Catastrophic Failure of a Historic Masonry Brick Wall

By Larry Smith, Butte CPR Brick Guy • Back to Brick Workshop page
Click on images below for full-sized pictures.
Masonry is generally known for its permanence. However, many owners of historic structures know that maintenance is required. A catastrophic failure of a brick wall a block from my house in Butte, Montana, on December 29, 2008, brought the need for maintenance of brick structures to dramatic attention (see photograph showing overview of historic wall failure, left). This article and these photographs record my cursory analysis of this failure and the various factors that led to a very costly, and what could have been a very dangerous, situation.

Failure analysis

The failure was apparently due to foundation settling that put part of the wall into downward tension, and possibly outward rotation. The historic use of ~5-inch-long nails as ties between the mortar joints and wooden sheathing provided inadequate attachment to the wall (see photograph of historic brick wall ties, right). The triggering event for failure was apparently wind loading.

A fast-moving winter storm passed through Butte between 5:00 and 7:00 p.m. Monday December 29th, bringing sustained winds of 35 mph and gusts clocked to 51 mph (three miles away from the building). At about 6:00 p.m. I heard a loud boom that sounded like a building blowing up when I was on the east side of the building. As I returned in about a half hour, I saw the source of the sound, the collapsed west-facing wall of the building (see night-time picture of historic brick wall failure, above left). The photograph shows the great force with which the bricks flew. Apparently wind pressure and relaxation (or differential pressure inside and outside the building) during the gusty wind event caused the slightly bulging single wythe (facing brick wall) to fail catastrophically. Some of the bricks flew halfway across the residential street, while others rotated nearly intact onto the sidewalk.

The distribution of the bricks and the remaining brick on the wall (see photograph looking along the wall at bulging brick, left) suggest that a pre-existing bulge in the wall rapidly expanded away from the building, with the brick on the lower portion of the bulge rotating downward, and the upper bricks flying away from the wall under the force of the weight of the brick between the bulge and the parapet.

Conditions that led to failure

A post-mortem of the failure strongly suggests that relatively minor differential settling of the foundation led to instability in the wall. In the direct photograph of the remaining wall (overview of cracks in foundation, right), the two inadequately patched fractures in the lower portion of the wall directly border the failed portion above (crack patch with high-Portland mortar, below left). The lower portion of the wall is apparently a stucco coat over the common brick wall. The nearly vertical fracture on the left and the diagonal fracture on the right both show tensional movement where the center portion has moved down. It is unknown to me at this time what is causing settling of the foundation. The photograph looking down the wall suggests that the intact bricks are also rotating away from the wall by a fraction of an inch, which could be part of the reason for a bulge that failed. Simple downward movement of the brick in the center could also have caused the brick to bulge slightly.

Maintenance that could have saved the wall

Clearly, recognizing bulges in this brick wall would have been the first step in saving it. Repeated fracturing and patching of the two large cracks should have clued the building owner that movement due to foundation settling is continuing and should be addressed. This may involve excavating inside and outside of the wall and stabilizing an inadequate foundation, replacing rotten beams with concrete, or any number of problems (it may even be due to settling over a near-surface mining stope [cavern] which are common in our mining city).

Stabilization should have also included repointing of failed mortar joints, and addition of more substantial masonry ties. Anything from specialized expansion bolts, to standard masonry ties (which involve some brick removal), to bedding the nails or new screws into new proper mortar could have helped.

Conclusion

Analysis of this failed historic masonry wall is helpful to all of us that work with old brick structures. A few of the things that come to my mind are:

  • 1) Brick walls, especially large expanses of single-wythe facing brick, inadequately tied to building sheathing, can fail catastrophically with great force and violence. Pedestrians could be injured or killed and parked cars could sustain significant body and glass damage.
  • 2) Wind loads, such as those resulting from winter storms or summer thunder storms, are sufficient to trigger failure of walls that are only slightly out of plumb.
  • 3) Bulges in brick walls should be judiciously stabilized and the source of sagging or settling should be identified and corrected. It is clear that the costs of stabilization are well justified if they are compared to correcting a failed wall, outside of the safety hazards.
BUTTE CPR • P.O. Box 164 • Butte, Montana 59703 • E-mail