when to use joist girders vs wide flange

huangyhg

when to use joist girders vs wide flange
i've never really run into anything saying when it's better to use joist girders vs just using a wide flange section.  just for simple roof framing.  i feel joist girders should really only be used when you can use a large amount of them to gain the economy.  for small structures, i think wide flange girders are the way to go.  what do you guys think?
joist girders are typically used for long spans that require a heavy section i-shape.  the economy comes because the joist girder is lighter (less steel) and cheaper to build than to buy a heavy i shape.  too really get some economy use as deep a joist girder as possible.  that is the trade off in some cases, the joist girder is deeper than the i shape.
i will use rolled beams when:
1) there is a possibility that the girder will need to be reinforced for added loads in the future. (have you ever tried to reinforce a joist girder?)
2) short schedule and quick delivery is important.
3) where minimizing the depth of the girder is important.
4) varying magitude of loads.
5) other special situations.
although joist girders can be designed for varying loads, pass thru ductwork, end moments, etc., the place that they really shine are equal loads and equal panels with no depth restriction.
query... a joist girder is simply a truss   
both the joist and joist girder are truss. the joist support the roof, itself is supported by the joist girders at the ends.
dik, the steel joist manufacturers here in the us also provide a product called a "joist girder" which is really just a big joist, but is designed with vertical web   
thanks... dik
jae brings up another reason to use a joist girder instead of a wide flange--they work better as part of a moment (rigid) frame, because they are deeper.
daveatkins