This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
The outer curve of an arch ring is called the extrados or back. The term back is also applied to the whole of the upper surface of an arch.
The extremities of the intrados and extrados are called springing points.
The horizontal line drawn through the springing points of the intrados is called the springing line. This term is also applied to the line formed by the junction of the under surface of the arch with its support.
The horizontal distance between the springing points of the intrados is called the span.
The vertical distance from the springing line to the highest point of the soffit is called the rise.
The highest point on the extrados is called the crown.
-The lower portion of an arch from the springing to a point midway between the springing and the crown are called the haunches.
The triangular spaces above the haunches of an arch are called the spandrels.
A series of arches with its supports is called an arcade.
An isolated mass of masonry supporting one end of an arch is called an abutment, or more correctly an abutment pier. The term abutment is usually applied to the end support of an arcade.
The intermediate supports of arcades are called piers.
A pier whose horizontal section is a circle or a regular polygon is called a column.
The ornamental head of a column is called a capital.
The moulded foot of a column is called the base.
The portion of a column between the base and the capital is called the shaft.
Arches may be classified, according to the shape of their intrados, into three groups - (1) flat arches; (2) arches struck from centres; (3) arches whose intrados forms a curve other than a segment of a circle.
1. An example of a flat arch is shown on the left-hand side (exterior) of Fig. 176.
2. Arches struck from centres may themselves be classified according to the number of centres used in striking them, as shown in Fig. 177.
Arches struck from two, three, and four centres are met with in Gothic work.
Semicircular arches are met with in Classic, Romanesque, and Renaissance work.
Segmental arches are used considerably in Renaissance work, and are not infrequently used in Gothic work.
The horseshoe arch is of Sarasenic origin.
The three-centred arches are often considered as particular cases of the four-centred arch.
3. Arches in this group are usually elliptical, parabolical, hyperbolical, etc.; but the elliptical arch is practically the only one used in architectural work.
Arches may again be classified into three groups according to their workmanship, as follows: -
(a) Rough arches.
(b) Axed arches.
(c) Gauged arches.
Rough Arches are used when the appearance of an arch need not be considered, or when it is intended to plaster over the face of the arch, as in internal doorways. The bricks are not cut to a wedge shape, but the necessary radiation is obtained by setting them with the edges close together on the soffit and open on the extrados, so that the mortar joint becomes wedge shaped, and radial lines would pass through the centre both of the joints and the bricks. Fig. 178 shows a rough semicircular arch, suitable for an internal door opening, composed of four concentric or rowlock rings, 4 1/2 inches deep.
It will be noted in Fig. 178 that there is no tie between the concentric rings save that of the adhesion of the mortar. This defect is remedied by laying courses of headers, called Lacing Courses, to bind the rings together at intervals where the joints in the rings most nearly coincide, as in Fig. 179. In arches of quick curvature it is better to roughly cut the bricks of the lacing courses to the proper wedge shape in order to avoid very large joints on the extrados. In large arches the bricks of the lacing courses need not be cut and need not pass right through the entire thickness of the arch, but pairs of rings may be laced together, as in Fig. 179.
Rough arches of small rise are used in conjunction with lintels to relieve the latter of the superincumbent weight; these are termed Relieving or Discharging arches. The most frequent position for such arches to occupy is over door and window openings. The usual method of construction for door openings is shown in Fig. 180. The opening is first bridged by a wood lintel, which should project at least 4 1/2 inches on either side of the opening. The depth of' the lintel in inches should be at least equal to the span of the opening in feet, but no lintel should be used less than 3 inches deep. Lintels are usually cut out of 4 1/2-inch battens, as this size is most convenient for brick walls, whose thicknesses are multiples of 4 1/2 inches. A brick core is now built on top of the lintel. To do this a 1/2-inch board is cut to the required curve; bricks are then laid on top of the lintel, the mould laid against them, and the curve marked upon their faces. The bricks are then cut with an axe and set. The skew-backs are then formed and the arch built up, using the core as a turning piece. The span of a relieving arch should be at least as long as the lintel.