why limit on light gauge backed masonry veneer heigh

huangyhg

why limit on light gauge backed masonry veneer height?
in aci 530 table 6.2.2.3.1 there are height limits for clay masonry veneer backed by light gauge framing.  the commentary does not explain what drives these limits.  thermal and moisture expansion of course have to be considered in the design and detailing of the veneer but i wondered what if any other factors drove this requirement.
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generally speaking, if a clay masonry veneer is relatively much stiffer than the steel studs behind it, the clay masonry veneer will try to resist wind or seismic load without transferring the load to the studs (load always takes the stiffest path).  with flexible studs, the clay masonry veneer will take lateral load until it fails, at which time it will crack horizontally and begin transferring the lateral load to the studs.
stiffening studs is an attempt to prevent cracking of the veneer.
i am more liberal than most engineers, in that i generally allow l/360 deflection of wood or steel studs behind clay masonry veneer.  many engineers limit deflection to l/600.  i once heard a speaker say that no mattter how stiff you make the studs, they are still not stiff enough.
daveatkins
david thank you for your answer.  however i think something else is driving this code requirement.  the chart i cited limits the height above the foundation for which the veneer can be self-supporting to 30'.  the back-up walls need to be stiff enough, no doubt, but i believe that is a separate issue.
am looking at a facade design for a 4 story building with a floor-to-floor height of 11 feet.   the light gauge stud walls will be designed to carry vertical floor loads and also limit lateral deflection to l/600+- under wind/seismic.  but, because of the code requirement cited previously, i will also need to vertically support the veneer at at least one of the intermediate floor levels.  will have a soft hor joint at the support level(s).  the architect is going to ask why the code requires this and i don't have a good answer (yet!).
also, in aci 530 sec 6.2.2.3.1.3, "the weight of the veneer shall be supported by noncumbustible construction for each story above the height limit given in table 6.2.2.3.1."  so it sounds to me like the vertical support has to be provided at each floor level above the 30' limit.  theres an obvious cost here and i wonder what is driving this requirement.
any additional comments appreciated.
it may be a more empirical judgement thing on the part of the code writers.  having more than 30 feet, all supported below, could be considered a dangerous condition in that an impact or other distress below might cause a huge amount of brick to fall.
these limits have to do with the vertical growth of the clay masonry relative to the structural frame. if i recall correctly, the estimated growth is about an 1/8" per 10 feet of height. when used in combination with a concrete frame, the differential between the growth of the brick and the shortening of a concrete frame due to shrinkage and creep can be quite dramatic.
if shelf angles are not used on taller buildings, it can cause failure of sealants, cracking of brick, drywall, windows, etc.
see
here is a link to valley block and brick website.  at the bottom of the site is a link to ncma tek notes.  there are guidelines for masonry veneers, including control joint requirements.