An arch is a structure that covers part of the space between two walls or between two columns. It serves as a decorative element in buildings. Arches are built with wedge-shaped stones called voussoirs. These are placed on top of structural supports.
The parts of an arch are the span, the rise and the springing line.
The voussoir at the top of the arch is called the keystone. It receives the weight of the wall and distributes it sideways.
Each arch has a different name, depending on its shape:
Round arch
The round arch or Roman arch is one that is shaped like half a circumference. Its centre point is on the springing line.
Segmental arch
The segmental arch is one that is shaped like half a circumference. Its centre point is below the springing line.
Equilateral pointed arch
In an equilateral pointed arch, the centres of the arcs are the points of support called the springing points. To construct a pointed arch, given the springing points A and B, follow these steps:
Step 1. Use centre point A and radius AB. Draw an arc.
Step 2. Use centre point B and radius AB. Draw an arc that intersects the previous arc, forming a pointed arch.
Horseshoe arch
The horseshoe arch consists of an arc of a circumference. It is longer than the corresponding half a circumference from the same springing points. So, its shape looks like a horseshoe.
To construct a horseshoe arch from its springing points A and B, and with radius r, follow these steps:
Step 1. Use centre point A and radius r. Draw an arc.
Step 2. Use centre point B and radius r. Draw an arc that intersects the previous arc at point C.
Step 3. Use centre point C and radius r. Draw the horseshoe arch from point A to point B.
Three-centred arch
The three-centred arch is formed by a segmental arch, with two arcs of a circumference at its ends.
To construct a three-centred arch from its springing points A and B, follow these steps:
Step 1. Draw the perpendicular bisector of the springing line AB. Label the midpoint C.
Step 2. Find the midpoint of AC and label it M. Then find the midpoint of CB and label it N.
Step 3. Use centre point M and radius MN. Draw an arc. Then use centre point N and the same radius. Draw an arc that intersects the previous arc at point P.
Step 4. Draw the equilateral triangle MP. Draw the rays PM and PN.
Step 5. Use centre point M and radius MA. Draw an arc from A that intersects the ray PM at point F.
Step 6. Use centre point N and radius CN. Draw an arc from B that intersects the ray PN at point G.
Step 7. Use centre point P and radius PF. Draw the arc FG that completes the three-centred arch.
Ogee arch
The ogee arch consists of four arcs of a circumference. Two of the arcs meet at a point at the top in the centre.
To construct an ogee arch from its springing points A and E, follow these steps:
Step 1. Divide the segment AE into four equal parts. This determines the points B, C and D.
Step 2. Draw perpendicular lines to AE at points B and D.
Step 3. Use centre point B and radius AC. Draw an arc to determine point F.
Step 4. Use centre point D and radius AC. Draw an arc to determine point G.
Step 5. Use centre point B and radius AB. Draw the arc AM. Then use centre point D and the same radius. Draw arc NE.
Step 6. Use centre point F and radius AB. Draw arc MP. Then use centre point G and the same radius. Draw the arc PN that completes the ogee arch.
Trefoil arch
The trefoil arch consists of three arcs of a circumference that form the shape of a leaf of clover.
To construct a trefoil arch from its springing points H and L, follow these steps:
Step 1. Divide the line HL into four equal parts. This determines the points /, J and K.
Step 2. Use centre point / and radius /K. Draw an arc. Then use centre point K and radius KI. Draw an arc that intersects the previous arc at point M.
Step 3. Draw the equilateral triangle MIK.
Step 4. Use centre point / and radius /H. Draw an arc from H that intersects MI at R. Then use centre point K and the same radius. Draw an arc from L that intersects MK at point S.
Step 5. Use centre point M and radius MR. Draw the arc RS that completes the trefoil arch.
