Masonry Magazine October 1995 Page. 36
TABLE 1
Minimum Radius for Uncut Arch Brick12
| Nominal Face Dimensions of Arch Brick, in. (height by width) | Minimum Permissible Radius of Arch to Intrados, ft |
|---|---|
| a) 8 IN. (200 mm) ARCH | |
| b) 12 IN. (300 mm) ARCH | |
| c) 16 IN. (400 mm) ARCH | |
| d) THREE COURSE JACK ARCH | |
| e) FOUR OR FIVE COURSE JACK ARCH | |
Typical Arch Configurations
FIG. 6
arch, but can be more economical because it requires only one special-shaped brick.
Selection of tapered or rectangular brick can be determined by the arch type, arch dimensions and by the appearance desired. Some arch types require more unique shapes and sizes of brick if uniform mortar joint thickness is desired. For example, the brick in a traditional jack arch or elliptical arch are all different sizes and shapes from the abutment to the keystone. Conversely, the voussoirs of a semicircular arch are all the same size and shape. Arch types with many different brick shapes and sizes should be special ordered from the brickmanufacturer rather than cut in the field.
The arch span should also be considered when selecting the arch brick. For short arch spans, use of tapered brick is recommended to avoid excessively wide mortar joints at the extrados. Larger span arches require less taper of the voussoirs and, consequently, can be formed with rectangular brick and tapered mortar joints. The thickness of mortar joints between arch brick should be a maximum of % in. (19 mm) and a minimum of % in. (3 mm). When using mortar joints thinner than 4 in. (6 mm), consideration should be given to the use of very uniform brick that meet the dimensional tolerance limits of ASTM C 216, Type FBX, or the use of gauged brickwork. Refer to Table 1 for determination of the mini-
36 MASONRY-SEPTEMBER/OCTOBER, 1995
Based on ¼ in. (6 mm) mortar joint width at the intrados and ½ in. (13 mm) mortar joint width at the extrados. If the mortar joint thickness at the extra dos is ¾ in. (19 mm), divide minimum radius value by 2.
21 in. = 25.4 mm 1 ft = 0.3 m
mum segmental and semicircular arch radii permitted for rectangular brick and tapered mortar joints. Typically, the use of tapered brick and uniform thickness mortar joints will be more aesthetically appealing.
Depth. The arch depth will depend upon the size and orientation of the brick used to form the arch. Typically, the arch depth is a multiple of the brick's width. For structural arches, a minimum arch depth is determined from the structural requirements. If the arch is supported by a lintel, any arch depth may be used.
The depth of the arch should also be detailed based on the scale of the arch in relation to the scale of the building and surrounding brickwork. To provide proper visual balance and scale, the arch depth should increase with increasing arch span. Because aesthetics of an arch are subjective, there are no hard rules for this. However, the following rules-of-thumb will help provide an arch with proper scale. For segmental and semicircular arches, the arch depth should equal or exceed 1 in. (25 mm) for every foot (300 mm) of arch span or 4 in. (100 mm), whichever is greater. For jack arches, the arch depth should equal or exceed 4 in. (100 mm) plus 1 in. (25 mm) for every foot (300 mm) of arch span or 8 in. (200 mm), whichever is greater. For example, the minimum arch depth for an 8 ft (2.4 m) span should be 8 in. (200 mm) for segmental arches and 12 in. (300 mm) for jack arches.
The depth of jack arches will also be a function of the coursing of the surrounding brick masonry. The springing and the extrados of the jack arch should coincide with horizontal mortar joints in the surrounding brick masonry. Typically, the depth of a jack arch will equal the height of 3, 4 or 5 courses of the surrounding brickwork, depending upon the course height.