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Definitions of Parts of Arches

arch, springing, crown, surface, line and rise

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Abutment: The outer wall that supports the arch, and which connects it to the adjacent banks.

Arch Sheeting : The voussoirs which do not show at the end of the arch.

Camber is a slight rise of an arch, as to inch per foot of span.

Crown : The highest point of the arch.

Extrados : The upper and outer surface of" the arch. Haunches: The sides of the arch from the springing line half way up to the crown.

heading Joint : A joint in a plane at right angles to the axis of the arch. It is not continuous.

Intrados or Soffit: The under or lower surface of the arch. Invert : An inverted arch, one with its intrados below the axis or springing line; e.g., the lower half of a circular sewer.

Keystone: The center voussoir at the crown.

Length: The distance between face stones of the arch. Pier: The intermediate support for two or more arches. Ring Course : A course parallel to the face of the arch. Ring Stones: The voussoirs or arch stones which show at the ends of the arch.

Rise : The height from the springing line to under side of the arch at the keystone.

Skew Back: The upper surface of an abutment or pier from which an arch springs; its face is on a line radiating from the center of the arch.

Span : The horizontal distance from springing to springing of the arch.

Spandrel : The space contained between a horizontal line drawn through the crown of the arch and a vertical line drawn through the upper end of the skew back.

Springing: The point from which the arch begins or springs. Springer : The lowest voussoir or arch stone.

String Course: A course of voussoirs extending from one end of the arch to the other.

Voussoirs : The blocks forming the arch.

Arches: The arch is a combination of wedge-shaped blocks, termed arch stones, or voussoirs, truncated towards the angle of the wedges by a curved surface which is usually normal to the surfaces of the joints between the blocks. This inferior surface of the arch is termed the soffit. The upper or outer surface of the arch is termed the back.

The extreme blocks of the arch rest against lateral supports, termed abutments, which sustain both the vertical pressure arising from the weight of arch stones, and the weight of whatever lies upon them; also the lateral pressure caused by the action of the arch.

The forms of an arch may be the semicircle, the segment, or a compound curve formed of a number of circular curves of different radii. Full center arches, or entire semicircles, offer the advantages of simplicity of form, great strength, and small lateral thrust; but if the span is large they require a correspondingly great rise, which is often objectionable. The flat or segmental arch enables us to reduce the rise, but it throws a great lateral strain upon the abutments. The compound curve gives, when properly proportioned, a strong arch with a moderate lateral action, is easily adjustable to different ratios between the span and the rise, and is unsurpassed in its general appearance. In striking the compound curve, the following con ditions are to be observed: The tangents at the springing must be vertical, the tangent at the crown horizontal, and the number of centers must be uneven, curves of 3 and 5 centers will be found to fulfil all requirements.

In designing an arch the first step is to determine the thickness at the crown, i.e., the depth of the keystone. This depth depends upon the form, and rise of the arch, the character of the masonry, and the quality of the stone; and is usually determined by Trautwine's formula, which is as follows for a first-class cut stone arch whether circular or elliptical.

in which D = the depth at the crown in feet.

•R = the radius of curvature of the intrados in feet. S = the span in feet.

For second-class work, the depth found by this formula may be increased about one-eighth part; and for brickwork or fair rubble, about one-third.

Table 13 gives the depth of keystone for semicircular arches, the second column being for hammer-dressed beds, the third for beds roughly dressed with the chisel, and the fourth for brick masonry.

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