sloped steel joist design - allowable girder deflection

huangyhg

sloped steel joist design - allowable girder deflection
hi all,
here we go again with the sloped member discussion with a new twist.  i read the threads related to this topic (see referenced threads below)
background information:
i am involved in designing some buildings for the federal government.  unfortunately they like roofs with 3/12 slope and the roofs are usually metal joist.  we have 2 buildings to design.
building 1:  work bay for trucks and equipment.  must be open floor plan.  width = 82', length=91'.  the architects want the ridge along the 92' length so i will end up with a 92' girder on the ridge and ~ 41' span joists at 3/12 pitch and gable ends to the wall.  i have lots of depth for the supporting joist girder - like 9' or 10'.  
building 2:  office type building, 2 stories with gable roof.  i am allowed a column line at the ridge, girder or beam spans ~ 24' to 31' with as much depth as needed, 3:12 sloped joist spans = 38'-6 and 50'. joists bear on masonry walls, masonry shear walls for lateral support.  (2nd story has additional columns that do not extend to the roof)
both buildings will have a metal roof diaphragm.
question:
general:  i'm looking for the most economical way to design the building (we do not have the job yet, overdesign could cost us the project).  would using the gider system at the ridge with 38'-50' single slope joist spans be the way to go?  we have also used light gauge joists in the past, but the government requires tedious repairs of any bent or dented joists which costs $$$.  
building 1:  for 91' girder span, l/360 is about 3".  although deflection rarely controls joist girders because of their depth, the deflection will still be quite high.  after reading threads about sloped members, horizontal thrust and deflection - how much deflection is too much?  if the low end of the joist is supported by a beam line with columns at 20' o.c., should i allow for horizontal deflection or design for thrust? should i specify a minimum deflection to the girder designer/supplier?
building 2:  for 31' girder span, l/360 is a little over 1".  again, is this too much?
my co-worker (who has 30 yrs experience and is the only other structural engineer here) is designing a gable roof for another government project.  he is insisting on using gable joist trusses that clearspan 60' @ 5' o.c. instead of using the sloped joist with a ridge girder/beam with 25' -30' spans.  i think special girder joists with that span sound insanely expensive. his thoughts about the sloped joists is that there is no horizontal thrust if simply supported.  i think there will be some type of force if there is a lot of vertical deflection at the ridge beam/girder.
fyi - i work for a design/build firm and have to validate anything they feel is "excessive" and they want the "cheapest" structure they can get away with to get the job.  rfp prefers masonry walls with brick facade and metal roof.
reference threads:
thread 507-167555
thread 507-189340
thread 507-162638 - i would like to hear from whodapookie about the trusses with 6-10" of deflection, the geometry of the building and the associated problems.
thread 507-140191
fyi -
i did check the expected l/360 live load deflection for the ridge girders.  it is as follows:
building 1:  1.64" for 92' span (l/673)
building 2:  0.63" for 31' span (l/609)
good problem, as a competitor in the same field i will study your final design solution. deflection of the joist/girder at midspan should be as listed, 1.5" (+/-).  the horizontal forces at the low ends of the joists will be easily resisted with some beam shape at the top of columns.  
there is no thrust since the joist bearings will be level.
the horizontal thrust will occur when live load deflection lowers the mid span of the joist girder. level bearings do not prevent this horizontal potential movement.
hi civilperson,
just curious, who do you work for?  you don't have to answer if you don't feel comfortable....  do you usually spec the internal axial force for the sloped joist?  i have seen a plans with the sloped joist but no axial force specified.  in fact, i have never actually seen it specified and have never been asked for it.
what do you think about the cost/performance of single sloped steel joist with girder at ridge vs. steel gable joists?  
just imagine, a sloping joist with level bearings. in an idealized state, it is modeled as a pin at one end and a roller on the other. this seems realistic because the outer beam has little or no restraint due to lateral motion.

the summation of external loads has to equal zero. all gravity loads are downward. applied loads plus reactions must sum to zero. if there are no horizontally applied loads, then there are no horizontally applied reactions.
is the "thrust" that you are refering to an internal force? if so it should be resolved internally so that the only reactions to gravity loads are upward.
try modeling your sloped joist on "risa" or some other program that you trust.
i hope this helps!