Components of an Arch in Building Construction
An arch is a curved beam in which horizontal movement at the support is wholly or partially prevented. Arch structures are constructed with wedge-shaped bricks or stones joined together with mortar.
Under the action of loads, the arch bricks are under compression load. Hence the induced tension or thrust is transferred to the supports. Here the supports are the abutments as shown in the figure-3.
In this article, we will discuss in detail the main components of arches and their behavior under the action of loads.
Table of Contents- Why are Arches Constructed in Buildings?
- Components of an Arch in Buildings
- General Stability of Arches in Buildings
Why are Arches Constructed in Buildings?
Walls and windows are an inevitable feature of any building. At the same time, these openings are the most critical elements of a building which require proper support to prevent collapse. Hence, to support a wall with an opening, we either provide a lintel or an arch above the opening. As shown in the figure below, an arch would transfer the load over it mainly through thrust and friction between its units. In addition, arches are also constructed for bridges to impart special design features or architectural aesthetics.
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| Fig.2. Arch as a Lintel Over Opening |
To summarise, arches are constructed in the following circumstances:
- Where the span is more
- Over the openings in walls
- When the loads are heavy
- For special architectural appearance
Components of an Arch
The major parts of an arch can be easily familiarized with the help of Figure 2 below. There are around twenty components for an arch structure. Here among various types of arches, we have taken segmental arch for an explanation of component parts.
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| Fig.3. Components of an Arch |
1. Ring
The circular course forming an arch is called an Arch Ring or Ring. An arch can be formed by one or a combination of rings.2. Voussoirs
Voussoirs are wedge-shaped or tapered units of bricks, stones, or concrete works, forming the courses of an arch.3. Extrados
It is the external curve of an arch.
4. Intrados
It is the inner curve of an arch.
5. Soffit
A soffit is the underside of a part of a building, like a protective covering under the surface of an arch or under the eaves of a house. Soffit is the inner surface of an arch
6. Crown
This is the highest point of the extrados or the highest point of an arch is called as crown.
7. Key
Key or keystone also called as capsule stone is a wedge-shaped stone at the apex of an arch and it is the last piece placed in construction. It locks all the stones of the arch so that it can bear the entire weight of the arch.
8. Abutment
The portion of the wall which supports the arch is termed as abutment. It forms the end support of an arch.
9. Arcades
Arcades are row of arches in continuation or succession that are supported on piers or columns and carry a wall over it. As shown in figure below, a series of arches are constructed and supported intermediately by columns or piers and at the end it is supported by abutments.
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| Fig.4. Arcades |
10. Pier
These are the intermediate supports of a row of arches i.e. the arcades.
11. Span
The clear distance between the supports of an arch is called as span.
12. Skewback
Skewback forms the inclined or splayed surface on the abutment from which the arch springs. It is a sloping surface against which the end of an arch rest by means the portion of support on which the first voussoir of arch placed.
13. Springer Points
The point in the arch from which the curve of an arch springs.
14. Springer Line
This is the imaginary line joining the two springing points of an arch.
15. Springer
The first voussoir at the springer point forms the springer. or it is the first voussoir at the springing level. It is immediately adjacent to the skewback.
16. Spandril
Spandril is the space between the extrados and the horizontal line through the crown. It forms a curved triangular space.
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| Fig.5. Spandril |
17. Haunch
Haunch forms the lower half of the arch between the crown and the skewback or springer.
18. Rise
Rise is the clear vertical distance between the highest point on the intrados and the springing line.
19. Centre of an Arch
The center of the arch is the geometric center of the curve that forms the arch.
20. Depth or Height of Arch
The depth or height of the arch is the perpendicular distance between the intrados and the extrados.
21.Thickness or Breadth of Soffit
The horizontal distance measured perpendicular to the front and back faces of the arch is termed as thickness of arch or breadth of soffit.
Stability of Arches
Stability is an important feature of arches. However, as the arch transfers the load to abutment by thrust, there are chances of failure in the following ways.
1. Crushing of masonry
The materials used in arches should be of good quality, otherwise there are chances of the masonry being crushed under the load. Voussoirs should be designed well to transmit the load. The height of voussoirs should not be less than 1/12th the span.
2. Sliding of voussoirs
Though the load transfer in arches is mainly by thrust, some shear is unavoidable. The shear force makes the units slide one over the other. To prevent such failures, voussoirs of sufficient height should be provided.
3. Rotation of some joint about an edge
To prevent rotation, the line of the thrust or resistance should lie within the intrados and the extrados. Also, the line of the thrust may be made to cross each joint away from the edge to avoid crushing at an edge.
4. Uneven settlements of abutment/pier
Abutments are subjected to compressive forces and thrust, causing settlement. To avoid settlement, the abutment/pier should be strong enough to withstand loads. Uneven settlement is avoided if the arches are symmetric.
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