huangyhg

wind load transfer through masonry to a stud wall
in looking over various wall tie devices used for connecting brick or stone to metal stud walls covered with sheathing, it seems to me that all these wall tie devices don't offer much resistance to a compression load. many wall tie devices are just wire square loops that slide up and down in a screw fastened bracket. they seem to offer plenty of resistance to keep the masonry from falling away from the wall sheathing, but not so much resistance from the masonry falling into the wall sheathing. are masonry wall ties designed to transfer both compressive and tensile loads on an equal basis from wind forces pushing or pulling on the masonry?
since masonry is a fairly rigid material with low ductility, this made me wonder what effect a compressive wind load has on masonry and how much of the load is really transferred to the wall frame and how much is just absorbed by a very slight deflection in the masonry itself.
it seems to me that a single story residential brick veneer wall of say an 8' height might never transfer any compressive load to the stud wall frame behind it, especially for wind loads below the maximum design. but even at maximum design wind load for an average region, would an 8' brick veneer wall transfer much compressive load to the wall frame? and if not, are the wall ties in a typical residential situation mainly needed to keep the brick from falling away from the wall whenever the brick might be pulled or "sucked" on by the wind?
it seems to me that a brick veneer wall would never be pulled evenly away from a wall (like metal r panel from a metal frame), but rather a suction force at the top of the brick wall might create a tipping force at the top that translates into a bending moment at the bottom of the wall sufficient to break the masonry bond.
and i guess this tipping force can happen in both directions, inward or outward to the wall frame. but it sure looks like wall ties are designed more to prevent walls from falling away from the wall frame rather than from falling inward. can someone enlighten me more about masonry wall tie design and theory?
bill

i see your point here, but considering the spacing of the wire ties or corrugated metal strips at a 2 foot grid, the short axial length of compression (1/2" to 1" air gap) and the fixity of the ties into the brick, i would consider compression not only possible, but actual reality.
design for the force both ways, and don't forget seismic as, depending on where you are, it may bovern.
mike mccann
mmc engineering
the ties should be specified to take both compressive and tensile forces. in residential applications, i would bet the the brick wall is stiffer than the studs and so probably no matter how you build it, it will work. in commercial construction, we make sure the ties can take both compression and tension.
one big caution is not to specify ties with drips. this is like specifying something that already is buckled and virtually has little to no compressive strength.
you could always do a cracking check on the brick at mid-height to determine if the section is cracked or not under full wind or seismic load.
if it is cracked, then rely on the metal studs. if not, then the brick is laterally self-supporting with the studs a back-up.
mike mccann
mmc engineering

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