Masonry Magazine August 1975 Page. 14
FIG. 2
High School in Columbus, Indiana
Harry Weese & Associates, Architects
plied directly upon the arch for design purposes. Heavy concentrated loads should not be allowed to bear directly on minor arches. This is especially true of jack arches. Minor concentrated loads bearing on, or nearly directly on the arch may safely be assumed to be equivalent to a uniformly distributed load equal to twice the concentrated load.
Figure 2 shows the use of minor arches in contemporary architecture.
MINOR ARCH DESIGN
There are three methods of failure of unreinforced masonry arches: (a) by rotation of one section of the arch about the edge of a joint; (b) by the sliding of one section of the arch on another or on the skewback; (c) by crushing of the masonry.
(a) Rotation. The assumption for the design of minor arches, that the equilibrium polygon lies entirely within the middle third of the arch section, precludes the rotation of one section of the arch about the edge of a joint or the development of tensile stresses in either the intrados or extrados.
(b) Sliding. The coefficient of friction between the units composing a brick or tile masonry arch is at least 0.60, without considering the additional resistance to sliding resulting from bond between mortar and the masonry units. This corresponds to an angle of friction of approximately 31 deg. If that angle, which the line of resistance of the arch makes with the normal to the joint between arch sections, is less than the angle of friction, the arch is stable against sliding. This angle can be determined graphically, as illustrated in Technical Notes, No. 31, or may be determined mathematically by the following formula:
B = tan-1(W/H) - Y
where:
B = angle between line of resistance and normal to joint,
W = total equivalent uniform load on arch,
H = crown thrust and
Y = angle of joint with vertical.