huangyhg

lintel/window head deflection limit?
my situation:
1. tube lintel spanning 42.5 ft.
2. carrying about 15' eifs & 8" metal studs.
3. window head height is about 11'
4. cc wind load of about 15 psf.
5. eifs deflection criteria is l/240, which for this span is a little over 2".
6. i want to use 8" wide tube to hide it in wall and post off of face of columns.
i am worried about the head joint at the window for horizontal deflection. what is the maximum absolute horizontal deflection permissible at top of window? i have calls in to a couple of local window mfrs, but am interested to hear others' opinions.
my client has agreed that we can put in some additional vertical steel, but we are trying to avoid kickers. my preference is the tube lintel for ease of fabrication and erection.
thanks in advance.

i would say that the answer is depending on how the window-wall system is built, all, mullions and frames respect bearing structure or bearing points, and glazing itslef within the frames. contact between the glass and the frame is understood must be prevented because both impact and heat source, heat sink there are known contributors to glass breakup.
in general, most well designed window framing should accommodate well serviceability deflections of about 1/400 in most circumnstances if only because this or 1/500 are both ordinary deflection limits at service level both for vertical deflection and sway in whichever the direction. since they must accompany the building in such excursions, most window systems shouldn't have problem with that.
note many codes (say ehe) and texts choose to establish as well a maximum deflection of 1 inch or even 1 centimeter at service level (not even under probabilistic loads, but characteristic ones). this means such texts and codes want the thing rigid and straight, and both camber and attent consideration of the excursions or the elements likely to suffer damage must be made. these requirements may force to select alternative structural solutions that suit better the requirements.
l/240 is too liberal for dealing with glass. i would not use anything less than l/360, the same that is usually applied for plaster. i would contact the technical services dept. of a window wall manufacturer and check what they expect for deflection limits. my guess is l/360 is the mininum and more likely l/500, as suggested by ishvaaag.
another thought is that most manufacturers use a slip channel detail around the window frames to help accomodate the expected building deflection.
by the way, you don't have to worry only about the glazing. i've seen steel stud partitions installed tight to a building from buckle due to deflection from the frame. they too need to allow for deflection.
although building codes allow a deflection limit of l/240 (maximum deflection limit,dead load), a deflection of 2.13" at what i would consider a "critical component" is excessive.
deflection at critical components should be limited from 1/2" to 1" for safety factors and component reliability. i would limit deflection to l/600 (0.85"), insuring component reliability.
porkchopbobby,
in addition to the comments about limiting the deflection, make sure you also consider how the deflections affect other components. a horizontal deflection of 1/2" or 3/4" might sound okay to a window manufacturer, but what happens if the window head is 12" from the ceiling or underside of the slab? what happens to the portion of stud above the slab if the rotation about the support is that great?
your wind load for components and cladding is also at the extreme low end of the spectrum. in the northeast usa, we seldom see walls under 25 psf, and more often than not, we are working above 30psf.
to date, i have not seen a 40ft + span on a wind girt like this. i am working a project right now at 30psf where the tubes are hss8x4x3/8 and spanning 24ft with something like 10ft of tributary wind area. i can't imagine stretching that another 15ft+. if you can make it work, be sure that it can support itself under its own weight.
on another thought, is this at the underside of a roof span? maybe you could use kickers above and cantilever the studs below the slab? similarly, a piece of tube at the top of the studs could also help counteract the forces at the bottom, if the necessary steel is there to connect it to.

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