What Is An Arch In Architecture-Everything You Need To Know
What Is An Arch-Definition?
An arch is a structure constructed of wedge-shaped units ( made by brick or stone), joined together with the mortar, and spanning an opening to support the weight of the wall above it along with the other superimposed loads. Due to wedge-like form, the unit supports each other, and the load tends to make them compact and enables them to transmit the pressure towards their support.
Some Important Facts To Know About Arches [ A Short History ]
Arches were known in ancient Egypt and Greece but were considered unsuitable for monumental architecture and seldom used. The Romans, by contrast, used the semicircular arch in bridges, aqueducts, and large-scale architecture. In most cases they did not use mortar, relying simply on the precision of their stone dressing.
The Arabs popularized the pointed arch, and it wa
s in their mosques that this form first acquired its religious connotations. Medieval Europe made great use of the pointed arch, which constituted a basic element in Gothic architecture.
In the late Middle Ages, the segmental arch was introduced. This form and the elliptical arch had great value in bridge engineering because they permitted mutual support by a row of arches, carrying the lateral thrust to the abutments at either end of a bridge.
What Are The Various Components Of An Arch
The designing and construction of an arch is an extremely difficult and delicate job, for a mason and it needs tight supervision during construction. It is made up of various components and each component has its own unique architectural and structural features. So here is the list of different components, that will help you to understand the anatomy of an arch.
|#1||Abutment||This is the support at the end of an arch.|
|#2||Arcade||It is a row of arches in continuation.|
|#3||Bed Joints||These are the joints between the voussoirs which are radiate from the center.|
|#4||Center or Striking Points||This is the geometrical center point from where the arches from the extrados, arch rings, and intrados are described or struck.|
|#5||Crown||It is the peak of an extrados.|
|#6||Depth||It is the perpendicular distance between the intrados and extrados.|
|#7||Extrados||It is an external curve of an arch, the upper surface of an open arch.|
|#8||Haunch||It is the lower half of the arch between the crown and skewback.|
|#9||Intrados||This is a curve inside of an arch|
|#10||Impost||It is projecting the course of the upper part of the pier or the abutment to stress the springing line|
|#11||Key||It is a wedge-shaped unit of an arch fixed at the crown.|
|#12||Pier||This is the intermediate support of an arcade.|
|#13||Ring||It is a circular course forming an arch. An arch may be made of one ring or more than one ring.|
|#14||Rise||It is the clear vertical distance between the highest points of intrados and the springing lines.|
|#15||Soffit:||It is the inner surface of an arch. Sometimes intrados and soffits named simultaneously.|
|#16||Spandril||This is a curved space in form of a triangle between the extrados and the horizontal line through the crown.|
|#17||Skew Back||This is an inclined or splayed surface on the abutment, which is so prepared to receive the arch and form which the arch springs.|
|#18||Springer||It is the first voussoir at the springing level; it is immediately adjacent to the skewback.|
|#19||Springing points||These are the points from which the spring of the arch swings.|
|#20||Springing line||It is an imaginary line joining the springing points of either end.|
|#21||Span||It is the clear horizontal distance between the support.|
|#22||Thickness||This is the horizontal distance, measured perpendicular to the front and back faces of an arch.|
|#23||Voussoirs||These are wedge-shaped masonry blocks, forming an arch.|
Why Archs Are Stronger?
An Arch transmits the superimposed load to the side of the wall or abutments through friction between the surface of voussoirs and the cohesion of mortar. Every element of the arch remains in compression. And it also has the ability to bear the transverse sear loads. this is why they provide extra stability than a beam.
Causes Of Failure Of An Arch
Though the arch is a safe technique of construction of any load-bearing structures above it, whether it is a large spaned arch bridge or small arch window opening but there are still have some reasons for the failure of an arch.
- Crushing of the masonry
- Sliding of voussoir
- Rotation of some joint about and edge
- Uneven settlement of abutment/pier
Construction Of An Arch
As we stated before the construction of an arch is a very delicate job for which highly skilled masons are required, here in this section we will discuss the various steps involved in arch construction.
The construction of arches, of all type of materials ( i.e. bricks, stone, concrete) is carried out in three steps:
- Installation of formwork or centering
- Laying of the casting of the actual arch.
- Striking or removal of formwork.
Installation Of Formwork or Centering
Centering is the temporary structure needed to support the brick, stone, or concrete arch during the construction until it has gained sufficient strength. The centering is installed in such a way its upper surface corresponds with the intrados of the arch.
For minor works, centering may be of mud masonry constructed to match with the inner soffit of the arch and then plastered. This masonry is dismantled later when the arch has been constructed or cured.
The usual centering is made of timber or steel. Woden centering is the simplest and the cheapest, used for a moderate span. It is easy to construct and easy to dismantle and it can be used several times. A thick wooden plank is, with a horizontal bottom and an upper surface shaped to the underside of the soffit. Such a plank is known as center or turning pice. Its width is normally 10 cm and is supported on the vertical timber posts called props, with wooden wedges to tighten or loosen the centering.
If the soffit is wider than 10 cm, two ribs, suitably spaced and suitably shaped at the top may be used. These ribs may be connected by 4 x 2 cm wooden called laggings. At the ends, the ribs are supported by bearers, wedges, and posts.
For wider soffits, and for larger spans, a built-up centering of cut wood ribs is used. The upper surface of the ribs is given the shape of the soffit of the arch. Laggings (or cross-battens) are nailed across the ribs at close intervals to support the voussoirs at their top. Ribs are kept 25 to 40 mm thick, with widths varying from 20 to 30 cm. The distance between ribs depends upon the thickness of the wall supporting the arch. The ribs are connected by braces and struts to strengthen them. Horizontal ties are provided at the lower ends of the ribs to prevent them from spreading. The ribs are supported on bearers, and a pair of folding wedges is provided at the top of each prop to tighten or loosen the centering.
Laying of Arch
After the erection or installation of centering, skewbacks are first prepared. Voussoirs are then arranged in proper and required forms, starting from skewbacks and proceeding towards the crown. Finally, a keystone (or key brick) is inserted so that all the voussoirs are locked in position. The voussoirs should be properly bedded. After that, the center or turning piece is eased by slackening the wedges so that it is lowered by a height of 2 to 3 mm. Such a process is an essential requirement in stone arches since it permits the voussoirs to settle upon their beds properly.
Removal of Centering
When the arch has developed sufficient strength, the centering can be removed. No load should be placed on the arch unless the centering has been removed. For small spans, the removal of centering is done by loosening the folding wedges. When the span is more than 7 m, the sandbox method can be used for loosening, so that shocks
are avoided. A sandbox, shown in Fig. 13.30, is placed below the prop. Sand is filled in a box with a plugged hole at its bottom. Prop rests on the steel plate placed on the top of the sand. In order to lower the centering, the plug is taken out due to which the sand flows out and lowers the prop gradually.