Masonry Magazine October 1995 Page. 39
Detailing Expansion Joints
Expansion joints should be considered in the design of brick masonry arches to relieve stresses caused by thermal and moisture movements. Expansion joints are particularly important in long span arches or runs of multiple arches. Fig. 10 illustrates a sectional view of the arch as a structural element. Refer to Fig. 10 for suggested expansion joint locations for structural and non-structural arches.
Detailing of expansion joints can be difficult with very long span arches or runs of multiple arches along an arcade. Structural analysis of the arch should consider the location of expansion joints. For the particular case of multiple arches closely spaced, vertical expansion joints should be detailed at a sufficient distance away from the end arches so that horizontal arch thrusts are adequately resisted by the abutments to avoid overturning of the abutments. For long arcades, expansion joints should also be placed along the centerline of abutments between arches when necessary. In this case, horizontal thrusts from adjacent arches will not be counteracting, so the effective abutment length should be halved and overturning of each half of the abutment should be checked. Refer to Technical Notes 31A for further discussion of abutment design for adequate stiffness.
MATERIAL SELECTION
To provide a weather resistant barrier and maintain its structural resistance, the arch must be constructed with durable materials. The strength of an arch depends upon the compressive strength and the flexural tensile strength of the masonry. Selection of brick and mortar should consider these properties.
Brick
Solid or hollow clay brick may be used to form the arch and the surrounding brickwork. Solid brick should comply with the requirements of ASTM C 216 Specification for Facing Brick. Hollow brick should comply with the requirements of ASTM C 652 Specification for Hollow Brick. Refer to Technical Notes 9 Series for a discussion of brick selection and classification. The compressive strength of masonry is related to the compressive strength of the brick, the mortar type and the grout strength. For structural arches, brick should be selected with consideration of the required compressive strength of masonry. Typically, compressive strength of the brick masonry will not limit the design of the arch.
Tapered voussoirs can be cut from rectangular units at the job site or special ordered from the brick manufacturer. Before specifying manufactured special arch shapes, the designer should determine the availability of special shapes for the arch type and brick color and texture desired. Many brick manufacturers produce tapered arch brick for the more common arch types as part of their regular stock of special shapes. Be sure to contact the manufacturer as early as possible if special shapes are needed. In many instances, production of the special shapes may require a color matching process and adequate lead time for the manufacturer.
Mortar
Mortar used to construct brick masonry arches should meet the requirements of ASTM C 270 Standard Specification for Masonry Mortar. Consult Technical Notes 8 Series for a discussion of mortar types and kinds for brick masonry. For structural arches, the flexural tensile strength of the masonry should be considered when selecting the mortar. The flexural tensile strength of the masonry will affect the load resistance of the arch and the abutments.
CONSTRUCTION AND WORKMANSHIP
The proper performance of a brick masonry arch depends upon proper methods of construction and attention to workmanship. Layout of the arch prior to construction will help avoid poor spacing of voussoirs, which results in thicker mortar joints and unsymmetrical arches. Some arch applications, such as barrel vaults and domes, can be entirely self-supporting, even during construction. However, most applications of the masonry arch used today require proper shoring and bracing.
Centering
Both structural and non-structural arches should be properly supported throughout construction. Brick masonry arches are constructed with the aid of temporary shoring, termed centering, or permanent supports, such as a structural steel angle.
Centering is used to carry the weight of a brick masonry arch and the loads being supported by the arch until the arch itself has gained sufficient strength. The term "centering" is used because the shoring is marked for proper positioning of the brick forming the arch.