brick cladding deflection limits

huangyhg

brick cladding deflection limits
i have a project in winnipeg, canada with an exterior load bearing wood wall ranging in height from 20 ft to 30 ft.  in addition, there is a partial height exterior brick veneer from the foundation up an elevation of 7'-0" above the foundation.
to ensure the brick doesn't get unacceptable tensile stresses we need to limit the deflections of the structural backing to l/720.  now if the brick ran full height, i would check the mid span deflection and compare to l/720.  however, in this case how would i check that the wall deflections are acceptable?  i would think that l/720 at mid height of the wall is too restrictive, since the brick only runs between 1/4 and 1/3 of the way up the wall?
i have always felt l/720, or l/600, or even l/480 deflection is too stringent for studs (wood or steel) behind brick veneer.  i use l/360 -- testing done at clemson university in 1981 confirmed that l/360 is adequate stiffness to prevent cracking in the brick veneer.  having said that, the stud should be limited to the same deflection, whether or not the brick runs full height.  the brick has to assume the same curvature as the stud behind it.
daveatkins
thanks daveatkins
unfortunately by our code i have to meet l/720 deflection limits.  i agree that the brick veneer would follow the same curvature as the studs behind them, but the l/720 deflection limits are based on the brick veneer extending the full height of the stud wall, with the maximum deflection at the wall mid-height.
in this case the brick veneer would only tend to lean outwards and the actual deflection of the top of the veneer would be less than that at mid-span.  what i'm wondering about is if i calculate the stud/brick deflection at the top of the veneer, what "l" would i compare that deflection to for l/720?
karlt -
when we check deflections of "cantilevers" we double the cantilever length and then take the end-of-cantilever deflection as the d, limiting it to l/360 or l/600, or whatever, where l = 2 x cantilever length.
the reasoning behind this is that if you look at the curvature of a cantilever, and picture a mirrored image of it where the mirror is at the fixed end, you get a shape similar to a simple span   
these things opens the interesting debate of how a masonry panel (of variegate kinds) break under deflection and drift. all i have seen are partial portraits of the thing, that one by one should be satisfactory enough, but make a general poor and incomplete portrait of the behaviour.
jae...that's what i also assume for cantilevers and i have thought to use the same idea in this case.  however i was thinking over lunch...
what if one were to make up an excel spreadsheet, & plot the deflection of the wall using the standard deflection equations.  then graph it to scale and draw on the brick wythe.  then draw a straight diagonal line from the top of the brick wall to the bottom and scale off the deflection of the curved brick wall at the mid height of the brick wall relative to the diagonal line.  then ensure this deflection is less than l/720, where l = brick wall height. i am assuming that the bottom of the brick will be pinned and rotate about the foundation.  does this sound reasonable?
to me it seems that the maximum stress in the brick wall would be halfway up the brick height, based on the way the brick wythe follows the curvature of the wall behind it. the point of the l/720 deflection limit is to prevent cracking of the brick at the point of maximum stress.
the reason i am going through all this hassle is that i don't want to end up with a wall that is incredibly thick and expensive to build due to the l/720 restriction on the entire wall height.
i had the same question the other day so i again looked at several brick references and code prescriptions. they pretty much don't spell out deflection out of plane. they leave it to the engineer to calculate the deflection of the brick withe and compare it to stress - strain relations and geometric assumptions for crack widths.
when the brick is continuously - uniformlt tied to the backing, the deflection geometries must conform. so you can check and predict a crack width at the bottom of the wall. it becomes a matter of what will you live with.
aisi uses a similar approach in their recomendations for backing of masonry.
karlt -
i guess your method would offer the out-of-plane deflection of the brick wall from its original planar condition.  that does make sense.
i would think of it a different way. imagine that the brick runs the full length of the wall.  (lets take your 30ft wall). when your 30ft wall is deflected to l/720, your maximum allowable deflection is at the midspan, and is 0.5". this 0.5" is at 15ft. now, while the stud is deflected to the maximum, figure out what the 'out of plane' distance is at 7'-0" from the end. for the 7'-0" wall height of 7'-0", limit the deflection at the top wall to this distance. this allows the brick to deflect, and curve identically to an equivalent brick wall which runs the full height.
reading all the posts, no one has mentioned horizontal deflection of the wall. it has been my experience that horizontal deflection can be more of a problem than vertical deflection. for instance, if your wall is 30 ft tall and 60 ft wide, with l/720, horizontal deflection would be 1".
just something to consider.
erv - the whole post is about horizontal deflection.