Several of the walls on a gymnasium of a 5-year-old school are now bowing out. The walls are block on the inside and brick on the outside with mortar in the collar joint between. What would cause this bowing?
Without inspecting the building, it's impossible to determine exactly what caused the bow. But differential expansion of the two materials is probably the most common cause of bowing in these types of walls. If the collar joint is grouted solid, the two walls are forced to move together. The brick tries to grow from thermal and moisture expansion. The concrete block tries to shrink. Because of this, the walls bow outward horizontally and often crack near the corners. Eccentric loads on top of the walls, such as joists or other roof members, also may contribute to bowing in the vertical direction. Norbert V. Krogstad is a senior engineer at Wiss, Janney, Elstner Associates, Inc., Northbrook, Illinois.