June 23, 2006, Newsletter Issue #70: Out-of-Plane Failure

Tip of the Week

Out-of Plane Failure in URM walls is usually much more destructive and spectactular than in-plane failures. As the name implies, out-of-plane failure occurs when the shockwaves in the ground move in any direction other than the plane of the wall. Unsupported masonry walls are far more susceptible to failure caused by ground motion perpendicular to the wall than from motion parallel to the wall. Out-of-Plane failure can cause a wall to literally explode. Because the ground motion caused by an earthquake is usually not merely in one direction, a wall will often fail due to the combination of in-plane and out-of-plane forces. Cracking caused by an in-plane failure may occur first, considerably weakening the wall. The wall could still support the building, but would be very susceptible to a devastating out-of-plane failure, which often occurs moments later when the ground begins shaking in a different direction.

The risk of an out-of-plane failure is greatly reduced if the distances between wall supports are reduced. The floor and ceiling joists, if properly attached to the masonry walls, can provide protection against out-of-plane failure. On older buildings, the attachments between the joists and the walls are often inadequate and provide little support for the walls. If these connections are reinforced, the spans between masonry wall supports are effectively reduced, significantly strengthening the building.

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