portal frame in pre-engineered building

huangyhg

portal frame in pre-engineered building
i am looking at adding a mezzanine in an existing  pre-engineered steel rigid frame building. i only have erection drawings of pre-engineered building and there is no way to contact the original pre-eng. office.
can anybody clarify the followings:
1. why is there a portal frame along the perimeter? what is the reason to have this?
if it is for resisting lateral loads, then how it is attached to main building. the drawings i have shows portal frame within the bay (about 1'2" away from main columns on either side)of lesser height than main building roof height.
2. can i rest my mezzanine beam on one of the portal column (weak axis) or not. is there any stability issue if i use portal frame column.

the portal frame is for stability of the building to resist lateral loads.  it is common to locate it so that it does not interfere with the base plate on the main column, so that is probably why you have the 1'-2 dimension.  there will be a connection at the top of the portal frame to the main column to transfer load from the building into the portal.
i would recommend keeping the mezzanine framing independent of the building, or you must take responsibility for the stability of the building also.  
mikee55,
thanks for the reply. the building was designed originally to have load capacity for some future mezzanine.so, the building can carry the mezzanine additional load. what i am not sure is about the portal. the portal   
maybe.  maybe not.
what is the wind load?  what is the seismic load?  what load is being resisted by the portal frame?  what is the elevation of the portal frame?  how is the portal frame beam and column connected?  what is the vertical load from the mezzanine framing?  do you expect the mezzanine to be supported laterally by the portal frame columns?  what lateral load will be resisted?
portal frames in sidewalls are practically always a part of the main lateral force resisting system in the direction parallel to the ridge.  don't mess with it.  the fact that it's 8" doesn't mean anything because they probably used h/60 or h/90 for the drift limit.
even if the existing bldg was designed for a mezzanine, was that under an old bldg code?  if so, the seismic load is probably so much more now that it won't work.
one of the others recommended making it independent.  that's the way to go, imo.
thanks everybody.
we are going to make the mezzanine independent.