Tech Talk: Arches

Words:   Figure 1. Arch terminology from BIA Technical Notes 31. The arch is the most common, traditional method of spanning openings with masonry. The arch makes use of the high compressive strength inherent in masonry and, with proper geometry, doesn’t need reinforcing or supplemental support. The arch is an aesthetically pleasing form and one that just looks “right” in masonry. There are a fair number of arch specific terms that should be defined before we get into a discussion of masonry arch construction. Figure 1 below is taken from the Brick Industry Association’s (BIA) Technical Notes 31 and visually shows many of the commonly used terms related to arch construction. There are a number of common arch types including the jack arch (no rise), the segmental arch (pictured above), the semi-circular arch, the gothic arch, various multi-centered arches and several other less commonly used arches. For further information, BIA’s Technical Notes 31 provides examples of most masonry arch shapes. Arches can be formed with standard units with rectangular faces by tapering the mortar joints. Visually, arches constructed with special tapered units and uniform mortar joints have more aesthetic appeal. Figure 2. Garage openings on this project ranged from 9’ 8” to 12’ 8” in the veneer, however the architect specified a constant rise of 7 inches and a single soldier course depth for all the arches. As a result, many arches over the larger openings are failing. Note the wide repointed bed joints over the crown and the distorted geometry of the arch. DEPTH In general, segmental and semi-circular arches should have a minimum depth of 1 inch for every foot of span with a minimum depth of 4 inches for any opening smaller than 4 feet. For jack arches, BIA recommends the greater of 4 inches plus 1 inch per foot of span or 8 inches minimum. For example, a 3-foot wide opening would require a jack arch depth of the greater of 4” plus 1” per foot (7” inches) or 8” minimum. Therefore, the minimum jack arch depth of 8 inches would control for openings less than 4 feet in width. Jack arches over 6 feet in width, should typically be supported by a lintel unless specifically engineered for the span. Figure 3. Rotation and settlement of the right-side abutment caused slippage between voussoirs (right) and settlement of the key stone. This condition required temporary shoring until repairs could be made. ABUTMENTS Masonry on either side of an arch serves, essentially, as an abutment for the arch and must be capable of resisting the horizontal force (thrust) generated by the arch. Keep in mind that arches with minimal rise, such as the jack arch, will generate more thrust and, as a result, the abutments must be larger. The Brick Industry Association’s recommended abutment widths, in terms of the arch span, are given in Table 1 below for several common arch types. Abutments are considered full height when the adjacent veneer extends from the base of the wall or floor line to the crown of the arch. Partial height abutments may arise where the veneer adjacent to the arch terminates at the height of the arch spring line or other exterior finishes are used below the arch spring line and masonry veneer starts at the spring line, typically supported by a lintel (see Figure 4). Table 1. Minimum abutment widths for typical arches and varying abutment height. Expansion or control joints should not be installed within the minimum abutment width requirements listed above. Figure 4. Examples of full height and partial height abutments. The surfaces indicated in the figure above are potential shear lines that resist the arch thrust (Figure from BIA Brick Brief, May 2012). CONSTRUCTION Arches are traditionally built on temporary forms, termed “centering”, that support and provide shape to the arch until mortar joints have cured. Typically, the centering is removed at around 7 days after construction although more time may be required during cold weather or if high lime content mortars are used. Pre-manufactured arches are also available in some markets as an option that eliminates the delays associated with mortar curing. Curved steel lintels or concealed lintel systems are sometimes used as both centering during construction and permanent support for the arch. Figure 5. Veneer and structural semi-circular arches in concrete masonry unit construction. ARCHES IN LOAD-BEARING MASONRY In modern masonry construction, arches in load bearing masonry are not as common as arches in veneers. However, the basic principles are similar. Arches with little rise are typically constructed with bond beam or other structural units that can be reinforced as needed (technically, these are also lintels). Semicircular arches or multi-centered arches may or may not need reinforcing depending on the geometry of the arch and the loads imposed. Arches in load-bearing masonry must be engineered and empirical or rule-of-thumb guidelines are not applicable. Again, placement of control or expansion joints adjacent to the arch should be carefully considered.
Words: Masonry Magazine and Mason Contractors Association of America Photos: Mason Contractors Association of America
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