Arches built either in the form of an ellipse or oval, or pointed at the centre and elliptical at the springing, are often used for architectural effect in buildings, although very seldom in engineering works. Such arches are very liable to either open at the centre and "kick up" at the haunches, or to fail by the centre voussoirs being forced down. An elliptical arch, especially if very flat, is undesirable for spans of over 8 feet, and should never be used without ample abutments unless beams are placed above the arch as described in Section 196.
The joints of an elliptical arch should be exactly normal (at right angles) to the curve of the soffit. If the line of the soffit is not a true ellipse, but is made up of circular arcs of different radii, the joints in each portion of the arch should radiate from the corresponding centre. Fig. 106 shows an easy method for laying out the joints where the curve of the soffit is a true ellipse. Let M1, M2, M3, etc., be points on the ellipse from which it is desired to draw the joints. Draw tangents to the ellipse at the points A and B intersecting at C. Draw the lines AB and OC. Draw lines from M1, M2, M3, etc., perpendicular to OA and intersecting OC at L1, L2, L3, etc. From these points draw lines perpendicular to AB, intersecting OA at N1, N2, N3, etc. Lines drawn through N1M1, N2M2, etc., will then be normal to the curve and give the joints desired.