An arch is a structure constructed of wedge-shaped units (bricks or stone), jointed together with mortar and spanning an opening to support the weight of the wall above it along with other super-imposed loads. Due to wedge-like form, the units support each other, the load tends to make them compact and enables them to transmit the pressure downwards to their supports.

The following technical terms are used in arch work :1. Intrados : This is the inner curve of an arch.2. Soffit : It is the inner surface of an arch. Sometimes, intrados and soffit are used synonymously.3. Extrados : It is the outer curve of an arch.4. Voussoirs : These are wedge-shaped units of masonry, forming an arch.5. Crown : It is the highest part of extrados.6. Key : It is the wedge-shaped unit fixed at the crown of the arch.7. Spandril :This is a curved-triangular space formed between the extrados and the horizontal line through the crown.8. Skew back: This is the inclined or splayed surface on the abutment, which is so prepared to receive the arch and from which the arch springs.9. Springing points: These are the points from which the curve of the arch springs.10. Springing line: It is an imaginary line joining the springing points of either end.11. Springer : It is the first voussoir at springing level; it is immediately adjacent to the skewback.12. Abutment : This is the end support of an arch.13. Pier: This is an intermediate support of an arcade.14, Arcade: It is a row of arches in continuation.15. Haunch: It is the lower half of the arch between the crown and skew back.16. Ring : It is a circular course forming an arch. An arch may be made of one ring or more than one ring.17. Impost : It is the projecting course at the upper part of a pier or abutment to stress the springing line.18. Bed joints : These are the joints between the voussoirs which radiate from the centre.20. Span : It is the clear horizontal distance between the supports.21. Rise: It is the clear vertical distance between the highest point on the intrados and the springing line.22. Depth or height : It is the perpendicular distance between the intrados and extrados.23. Thickness (or breadth of soffit) : This is the horizontal distance, measured perpendicular to the front and back faces of an arch.

STABILITY OF AN ARCHAn arch transmits the super-imposed load to the side walls (or abutments) through friction between the surfaces of voussoirsand the cohesion of mortar. Every element of arch remains in compression. It has also to bear transverse shear. An arch may therefore fail in the following ways(i) Crushing of the masonry(ii) Sliding of voussoir(iii) Rotation of some joint about an edge(iv) Uneven settlement of abutment/pier.If the compressive stress or thrust exceeds the safe crushing strength of the materials (i.e., masonry units and mortar), the arch will fail in crushing. Hence, the material used for construction should be of adequate strength, and the size of voussoirs should be properly designed to bear the thrust transmitted through them. The height of voussoirs should not be less than 1/12th the span. For span upto 1.5 m, 20 cm thick arch ring is provided, while for span between 1.5 to 4 m, 30 cm thickness is sufficient. For span between 4 to 6.5 m, 40 cm thickness should be provided whilefor span more than 6.5 m, the thickness at springing may be increased by about 20% of the thickness at the crown. Sometimes, voussoirs of variable heights are provided—less height near crown and more height at skewback. To safeguard against sliding of voussoirs past each other due to transverse shear, the voussoirs of greater height should be provided. Also, the angle between the line of resistance of the arch and the normal to any point should be less than angle of internal friction. Rotation can be prevented if the line of resistance is kept within intrados and extrados. Also, the line of thrust should be made to cross the joint away from the edge to prevent the crushing of that edge. It should be within middle third of the arch height.The uneven settlement of abutment may cause secondary stresses in the arch. Hence the abutment, which has ultimately to bear all the loads transferred to it through the arch, should be strong enough. Also, the arch should be symmetrical, so thatunequal settlements of the two abutments is minimised. Also, the abutment should be strong enough to take the thrust.

CLASSIFICATION OF ARCHES:1. Shape2. Number of Centres.3. Materials and Workmanship

CLASSIFICATION BASED ON SHAPE

ISOCELES POINTED ARCH FLAT ARCH

SEGEMNTAL ARCH

HORSE SHOE ARCHEQUILATERAL ARCH

RELIEVING ARCH

FLORENTINE ARCH

STILTED ARCH

VENETIAN ARCHSTILTED ARCH

CLASSIFICATION BASED ON NO. OF CENTRES

ONE CENTERED ARCH

ONE CENTERED ARCH

TWO CENTERED ARCH (EQUILATERAL ARCH)

THREE CENTERED ARCH (BASKET HANDLE)

FOUR CENTERED ARCH

FIVE CENTERED ARCH

CLASSIFICATION BASED ON MATERIAL AND WORKMANSHIP

A) STONE ARCHES – 1. RUBBLE 2. ASHLAR

RUBBLE ARCHES

B) BRICK ARCHES– 1. ROUGH BRICK ARCH, 2. AXED BRICK ARCHES, 3. GAUGED BRICK ARCH, 4. PURPOSE MADE BRICK ARCH.

AXED BRICK ARCH

ROUGH BRICK ARCH

GAUGED BRICK ARCH

C) CONCRETE ARCHES – 1. PRE CAST, 2. CAST IN SITU.

CONSTRUCTION OF ARCHES 1. INSTALLATION OF CENTERING OR FORMWORK.2. LAYING OR CASTING OF ARCH.3. STRIKING OR REMOVAL OF FORMWORK.

CENTERING OR FORMWORK

LAYING OF AN ARCH.

1. Voussoirs are arranged in proper and required forms.2. Laying proceeds from skewbacks towards the crown.3. Before inserting key the turning piece or the center is slightly slackened

(2-3mm) to lower the height so that voussoirs settle on their beds properly.

4. Key stone is inserted in the end so that all the voussoirs get locked.