Because masonry chimneys are one of the most visibly damaged features of a building, scientists sometimes calculate the percentage of chimneys that have fallen in a given area to help determine the severity of an earthquake. Most chimneys left standing were reinforced in some manner. Reinforcing chimneys not only is sound practice in areas of seismic risk, but also in areas subjected to high winds. REINFORCING NEW CHIMNEYS Three methods of reinforcement can be used to help ensure the structural stability of new chimneys; vertical reinforcement grouted inside the chimney, horizontal reinforcement placed in the mortar bed, and anchor straps securing the chimney to the building's frame. VERTICAL REINFORCEMENT On chimneys 40 inches wide or less, use four #4 vertical reinforcing bars. Place one bar in each of the chimney's inside corners. The reinforcement should extend the full height of the chimney. For chimneys wider than 40 inches, use two additional vertical reinforcing bars for each additional flue in the chimney or for each additional 40 inches in width or fraction thereof. HORIZONTAL REINFORCEMENT Place horizontal reinforcement in mortar beds at 18-inch intervals vertically and tie it to vertical reinforcement using steel ties at least 1/4 inch in diameter. CHIMNEY ANCHOR STRAPS Anchor chimneys to the building's frame at each floor or ceiling level that is more than 6 feet above grade. Anchors should consist of two 3/16x1-inch steel straps embedded at least 12 inches into the chimney. REINFORCING EXISTING CHIMNEYS Before repairing a damaged chimney, always inspect the firebox and thoroughly look for cracks or voids that might allow flames or hot gases to contact wood or other flammable materials on the exterior of the firebox or chimney.