Multicentred Arches
multicentered arch
An arch having a shape composed of a series of circular arcs with different radii, giving an approximation to an ellipse. These arcs are symmetrically disposed about a vertical axis and occur in odd numbers.
Construction known as an ‘arch ring’, made of truncated wedge-shaped voussoirs that by mutual pressure stay in place, set out in a curved form to span an opening and carry a superimposed load.
Installation of flashing with other arch types, such as segmental and semicircular arches, can be more difficult.
This is because most rigid flashing materials are hard to bend around an arch with tight curvature. If the arch span is less than about 3 ft (0.9 m), one section of tray flashing can be placed in the first horizontal mortar joint above the keystone, as illustrated in Fig. 5b. For arch spans greater than 3 ft (0.9 m), flashing can be bent along the curve of the arch with overlapping sections, as illustrated in Fig. 4. Alternately, a combination of stepped and tray flashing can be used, as shown in Fig. 5c. To form a step, the end nearest the arch should be turned up to form an end dam, while the opposite end is laid flat. A minimum of No. 15 building paper or equivalent moisture resistant protection should be installed on the exterior face of the backing over the full height of the arch and abutments. The building paper or equivalent should overlap the arch flashing.
The design of a structural masonry arch should include consideration of the effect of flashing on the strength of the arch. Flashing acts as a bond break. If flashing is installed above the arch, the loading on the arch will likely be increased, and the structural resistance of the arch will be reduced. Installation of flashing at the abutments will affect their structural resistance and should also be considered. Consult Technical Notes 31A for a more extensive discussion of arch loads and structural resistance of brick masonry arches.
DETAILING
An arch having a shape composed of a series of circular arcs with different radii, giving an approximation to an ellipse. These arcs are symmetrically disposed about a vertical axis and occur in odd numbers.
Construction known as an ‘arch ring’, made of truncated wedge-shaped voussoirs that by mutual pressure stay in place, set out in a curved form to span an opening and carry a superimposed load.
Installation of flashing with other arch types, such as segmental and semicircular arches, can be more difficult.
This is because most rigid flashing materials are hard to bend around an arch with tight curvature. If the arch span is less than about 3 ft (0.9 m), one section of tray flashing can be placed in the first horizontal mortar joint above the keystone, as illustrated in Fig. 5b. For arch spans greater than 3 ft (0.9 m), flashing can be bent along the curve of the arch with overlapping sections, as illustrated in Fig. 4. Alternately, a combination of stepped and tray flashing can be used, as shown in Fig. 5c. To form a step, the end nearest the arch should be turned up to form an end dam, while the opposite end is laid flat. A minimum of No. 15 building paper or equivalent moisture resistant protection should be installed on the exterior face of the backing over the full height of the arch and abutments. The building paper or equivalent should overlap the arch flashing.
The design of a structural masonry arch should include consideration of the effect of flashing on the strength of the arch. Flashing acts as a bond break. If flashing is installed above the arch, the loading on the arch will likely be increased, and the structural resistance of the arch will be reduced. Installation of flashing at the abutments will affect their structural resistance and should also be considered. Consult Technical Notes 31A for a more extensive discussion of arch loads and structural resistance of brick masonry arches.
DETAILING