snow drift on existing roof

huangyhg

snow drift on existing roof
i am working on a project where it is proposed that an addition will be approximately 12 feet taller than the existing building.  due to the length of the existing roof, there is potential for a 4.5' drift to form at the junction.
unfortunately, the existing structure is joists and joist girders, which were not designed for this type of loading.  since reinforcement of the girders is almost impossible i was considering another avenue.  i have heard of similar cases where a sloped roof, mono-slope type structure was erected on the existing roof that represents the anticipated shape of the potential drift.
have any of you heard of this practice?
check out our whitepaper library.
adding more girder(s) is not an option?
what do you mean by adding more girders?
this is an industrial facility and is currently under production.  it is not viable to add columns, etc.
i think coengineer means can you add more joists in the drift area.
i cant think of any other option. adding a section of sloped roof will just transfer your drift to another part of the roof.
csd
adding more joists doesn't solve the issue of the overloaded joist girder.
ok, what do you mean my joist girder?  dropped beam right?  if you have dropped beam, isnt it easy to sister another beam or two next to it? or perhaps add another one somewhere else.  usually you are ok with the shear and bearing at the support (usually the deflection controls) so you can just sister on another beam on both side and bolt them together without supporting the end of the new   
sorry i just assume it was wood, but if it was steel then add channels on both side of the existing beam and maybe at the bottom.  do shear flow analysis if necessary.
coengineer-
the joist girder is most likely a steel bar joist (just heavier), and it is probably at the same elevation.  
i don't think you can add channels to this condition, but possibly a king post on the bottom chord will help?  this is done for vibrations quite frequently, maybe it will help for your strength concerns.
how about running the channels either side between the diagonals of the joists, then put angles between to support the top chord of the joist girder.
csd
that is correct, i am sorry i was not clearer.  it is a steel structure, the joist girder is an open web steel bar joist.
bubba1:
i have seen what you are describing done. i'm not sure i agree with the theary.
one solution to a joist and joist girder roof that needs to be upgraded to handle new snow drifting loads is to build a secondary roof system over the area in question. therefore by passing the existing joists and joist girders which are very difficult to strengthen.
come out from 1 bay deep over your existing wall, and set new small post directly on top of the existing columns (poking up through the existing roof), and span a beam between them, and then frame bewteen this beam and your new structure with joists, beams, light gage studs, etc.
this has the effect of putting most of the new drift load into the new structure, where you can control the size of memebrs. the existing interior columns only see a small bit of the increase load, and if they need streghtening, it is usually easier to streghten a column that a joist or joist girder.
usually have to talk yourself into that fact the footings will be ok, since the drift load is only a short duration loading.
just some thoughts.