Copyright Goodheart-Willcox Co., Inc. 300 Section 4 Construction Details Centering for Arches Arches are constructed with the aid of temporary shoring, or centering, which supports the masonry while the arch is being constructed. The centering carries the load of the arch until it has developed sufficient strength to support itself and the imposed load. The centering should not be removed for at least seven days after the arch has been completed. The centering is usually made from wood. The ribs are 2″ lumber and the lagging is usually 3/4″ × 2″ strips of wood. See Figure 14-49. The lagging should be cut 1″ shorter than the width of the masonry wall so it does not interfere with the mason’s line. The ribs are cut to the shape of the arch. The centering must be supported adequately for the weight it is to bear. Jack Arch Construction The jack arch is relatively weak and should be supported by steel if the opening is over 2′ wide. The steel must be bent to the camber (small rise of the arch), which is generally 1/8″per foot of span. See Figure 14-50. For best appearance, each joint should be the same width as the entire length of the joint. Special bricks are required or regular bricks must be cut to have a perfect job. The bricks should be shaped so the end joints are horizontal rather than perpendicular to the radius of the arch. See Figure 14-51. When constructing any arch, the bricks should be laid out to determine the number required, inclination, and spacing. Segmental Arch Construction The rise and span are the most important features to be considered when constructing a segmental arch. Refer to Figure 14-50. The length of the arch must be found in order to determine the number of courses and size of bricks needed. • The number of courses of bricks in an arch is determined by finding the length of the extrados. Goodheart-Willcox Publisher Figure 14-49. Centering for jack and segmental arches. Lagging Rib Jack Arch Center Segmental Arch Center Goodheart-Willcox Publisher Figure 14-50. The jack and segmental arches are frequently used for short spans. Camber Jack Arch Camber = 1/8" per foot of span Rise Segmental Arch Minimum Rise = 1" per foot of span Goodheart-Willcox Publisher Figure 14-51. Construction layout for a jack arch. Extrados line Intrados line Radial center Centering line