Copyright Goodheart-Willcox Co., Inc. 298 Section 4 Construction Details Trade Tip A Gothic arch is also referred to as a drop arch, equilateral arch, or lancet arch, depend- ing on whether the spacings of the centers are respectively less than, equal to, or more than the clear span. An arch is normally classified by the curve of its intrados (curve that bounds the lower edge of the arch) and by its function, shape, or architectural style. Arches are also classified as major and minor arches. Minor arches have spans that do not exceed 6′ with maximum rise-to-span ratio of 0.15. The rise of an arch is the maximum height of the soffit, or lower surface of the arch, above the level of its spring line. The spring line is the horizontal line that intersects the springing (point where the skewback intersects the intrados). The span of an arch is the horizontal clear dimension between abutments. Refer to Figure 14-45. The following types are most commonly used for building arches: • Jack arch. A flat arch with zero or little rise. • Segmental arch. An arch with an intrados that is circular but less than a semicircle. See Figure 14-46. • Semicircular arch. An arch with an intrados that is a semicircle (half circle). Because of its structural efficiency, this arch is often used for bridges. • Multicentered arch. An arch with a curve that consists of several arcs of circles that are normally tangent at their intersections. Function of Arches The curvature of an arch provides structural efficiency by transferring vertical loads laterally along the arch to the abutments at each end. Compression is distributed through the arch form and weight is diverted onto the abutments. An arch is designed to keep stresses primarily compressive. The compressive strength of brick masonry allows brick masonry arches to support considerable loads. Most brick masonry arches are built with unre- inforced masonry. However, arches with a small rise and very long span arches may require steel rein- forcement to resist tensile stresses. Reinforcement may also be required if abutment size and arch thickness have been reduced for economy. An arch requires support if it is not structurally adequate. This support is typically provided by a steel angle, which is bent to the curvature of the intrados of the arch. Curved sections of steel angle are welded to horizontal steel angles to form a continuous support. See Figure 14-47. Arch Design Arches can be built using a variety of depths, brick sizes and shapes, and bond patterns. An odd number of units are used so the keystone falls at the center of the arch. The keystone is the masonry unit located at the crown of the arch. See Figure 14-48. Arch voussoirs (single masonry units) are usually laid in radial orien- tation. The voussoirs may be similar in size and color to the surrounding brickwork, or the arch brick can be thinner, wider, or a different color for variation. Goodheart-Willcox Publisher Figure 14-45. Arch terminology. Arch axis Crown Extrados Intrados Depth Soffit Rise Spring line Abutment Skewback Span Goodheart-Willcox Publisher Figure 14-46. A typical segmental arch in modern construction.