Masonry Magazine October 1995 Page. 38
Arches
When two arches are adjacent, such as with a two-bay garage or building arcades, intersection of the arches may occur at the skewback. Attention should be given to proper bonding of the arches for both visual appeal and structural bonding. Creation of a vertical line between arches should be avoided. Rather, special shape brick should be used to mesh the two arches properly. One example is illustrated in Fig. 9.
Abutments
An arch abutment can be a column, wall or combination of wall and shelf angle. Failure of an abutment occurs from excessive lateral movement of the abutment or exceeding the flexural, compressive or shear strength of the abutment. Lateral movement of the abutment is due to the horizontal thrust of the arch. Thrust develops in all arches and the thrust force is greater for flatter arches. The thrust should be resisted so that lateral movement of the abutment does not cause failure in the arch. If the abutment is formed by a combination of brickwork and a non-masonry structural member, rigidity of the non-masonry structural member and rigidity of the ties are very important. Adjustable ties or single or double wire ties are recommended. Corrugated ties should not be used in this application because they do not provide adequate axial stiffness. Consult Technical Notes 31A for further discussion of abutment and tie stiffness requirements.
Lateral Bracing
In addition to gravity loads, out-of-plane loads should be considered when designing a masonry arch. The arch should have adequate resistance to out-of-plane loads or lateral bracing should be provided. In veneer construction, lateral bracing is provided by the backing through the use of wall ties. Arches which are not laterally braced may require increased masonry thickness or reinforcement to carry loads perpendicular to the arch plane in addition to vertical loads.
Expansion Joints
Thermal and moisture movements of brick masonry are controlled by the use of expansion joints. Expansion joints avoid cracking of the brickwork and also reduce the size of wall sections. Reduction of wall size has a very important effect upon the performance of structural brick masonry arches. The state of stress in a structural brick arch and the surrounding masonry is very sensitive to the relative movements of the abutments. If an inadequate number of expansion joints are provided, the differential movement of abutments can cause cracking and downward displacement of brick in the masonry arch and surrounding masonry. Proper size and spacing of expansion joints is discussed in Technical Notes 18A Revised.
If the arch is structural, care should be taken not to affect the integrity of the arch by detailing expansion joints too close to the arch and its abutments. Vertical expansion joints should not be placed in the masonry directly above a structural arch. This region of masonry is in compression, so an expansion joint will cause displacement when centering is removed and possible collapse of the arch and surrounding brickwork. In addition, vertical expansion joints should not be placed in close proximity to the springing. The expansion joint will reduce the effective width of the abutment and its ability to resist horizontal thrust from the arch. If the arch is non-structural, placement of expansion joints may be at the arch crown and also at a sufficient distance away from the springing to avoid sliding. While permitted, placement of an expansion joint at the arch crown is not preferred because it disrupts ones tradi-