ARCHES

An arch is a mechanical arrangement of wedge-shaped blocks of stones or bricks mutually supporting each other and in turn supported at the ends by piers or abutments. However, arches made of steel and RCC are built as a single unit without the use of wedge-shaped units.

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.

19.2.2 Terminologies in Arch Work

Following are the terminologies used in arch work (Fig. 19.1).

Figure 19.1 Various parts of an arch

1. Abutments

These are the outermost supports of an arch from which the arch starts. That is, this is the wall which supports the arch.

2. Springing Line

It is the horizontal line joining the two springing points.

3. Span

This is the width of the opening covered by the arch.

4. Intrados

The inner curve of the arch.

5. Extrados

The outer curve of the arch.

6. Arch ring

This is the curved portion of masonry forming the arch.

7. Voussoirs

Wedge-shaped blocks of stone or brick masonry of which the arch ring is made.

8. Key Stone

The central voussoir at the central point (crown) of the arch.

9. Crown

The highest part of the extrados.

10. Soffit

The inner or under surface of the arch.

11. Skewbacks

This is the springing point of an arch at the support.

12. Springing Points

The points at the intersection between the skewbacks and the intrados.

13. Springer

The lowest voussoir next to the skewback.

14. Haunch

The lower half of the arch between skewback and crown.

15. Rise

The vertical distance between the highest point of intrados and springing line.

16. Spandrill

The triangular space between the back of the arch ring and a horizontal plane tangent to it at the crown.

19.2.3 Types of Arches

Intrados is a combination of arcs of various radii. Based on the number of centres, arches may be classified as follows:

1. One-Centred Arch

Under this category, semi-circular arch, segmental arch (which is less than a semi-circle), horse-shoe arch (which includes more than a semi-circle) and stilted arch (which consists of semi-circular arch with two vertical portions at the springings) are included. Figure 19.2(a–d) schematically represent these arches.

Figure 19.2 Types of arches

2. Two-Centred Arches

There are different types of two-centred arches. Three of them are shown in Figs. 19.2(e), (f) and (g). They are as follows:

(i) Blunt Arch

The two centres of the arch are within the arch itself.

(ii) Gothic Arch

This is also called as equilateral arch. Here the radius of intrados are equal to the span length and the centres are on the springing line.

(iii) Acute Arch

In this type, the centres of the arch are outside the arch.

3. Three-centred Arches

Here there are two types. In the first type, the central portion of the arch is drawn with one centre and the other two centres are used for making up the ends of the arch (Fig. 19.2(h)). In the other type, the procedure is reversed (Fig. 19.2(k)).

4. Four-centred Arches

Here the centres for the lower section do not coincide as in the case of three-centred arches (Fig. 19.2(i)).

(i) Two Cusped Arch

It is used mainly for decorative purposes (Fig. 19.2(j))

(ii) Elliptical Arch

It has an elliptical shape for its intrados.

(iii) Flat Arch

In this type of arch there is no size (Fig. 19.2(k))

Materials Used for Arches

Materials used for arches are stone, brick and concrete. A classification based on number of centres is discussed in Section 19.2.3. A classification of arches based on materials is also feasible which are discussed below.

1. Stone Arches

Ashlar and rubble masonry are best for arch construction.

Ashlar arches are constructed of stones which are cut and dressed to their true wedge shapes. Ashlar arches are used for superior work and have a good appearance. Ashlar arches can also be made to give a flat arch appearance (Fig. 19.3). These flat arches are strengthened by related or jogged joints. For excessive loads, relieving arches can be built.

Figure 19.3 Flat arch of ashlar masonry

For rubble masonry arches, roughly dressed stones are made. They are arranged and fitted into a definite arch shape using cement mortar. Because of unevenness of individual stones, the points are thicker. These arches comparatively have less strength than that one made of ashlar masonry (Fig. 19.4).

Figure 19.4 Relieving rubble masonry arch

2. Brick Arches

The following four types of brick arches are in use:

1. Gaughed arches
2. Axed or rough cut arches
3. Rough brick arches
4. Brick flat arches

The above classification is based on the type of bricks used in the arch construction. The different types of bricks which can be used for arches are as follows:

1. Ordinary standard bricks.
2. Ordinary bricks cut to a wedge shape.
3. Special bricks of different sizes and shapes called as purpose-made bricks.
4. Soft bricks which are made of diatomaceous earth. They can be sawn and rubbed to the required shapes.

In gaughed arches, bricks are cut to the required wedge shapes so as to suit the various voussoir sizes. True radii joints are made with thin joints. The required centring is erected and each voussoir is fitted in its position with mortar. After all the voussoirs are in position, the key stone is fitted and hammered slightly. Figure 19.5 shows gauged flat arch and semi-circular arch.

Figure 19.5 Gauged brick arches

For axed arches, bricks are cut with a bricklaying axe to make wedge-shaped bricks. They show rough joints. Construction procedure is same as that of gauged arches.

Rough brick arches are constructed using ordinary bricks. Here, the length of extrados is greater than that of the intrados. Thus the joints near the extrados are wider. These arches do not give pleasing appearance.

Flat brick arches are constructed using cut bricks made to wedge shape and arranged such that to have a flat under surface (Fig. 19.5(a)). All the joints are made to radiate from one point. One brick or half brick may be used. In case of large loads, segmental or semi-circular arches can be built above the flat arches in order to reduce the load on the latter (Fig. 19.6).

Figure 19.6 Relieving arch over flat arch

3. Concrete Arches

Now-a-days, only RCC arches are used. Here, the blocks are similar to stone voussoirs and of precast type. These arches are used only for ornamental purposes.

Methods of Construction of Arches

Construction of arches is done under the following three operations:

1. Installation of centring
2. Laying the arch
3. Removal of centring

1. Installation of Centring

Arches are generally built over a centring made to the shape of the arch needed. The centring is a temporary structure which has to support the arch till it develops strengths. For small spans, mud masonry is used. The masonry is made to the shape of the soffit of the arch to be built and plastered, over which the arch is constructed. After attainment of adequate strength, the masonry can be dismantled.

Timber centring is preferred generally as it is easier to erect and dismantle. Further, the material can be re-used. Thick wooden planks are used to get the shape of the soffit and are supported by vertical timber posts. Wedges are used to tighten or loosen the centring.

For heavy arches, the shape is got by suitable braces and struts. In such cases it is necessary to support the posts on boxes containing sand. A hole is kept plugged into the box. The post sinks slowly when the plug is removed and the sand flows out, thereby relieving the arch centring without any shock.

2. Laying of Arch

Skewbacks at the springing of the arch are prepared first and placed very carefully in the correct angles. Voussoirs are then laid on either side of the crown. In case of brick arch, bricks may be placed as showing stretcher-face or header-face. More than one ring may be used. Joints are to be filled with mortar not less than 5 mm and not more than 15 mm thick. Finally the key stone is fitted in to lock the whole arch in position. In order to counteract the thrust, the masonry above the skewback at the haunches should be loaded by filling up the spandrels up to the level of the crown of the arch or at least two-thirds of the height of the arch. This has to be done on both sides of the arch. Before hardening of the mortar, the centring is slightly slackened by means of easing the wedges thereby permitting the voussoirs to rest on their beds properly. The skewback is kept wet during the work. The above procedure is followed for stone and brick arches. For concrete arches the procedure followed for lintels may be followed.

3. Removal of Centring

Sandbox method may be followed, which permits removal of centring without any shock. Releasing of wedges in order may also be followed. Whatever may be the material used for centring, the design should be proper such that the centring is lowered with ease and without any shock.