Question: I'm involved in a repair project on a building where there are vertical cracks near the corners. However, the crack patterns on this project aren't like those I have seen before.

First, the building has vertical strip windows that alternate with 10-foot-wide masonry piers between the windows. With these piers being only 10 feet wide I think there would not be very much horizontal expansion.

Next, these cracks don't extend to the full height of the corner but run only at the level of the shelf angles.

Finally, this building was built during the 1960s, and the shelf angles were installed without any expansion joints beneath them. The top of the wall is restrained on the underside of the concrete roof system, which projects from the building as an architectural statement. It seems like unrelieved vertical expansion may be part of the problem. I am not sure what is causing these cracks. Any ideas?

Answer: From what you have described it does seem likely that these cracks were caused by unaccommodated vertical differential movement between the clay brick masonry and the structural frame of the building. Concrete frame buildings will shrink over time. Clay masonry, in contrast, will grow as a result of moisture expansion.

Large compressive stresses will cause horizontal tensile stresses to develop in the masonry. Often cracks will develop in narrow, vertical sections of masonry. However, because you stated the cracks are occurring at floor lines, the cracking may have been caused by outward displacement of bulging at the shelf angles. This outward bulging occurs because there is a mortar joint in front of the shelf angle.

When the walls were first built there likely was some gap underneath the angle. As this gap closes, due to vertical differential movement between the clay masonry and the concrete frame, the mortar that bridges the joint in front of the shelf angle will take the entire load. This eccentric load creates a movement in the exterior wythe that causes the masonry to displace outward.

The vertical movement also will lift the masonry off of the angle as the gap below the angle closes. This reduces or eliminates the friction on the angle and therefore eliminates the lateral restraint at this location. Although this bulging will be fairly small, it will be more than sufficient to develop vertical cracks that can be several feet long centered at the floor lines of each floor.

To correct this problem and prevent cracking from recurring, horizontal expansion joints should be installed to accommodate differential vertical growth. Prior to performing repairs the brick above the angle must be supported adequately to transfer the loads back to the building structure. Repairs should begin at the top of the building by first cutting a joint between the underside of the concrete beam and the top of the masonry. Next, perform shelf angle and expansion joint repairs at the highest floors first, then progress downward in order to reduce the weight to be carried by the temporary shoring. The angles likely will need to be removed, reset, or replaced because movement may have disrupted the anchorage of the angle to the concrete.