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Description of Arches

arch, intrados, ring and built

DESCRIPTION OF ARCHES.

Arch

: One in which the intrados resembles a semi-ellipse, but is composed of arcs of circles tangent to each other Circular Arch ; One in which the intrados is a part of a circle. Discharging Arch : An arch built above a lintel to take the superincumbent pressure therefrom.

Elliptical Arch :

One in which the intrados is a part of an ellipse.

' Geostatic Arch : An arch in equilibrium under the vertical pressure of an earth embankment.

Hydrostatic Arch :

An arch in equilibrium under the vertical pressure of water Inverted Arches are like ordinary arches, but are built with the crown downwards. They are generally semicircular or segmental in section, and are used chiefly in connection with foundations.

Plain or Rough Arches are those in which none of the bricks are cut to fit the splay. Hence the joints are quite close to each other at the soffit, and wider towards the outer curve of the arch; they are generally used as relieving and trinzer arches, for tunnel lining, and all arches where strength is essential and appearance no par ticular object. In constructing arches of this kind it is usual to form them of two or more four-inch concentric rings until the required thickness -is obtained. Each of the successive rings is built inde pendently, having no connection with the others beyond the adhesion of the mortar in the ring joint. It is necessary that each ring should

be finished before the next is commenced; also that each course be bonded throughout the length of the arch, Ďand that the ring joint should be of a regular thickness. For if one ring is built with a thin joint and another with a thick one the one having the most mortar will shrink, causing a fracture and depriving the arch of much of its strength.

Pointed Arch : One in which the intrados consists of two arcs of equal circles intersecting over the middle of the span.

Relieving Arch : See Discharging Arch.

Right Arch : A cylindrical arch either circular or elliptical, terminated by two planes, termed heads of the arch, at right angles to the axis of the arch.

Segmental Arch: One whose intrados is less than a semicircle. Semicircular Arch : One whose intrados is a semicircle; also called a full-centered arch.

Skew Arch : One whose heads are oblique to the axis. Skew arches are quite common in Europe, but are rarely employed in the United States; and in the latter when an oblique arch is employed it is usually made, not after the European method with spiral joints, but by building a number of short right arches or ribs in contact with each other, each successive rib being placed a little to one side of its neighbor.