Masonry Magazine October 1995 Page. 34
STRUCTURAL FUNCTION OF ARCHES
The brick masonry arch has been used to span openings of considerable length in many different applications. Structural efficiency is attributed to the curvature of the arch, which transfers vertical loads laterally along the arch to the abutments at each end. The transfer of vertical forces gives rise to both horizontal and vertical reactions at the abutments. The curvature of the arch and the restraint of the arch by the abutments cause a combination of flexural stress and axial compression. The arch depth, rise and configuration can be manipulated to keep stresses primarily compressive. Brick masonry is very strong in compression, so brick masonry arches can support considerable load.
Historically, arches have been constructed with unreinforced masonry. Most brick masonry arches continue to be built with unreinforced masonry. The structural design of unreinforced brick masonry arches is discussed in Technical Notes 31A. Very long span arches and arches with a small rise may require steel reinforcement to resist tensile stresses. Also, reduction in abutment size and arch thickness for economy may require incorporation of reinforcement for adequate load resistance. Refer to the Technical Notes 17 Series for more information on reinforced brick masonry. Elaborate and intricate arches are sometimes prefabricated to avoid the complexity of on-site shoring. Most prefabricated brick masonry arches are reinforced. Prefabricated arches are built off site and transported to the job or built at the site. Cranes are often used to lift the arch into place in the wall. Such fabrication, handling and transportation should be considered in the structural design of the arch. Refer to Technical Notes 40 for a discussion of prefabricated brick masonry.
If an unreinforced or reinforced brick masonry arch is not structurally adequate, the arch will require support. Typically, this support is provided by a steel angle. This is the most common means of supporting brick masonry arches in modern construction. The steel angle 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. The angle either bears on the brickwork abutments or is attached to a structural member behind the wall. One example is shown in Fig. 4. When an arch is supported by a steel angle, the angle is designed to support the entire weight of brick masonry loading the arch, and the structural resistance of the arch is neglected. Consult Technical Notes 31B Revised for a discussion of the structural design of steel angle lintels.
WEATHER RESISTANCE
Water penetration resistance is a primary concern in most applications of the building arch. In the past, the mass of a multi-wythe brick masonry arch was sufficient to resist water penetration. Today, thinner wall sections are used to minimize material use for economy and efficiency. Still, the arch must provide an effective weather resistant facade. Some arch applications do not require provisions for water penetration and insulation. For example, arch arcades and arches supported by porch columns typically do not conceal a direct path for water migration to the interior of the building they serve and may not require insulation. If this is the case, provisions for weather resistance need not be included in the arch design and detailing.
Preventing water entry at an arch in an exterior building wall is just as important as at any other wall opening. Water penetration resistance can be provided by using a barrier wall system or a drainage wall system. Refer to Technical Notes 7 Revised for definitions and discussion of barrier and drainage wall systems. A drainage wall system, such as a brick veneer or cavity wall, is the most common brick masonry wall system used today. For either wall system, the arch should be flashed, with weep holes provided above all flashing locations.
Flashing and Weep Holes
Installation of flashing and weep holes around an arch can be difficult. Installation of flashing is easiest with jack arches because they are flat or nearly flat. Flashing should be installed below the arch and above the window framing or steel angle lintel. Flashing should extend a minimum of 4 in. (100 mm) past the wall opening at either end and should be turned up to form end dams. This is often termed tray flashing. Weep holes should be provided at both ends of the flashing and should be placed at a maximum spacing of 24 in. (600 mm) on centers along the arch span, or 16 in. (400 mm) if rope wicks are used. An example of flashing a jack arch in this manner is shown in Fig. 5a. Attachment of the flashing to the backing and formation of end dams should follow standard procedures. If the arch is constructed with reinforced brick masonry, flashing and weep holes can be placed in the first masonry course above the arch.