This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
268. In place of cambering ox curving the under side of a flat brick arch over an opening, the soffit is often made flat and is supported on an iron angle bar. This form of construction is shown in Fig. 119; at (a) is given apart elevation of a window opening with 12-inch flat arch, supported on 1 1/2" x 1 1/2" X 1/4" iron angle as shown at a; (b), in the same Figure, shows a section through the arch, a being the iron angle, c the 4-inch brick arch, and b the wooden lintel behind the arch.
269. Sometimes, when the construction will admit, the wooden lintel may be cut on a segmental curve, as shown in Fig. 120. In this sketch, a shows the 4-inch wooden lintel cut to the required curve, and b the two-rowlock brick arch resting on the curve of the lintel. This arch is called a relieving arch, because if there is any shrinking of the wooden lintel, there will be no settlement of the brickwork, the arch carrying the weight of the wall placed on it. Arches are usually built over stone lintels to keep them from cracking.
In some cases the arch is turned over an ordinary lintel, with the spring of the arch starting from the ends of the lintel, and a core of brickwork between the under side of the arch and the top of the lintel.
This is shown in Fig. 121; a is the lintel, which may be either of wood or stone; b is the relieving arch, shown in this case as a two-rowlock arch; and c is the brick core between the lintel and the under side of the arch.