Q: I recall that earlier versions of the Brick Industry Association's Technical Note 28B recommended that mortar should be Type S for brick veneer over steel stud back-up walls when wind loads exceed 25 psf, and that only portland cement-lime mortars should be used. I noticed that in the December 2005 version this language was modified. Now it states that Type N mortar is recommended with Type S as an alternate.
Will Type N mortar have sufficient strength in this wall system?
A: The language in Brick Industry Association's Technical Note 28B was changed in 1999 and again in 2005. The recommendation that only portland cement-lime mortars be used was eliminated in 1999.
The recommendation that Type S mortar be used where wind loads are expected to exceed 25 psf was eliminated in 2005. The Brick Industry Association continues to recommend Type S mortar where a higher degree of flexural resistance is required; however, it does not state when this may be needed.
Brick veneer over steel stud back-up walls are not designed to prevent cracking, but to control the maximum crack width by using a deflection criteria of L/600. Technical Note 28B states that the maximum crack width should be no more than 0.015 in. in typical floor-to-floor dimensions using this deflection criterion.
Therefore, rather than increasing the bond strength by using cement rich Type S mortar in high wind load areas, the Brick Industry Association now recommends the softer Type N mortar, which makes the veneer wythe more flexible, even in high wind loads areas.
At first glance, it seems that any cracks in the wall system would represent a failure. However, brick veneer over steel stud wall systems are designed so that all lateral wind loads are transferred to the stud backup wall. Therefore, the back-up and not the veneer is the structural element that handles the wind.
The brick veneer only needs to be adequately tied to the back-up to prevent collapse or failure. Steel studs are not designed to prevent cracking in the masonry, but rather to limit the size of those cracks.
As long as the cracks are very small, they close up when wind blows against the masonry. Therefore, water penetration under wind-driven rain should not increase after cracking because when rain is blown against the face of the building, the wall deflects slightly and the cracks close.
Tests referenced in Technical Note 28B have shown that these small hairline cracks do not significantly impact the rate of water penetration.
Also, Type N mortar is more forgiving than Type S because it is softer and better able to accommodate brick expansion. Type N is less likely to develop shrinkage cracks and has better water retention.
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