Masonry Magazine October 1995 Page. 37
Keystone
Keystone. The keystone may be a single brick, multiple brick, stone, precast concrete or terra cotta. Avoid using a keystone which is much taller than the adjacent voussoirs. A rule-of-thumb is that the keystone should not extend above adjacent arch brick by more than one-third the arch depth. When a keystone is used that is larger than adjacent arch brick or formed with different material, one option is to use springers that match the keystone.
The use of a large keystone has its basis in both purpose and visual effect. With most arch types, the likely location of the first crack when the arch fails is at the mortar joint nearest to the midspan of the arch. Use of a large keystone at this point moves the first mortar joint further from the midspan and increases the resistance to cracking at this point. Aesthetically, a large keystone adds variation of scale and can introduce other masonry materials in the facade for additional color and texture.
If the keystone is formed with more than one masonry unit, avoid placing the smaller unit at the bottom. Such units are more likely to slip when the arch settles under load. Also, it is preferred to have the arch crown (the top of the keystone) coincident with a horizontal mortar joint in the surrounding brickwork to give the arch a neater appearance.
Soffit
A brick masonry soffit is one attractive feature of a structural brick masonry arch. Many bonding patterns and arrangements can be used to form the arch soffit. Deep soffits are common on building arcades or arched entranceways. In this case, it is common to form a U-shaped wall section, as illustrated in Fig. 7. The arches on either wall face should be bonded to the brick masonry forming the soffit. Bonding pattern or metal ties should be used to tie the brick masonry forming the soffit together structurally and to tie the arches on either wall face to the soffit. If metal ties are used to bond the masonry, corrosion resistant box or Z metal wire ties should be placed along the arch span at a maximum spacing of 24 in. (600 mm) on center.
Structural resistance of the arch should be evaluated at sections through the soffit, the exterior wall face and the interior wall face. Deeper soffits may require an increase in arch depth. If the arch is structural, connection of the brick masonry forming the soffit to interior framing members with wall ties or connectors may not be required.
Skewback
For flat arches and arch types that have horizontal skewbacks, such as jack and semicircular arches, respectively, the most desirable spring line location is coincident with a bed joint in the abutment. For other arch types, it is preferred to have the spring line pass about midway through a brick course in the abutment, as illustrated in Fig. 8, to avoid a thick mortar joint at the springing.