Q: I am working on a building that has 3-foot-high horizontal bands of brick veneer, supported by shelf angles attached to the edge of an 8-inch-thick concrete flat plate structure. The masonry occurs between horizontal ribbon windows.
The veneer has vertical cracks that are spaced at relatively regular intervals from 5 to 15 feet on center. The veneer is clay masonry, not concrete masonry. I have seen cracks closely spaced like this in concrete masonry due to shrinkage but I have not seen it in clay masonry. What could be causing this problem?
A: There are multiple causes for vertical cracking in masonry veneers. One common cause in horizontal bands of masonry supported by concrete floor systems is deflection of the floor system. Like all structural systems, concrete structures will deflect when floor loads are applied. Vertical cracks like you described may be caused by a relatively flexible floor system in combination with significant floor loads.
Concrete floors also undergo long-term deflections due to a process known as creep deflection. Creep is a time-dependent strain that occurs when concrete columns and beams are subjected to loads over a long period of time. The concrete columns will shorten and concrete floors and beams will sag. Because brick masonry is stiff and brittle, as the floor structure supporting the masonry deflects, the brick will crack, as shown in the diagram.
Assuming that the floor loads remain relatively constant over the life of the building, most of the creep and shrinkage deflection in concrete occurs early in the life of the building. (Short duration changes in loads have very little impact on creep and shrinkage deflection.)
Since future movements will generally be small, one method of addressing cracks caused by creep and shrinkage deflection is to create movement joints in the masonry veneer at roughly the locations of the cracks. Depending on how straight and vertical the cracks are, it may be possible to cut joints to follow the cracks. Where the crack falls outside the cut joints, the brick units can be replaced. By installing joints where the cracks have already occurred, small additional movements from temperature movements, deflections due to changes in floor loads, or additional creep can be accommodated.
In new design, the total deflection (initial deflection plus creep deflection) should not exceed the length divided by 600. In these situations, I also recommend considering closer spacing of expansion joints (20 to 25 feet on center) to reduce the likelihood of developing cracks from deflection.
A Principal at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.
