Stability of an Arch

The stability of an arch depends on the friction among the surfaces of wedge- shaped blocks known as voussoirs and the cohesion of mortar. Every component of arch remains within compression. It has also to bear transverse shear. An arch might, thus, fail in the following ways:

(a)        Crushing of the arch material.

(b)       Sliding of wedge-shaped voussoirs or blocks.

(c)        Rotation or overturning of some joint about an edge.

(d)       Differential settlement of abutment/pier or supports.

The arch will fail in crushing if the compressive stress or thrust exceeds the safe crushing strength of the materials. Therefore, the material used for construction should be of adequate strength, and the size of voussoirs should be correctly designed to bear the thrust transmitted by them. For little spans, the thickness of the arch ring is kept uniform from crown to the springing. Since a rule, the thickness of the ring might be taken either 1/12th the span or as follows (for brickwork in cement mortar, 1: 4).  For span up to 1.5 m - 20 cm; spans among 1.5 to 4 m - 30 cm; spans among 4 to 7.5 m - 40 cm, for span more than 7.5 m, the thickness at springing might be increased through about 20% of the thickness at the crown. For arch work, just first class blocks should be used, and in case of large spans, the arches might be strengthened through steel reinforcement, so in which the safe crushing strength is not exceeded.

Sometimes, voussoirs of variable heights are given along with less height near crown and more height at skew-back. For safeguard against sliding of voussoirs past each other because of transverse shear, a voussoirs of greater height should be given. Also, the angle among the line of resistance of the arch and the general to any point should be less than angle of internal friction. To avoid Rotation, the line of resistance is remaining within intrados and extrados and the line of resistance or thrust should be made to cross the joint away from the edge to avoid the crushing of that edge. It should be inside middle third of the arch height. The differential settlement of abutment might cause secondary stresses in the arch. Therefore the abutment, that has ultimately to bear all the loads transferred to it by the arch, should be strong enough. In addition, the arch should be symmetrical, so in which unequal settlements of the two abutments are minimized.