborate more?

Regards,
Lutfi

 

[apsix]

If you mean the principal axis orientation, I would expect the minor axis parallel to roof slope to be more efficient only if wind is the critical load.