The chimney shell thickness, through openings, shall not be less than 1/24 times the height of the opening over a vertical distance extending from 1/2 the height of the opening below the sill of the opening to 1/2 the height of the opening above the top of the opening. Properly designed buttresses or other means of lateral restraint may be permitted in place of this requirement. However, the buttresses or other means of lateral restraint shall not be included when calculating vertical strength.