I am building a 40&60 metal building I need to build a 60 foot center truss . Iwant to use a 18 inch 35w ibeam but I am not sure if the size is right . The roof pitch is 12/2 with 6 inch insulation and 26 gauge are panels

Are you going to be lifting off this center beam? I question the size as it isn't really carrying all that much area, only 20' per side.

Contacting a structural engineer would best suit your interest.perhaps a truss company with steel manufacturing,or maybe the steel mfgr themselves.

I am not going to be using it for lifting. A truss might not be the right name for it. It is only 1 ibeam running up from the eve to peak on each side like a regular steel buiding is constucted. Sorry for not being clear

So am I correct on the span of 20' per side or is the beam the other way,with 30' per side?

30 foot per side

Steel roof trusses? Contact truss manufacturer. It's either that or consult an architect or engineer.