Arches are one of the most elegant spanning systems used in buildings. They are quite common features. Yet a beautifully designed arch always makes us pause for a moment. With the concrete usually running in straight lines and pointed corners, it is always a break to find a free flow of curve in a building.
The Discovery of Arches- A structural need
Arches are employed in architecture for several purposes. It is not just one aesthetically sound building design. The discovery of arches has evolved from a very necessary requirement. This requirement was for building larger spanning systems.
The common trabeated systems of spanning that are lintels and post can only carry the structural load over smaller spans. Common building materials then like stone were poor in tensile strength hence could not support heavy load above the openings.
As human civilization grew, the demand for larger homes and even larger assembly halls emerged, however the system of roofing could not support large roofs. Columns were required to support the roof over any large space. This system of holding the roof with the help of numerous columns was a very clumsy idea. Moreover, a lot of space under the roof was eaten away with the placement of columns with large diameters.
This was the situation when architects came up with the idea to shift from trabeated systems to arcuated systems of spanning.
Arches are a very clever structural solution to covering larger spans as the distribution system of forces in arches are much efficient than in posts and lintels. Arches are purely compressive in nature.
Variation in forms of arches
There a several kinds of variations in the curve of the arch which took place over the time. And now we have a whole array of arches.
The flat arch
The flat arch is looks similar to straight lintel and its work is also similar to that of a lintel. However, the force distribution here is similar to that of an arch and it also can carry heavier load above than a lintel.
The relieving arch
The relieving arch as the term suggests is entirely designed to relieve the lintel of the above structural load. This arch is made above the lintel and is made for purely structural reasons and not aesthetics. The curve is visible under the masonry.
The Dutch or French arch
This arch is also flat and without a curve and is somewhat similar to that of a flat arch. However, the arrangement of bricks and method of construction differs. This arch is not suilable for very large openings.
The semi-circular arch
It is the most common type of arch that is seen. The arc of the arch is a simple semi-circle with the distance between the buttresses as the diameter.
The segmental arch
The curve of the arch in a segmental arch is an arc from the circumference of an entire circle. The arc subtends an acute angle at the centre. In this kind of arch the distance between the two buttresses is less than the diameter of the arc.
The horse shoe arch
The horse shoe arc as the name suggests is an arch flaunting a curvature resembling the shape of a horse shoe. These arches are seen in Islamic architecture therefore also known as the Moorish arch. These arches are also found with the certain modification of pointed heads.
The slited arch
The slited arch is similar to a semi-circular arch except that the buttresses start from a distance slightly below from where the arc ends. This arch exhibits the shape of a half-capsule.
The Florentine arch
The distance between intrados and extrados of a Florentine arch gradually increases from its lowest springing point. It is the greatest at the top. The intrados is near circular while the extrados gradually moves away from the intrados widening the arch ring.
The elliptical arch
The curvature of the elliptical arch is the symmetric half of the ellipse. Arches of this shape are often use in bridges. This is a shallower and wider arch.
Parabolic arch employs a parabolic curve. It employs maximum thrust at the base but can span large distances. It is used in bridges.
The gothic two-centred pointed arch.
It is a two-centred arc in which two arcs of equal length meet at a pointed end at the tip. This kind of arch is also called the gothic arch. Pointed arches evolved as a modification of the common roman arches. These are structurally more efficient and can carry sideways thrust better than circular arches.
There are equilateral, lancet and drop arches which fall under this category. Gothic arches are tall and slender.
The Venitian arch
It is a pointed arch with the arch ring gradually widening almost similarly like the Florentine arch.
The three centred arch
The three centred arch consists of a curve made up of the culmination of three different arcs. The arch is smooth. The three centred arc makes the circular arch shallower and wider.
The four centred arch
Four centred arch like the two centred arch has a pointed end at the top. The curve here is formed of four arc segments. It is an attempt to make the two centred arch more shallow and wide. This too is a Gothic arch.
The five centred and the seven centred arches
These arches are similar to the three centred arches in terms of logic. These arches are shallower and wider and consists of five and seven segments respectively. These arches like all odd number centred arches are also smooth.
The ogee arch
The ogee arch is composed of two S-shaped curves forming a pointed end at the top. There are single centred and two centred ogee arches. We also have reverse ogees which have a smooth head.
The tree foil arch and multi-foil arch
These aches are made up of several arcs from several circles. It may have a pointed or smooth head known as circular head tree foil arch or the pointed head tree foil arch. Such arches have several pointed features in its arch ring.
This is an asymmetric arch which has an unequal curve on both sides of the head.
Arcuated roofing systems
Arcuated roofing systems originated from the necessity for roofing large spaces.
Barrel vaults emerged from translating an arch perpendicular to its front face.
Pointed gothic arches have an advantage over circular ones as then can roof non-rectangular shapes like trapezoids. Roofs made from Gothic arches are higher and slender compared to the circular ones giving the architecture a taste of Gothism.
Groin vaults were the first attempt at dome shaped roofing by intersecting two barrel vaults perpendicular to each other.
Ribbed vaults were formed by intersecting several barrel vaults of different diameters. They formed some elaborate beautiful roofs for the churches. The intersecting lines were evident in the form of ribs, hence the name.
An arch can be rotated 180 degrees to form a dome shape.
Though now the structural requirement has altered due to the advent of steel reinforcements yet an arcuated form in your elevation can often steal the show.