Masonry Magazine June 1983 Page. 21
Masonry/metal stud wall systems
A critical review-updated
By JOHN A. HESLIP
President
Masonry Institute of Michigan
Remember that old adage about a picture being worth a thousand words? Well, here's a case (see below) where it really applies.
We think this illustration can go a long way in clearing up the ongoing controversy that surrounds the compatibility of the various elements used in the masonry/metal stud wall system.
But let's back up a minute.
You may recall that back in June, 1979, we first published a critical review of the masonry/metal stud system. This followed two Masonry Institute-sponsored seminars on the subject.
One of the major questions we raised at that time concerned the recommended maximum deflection allowables for the metal stud back-up.
While this is still a question that bothers some people, most designers now accept deflection values that reflect the behavior of masonry... in the range of L/600 to L/900 for brick, and a more conservative value for concrete masonry (as spelled out in Tek Note #114-A. "Structural Back-up Systems for Concrete Masonry Veneers," from the National Concrete Masonry Association.)
The question of deflection is an engineering question that should be resolved by accepted engineering practice.
The second question raised then and still, in our opinion, the most critical concern-was that of corrosion. By looking at this section drawing of a brick/metal stud detail, shown at half scale, you can see that all that holds the heavy masonry anchor to the metal stud is approximately one turn of the threads of the commonly used self-tapping screw. In all probability, the thread depth is ½" or less!
What's the likelihood of moisture entering the wall and getting at the screw threads of this anchor system? Unlike the question of relative stiffness of components, which can be resolved by engineering practice, this is more of a judgment call based on experience.
Certainly, we can anticipate some wind-driven rain to penetrate 4 of masonry. We also know that in this midwestern climate, the probability of experiencing condensation in the area of the screw threads of the anchor system is high. Whether from without or within, moisture around the pierced galvanized metal stud at the screw location can cause major problems if corrosion occurs.
The late, nationally recognized corrosion expert. Dr. Lewis W. Gleekman, predicted a service life of approximately seven years when moisture and chlorides are present in the system. In our opinion, that kind of service life is not acceptable.
Lately, we've noted that several major design firms have been reconsidering the compatibility of masonry with metal studs, and are returning to the conventional masonry cavity wall or masonry composite wall. This choice is based both on first cost and on life-cycle cost.
We encourage all design firms in Michigan and northwestern Ohio to make their own critical review of the masonry/metal stud system. For further details of this discussion, please contact us here at the Masonry Institute.
Reprinted from The Story Pole, April, 1983, published by the Masonry Institute of Michigan.
Corrosion problems in masonry/metal stud wall systems
3" BATT INSULATION
½" DRYWALL
3½" 18 GA. METAL STUD
1½" EXTERIOR SHEATHING
SELF TAPPING SCREW
-ADJUSTABLE VENEER ANCHOR
MASONRY VENEER
GALVANIZING PIERCED
AT EACH SCREW
MOISTURE FROM WITHOUT
OR FROM WITHIN (CONDENSATION)
SCREW HOLDING APPROX.
ONE THREAD
MASONRY-MAY/JUNE, 1983 21