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Voussoir Arches

arch, ring, voussoirs, joint, intrados, fig and composed

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VOUSSOIR ARCHES masonry arch is a structure of masonry spanning an opening and carrying its loads as longitudinal thrust, which exert outward as well as vertical thrusts upon the abutments. A voussoir arch is one in which the arch ring is composed of a number of independent blocks of stone or masonry.

Parts of an Arch.The principal parts of an arch are as follows: The under or concave surface of an arch is called the soffit. The outer or convex surface is the back.

The crown is the highest part of the arch ring (EF, Fig. S3).

The skcwbacks are the joints at the ends of the arch where it rests upon the abutments (CA, BD, Fig. S3).

The intrados is the intersection of the soffit with a vertical plane perpendicular to the axis of the arch (AIsB, Fig. S3).

The extrados is the intersection of the outer surface with a vertical plane perpendicular to the axis (CPD, Fig. S3).

The springing lines are the intersections of the skewbacl:s with the soffit.

The span is the distance between springing lines.

The rise is the perpendicular distance from the highest point of the intrados to the plane of the springing lines.

The roussoirs are the wedge-shaped stones of which an arch is composed.

The keystone is the voussoir at the crown of the arch The springers are the voussoirs next the skewhacks.

The haunch is the portion of the arch between the keystone and springers.

The arch ring is the whole set of voussoirs from skewback to skew back.

The ring stones are voussoirs showing on the face of the arch.

The arch sheeting is the portion of the arch ring not showing at the ends.

Backing is masonry above and outside the arch ring.

The spandrel is the space between the back of the arch and the roadway above. The walls above the ring stones at the ends of the arch are spandrel walls and the filling between these walls is spandrel filling.

Kinds of arch is one whose intrados is a semicircle. A segmental arch is a circular arch whose intrados is less than a semicircle. A pointed arch has an intrados composed of two cir cular arcs which intersect at the crown. A three-centered arch com posed of arcs tangent to each other is sometimes called a basket handled arch.

A right arch is one whose ends are perpendicular to its axis. An arch whose ends are oblique to its axis is called a skew arch.

Hinged arches are those in which hinged joints are used at crown and skewback. Those without hinges are called solid arches.

156. Theory of voussoir arch is supposed to he composed of a number of independent blocks in contact with each other and held in place by the pressures between them. In Fig. 84, let ABCD represent a voussoir at any part of an arch ring. If P is the pressure received from the voussoir above and 11' the external load carried by the voussoir, the resultant, B, of these forces will be the pressure transmitted to the voussoir below. If the line of action of this resultant should pass outside of the joint the arch will fail by the voussoir rotating about the edge of the joint.

If the point of application of B is outside the middle third of A-D, there will be a tendency for the joint to open on the opposite side, and the area of contact between the voussoirs will be reduced. If the line of action of R makes an angle with the normal to the joint A-D greater than the angle of friction for the surfaces upon each other, the voussoirs may slide upon each other, causing failure of the arch.

For stability of the arch: (1) The resultant pressures between voussoirs should act within the middle third of the joints.

(2) The components of the resultant pressures parallel to the joints (R sin a) should be less than the frictional resistance of the voussoirs to sliding upon each other. (3) The unit pressures at the surfaces of contact should be less than the safe compressive strength of the material of the voussoirs.

If b represents the width of the joint. AD, x the distance of the point of application of R from the nearest edge and a the angle made by R with the normal to the joint, the maximum unit compression will be represented by Usually the angle a is so small that cos a may he taken as 1 with out sensible error, or R may be considered as equal to its normal com ponent.

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