Stability of Arches

Basically the stability of an arch depends on the friction developed between the wedge-shaped blocks and the cohesion of the mortar.

The stability is disturbed by one of the following reasons:

1. Crushing of arch material
2. Sliding of wedge-shaped blocks
3. Rotation about an edge
4. Differential settlement of supports

1. Crushing of Arch Material

Crushing of arch material can be safeguarded by taking the following precautions:

1. The size of the wedge-shaped blocks should be adequate enough to withstand the pressure exerted by the adjacent blocks.
2. The thickness of the arch ring is accordingly adopted depending on the span. As a general rule, the thickness of the ring may be taken 1/12th of the span.
3. High quality stones or bricks should be used for the arch. In case of large spans, steel reinforcement may be used to increase the strength.

2. Sliding of Blocks

In order to prevent sliding of blocks, the following points are considered:

1. All the bed joints are made perpendicular to the line of least resistance. To satisfy this, they are made normal to the curve of the arch.
2. The depth of wedge blocks should be adequate to resist the tendency of the joints to open and slide upon one another.

3. Rotation About an Edge

This is prevented by adopting the following aspects:

1. The thrust on any block should be within the middle third of the arch.
2. The thickness of arch is such that the line of resistance atleast falls within the section and crosses each joint away from the edge.

4. Differential Settlements of Supports

This is taken care of by observing the following points:

1. The supports are to be adequately strong to withstand the thrust of the arch due to self-weight and the super-imposed loads.
2. Irrespective of the type of arch, the arch should be symmetrical.