Traditional Masonry Archives

Olana State Historic Site in Hudson, N.Y.

Brick Arches

The simplest way to span any opening is by means of a beam or lintel. If for some reason they are unobtainable, or the space to be bridged is beyond the length of a suitable beam, then a way has to be found to do so with an assembly of small stones or bricks. These have to be fitted together in such a way as to stay permanently in place, stabilize the opening, and safely carry and transmit loading through the walls to the foundations, and the very best way is by means of an arch.

Figure 1

Figure 2

Most arches have a characteristic curvature, though other shapes have been used as well. In a well-built arch the arch bricks or “voussoirs” will be wedge-shaped and remain in place by each one pressing in on, or squeezing, its neighbor.

In discussing the masonry arch it is important to have a basic understanding of the craft terms used for the various parts that are part of the overall construction, as well as of their function in it (Figure 1).

The development of the arch
How the true arch developed is lost in the mist of time, but from a natural and logical perspective it was, as stated earlier, a pragmatic solution to overcoming the limitations of masonry units of smaller dimensions than the span of an opening. Aside from the use of a single beam, the next option is to utilize two lengths of stone erected in such a way as to form a triangle, or “triangular arch,” a method that was used by the ancient Egyptians in parts of their pyramids (Figure 2).

With small stones and bricks these two options are not possible. Thus, the first and most logical method is to gradually corbel successive courses out from either side until they eventually unite at the center, which is also a method of great antiquity, having been employed by a number of ancient civilizations. When corbelling is carried out with rectangular masonry units it creates triangular steps with sharp edges facing into the opening. It would not have taken long for ancient builders to realize the practical and architectural benefit of shaping the underside of each stone or brick in the corbel to run smoothly — forming a curved line and creating an arch. A good example of this is the great corbelled gate in a megalithic wall at Arpino, which dates from the fifth century B.C. and is the work of the Etruscans.

At some point craftsmen began to realize that this arch shape could be made even stronger if, rather than corbelling the stones or bricks, they built them to follow the curve but laid them so that they were facing towards the radial point, locking them together with greatly increased strength.

Jefferson County Courthouse Clock Tower in Port Townsend, Wash.

It is to the Tigris and Euphrates valley of Mesopotamia 5,000 years ago, where timber was scarce and plenty of mud was available to make bricks, that we must first look for the origins of brick arches and of vaults, developed by the Assyrians and Babylonians. By building their arches and vaults in either a semicircular or two-centered pointed form against a gable wall and using a sloping ring technique, they eliminated the need for any support. The ancient Egyptians did not really use the arcuated arch like the Mesopotamians. Rather, they used trabeated, or post-and-lintel, construction, and occasionally utilized the triangular and corbelled methods of bridging openings.

It is frequently, and incorrectly, stated that the ancient Greeks did not use the arch in their buildings. Although they mainly made use of post-and-lintel (pillar-and-beam) construction, they did use the arcuated arch and vaulting when necessary. The remains of a segmental vault with cut stone voussoirs are still to be seen at the entrance to the stadium at Olympia, where the Olympic games were held every fourth year from 776 B.C. until A.D. 393. The entrance into the “Asklepios” at Kos is bridged with a semicircular arch with cut stone voussoirs that dates from the first century A.D. Another good example is a series of large-span segmental arches with cut stone voussoirs that are to be seen in the ruins of a water cistern in front of the theatre at Delos dating from the third century B.C.

In Europe between 700 and 100 B.C. the Romans developed ideas on arch construction that were borrowed from the Etruscans, who, it appears, were the first to use a radiating arch requiring a temporary support, or “center.” The Romans exploited the potential of the stone or brick voussoir arch, reaching high levels of technical complexity. They used shapes that were mainly based on the geometry of the circle, primarily the semicircle — famously utilized in three stories of colonnades to dramatic effect on the Colosseum (A.D. 70 to 82) — and the segmental form. This type of construction worked very well for them, so they were not keen to experiment with well-proven theory and rules. Such was the Romans’ faith in their arch construction that they had a saying: “When falls the Colosseum, Rome shall fall.”

It is interesting to note that the Romans also made occasional use of the flat, or “Jack,” arches of cut brick voussoirs too, as can be seen in several buildings in the ruins of Pompeii (destroyed A.D. 79). The flat arch is not really an arch, but rather a scheme of radiating voussoirs that spans an opening within a flat profile. It can therefore be weak and fail at the center. The Romans sometimes utilized this arch as a shuttering to receive a hydraulic concrete fill and thus cast it as a lintel. The concrete sticking tenaciously to the rear of the voussoirs helped secure the bricks in position across wide openings — which were then stuccoed to give the illusion of the stone beam the arch was replacing. After the fall of the Roman Empire it was not until the Renaissance that the flat arch of cut bricks was rediscovered and used again in Classical architecture.

Around A.D. 600 the Chinese were the first to use the large-span segmental arch for bridges. In a union of art and engineering they wanted to remove the need for a colonnades of semicircular arches across wide rivers, because the piers could form potentially dangerous dams against floodwaters. A large semicircular arch with a high rise-to-span ratio would have created a large central hump. The solution was found in flattening it by dropping the crown of the arch, thus creating a segmental arch with a low rise-to-span ratio. The Anji Bridge, also called the Zhaozhou Bridge, in Zhaoxian County, Hebei Province, was built around 1,400 years ago during the Sui Dynasty. It is China’s oldest surviving single-span stone arch bridge, and Chinese legend calls it “The First Bridge Under Sky.” The bridge is about 9 meters in width and the arch supporting it spans just over 37 meters with a rise of 7.23 meters above the “springing line,” or chord, to the underside of the keystone.

The masonry arch was developing independently elsewhere too. The Arabs based their arches on the oval, and as a result the effect on the construction of the building was quite spectacular. The pointed arch, in a crude form, is known to have been used by them during the eighth century for drains, but some authorities attribute the origin of the pointed arch to the building of the Mosque of Ibn Tulun, Cairo, Egypt, in the ninth century. Considered a marvel of ancient craftsmanship, the pointed arches, with spans of 15 feet, are technically correct. The development of prolonged curves and sweeps that narrow the pointed arches at the springing point of the arch, or impost, were to become a characteristic of what is frequently termed Moorish architecture.

Olana State Historic Site in Hudson, N.Y.

It cannot be a coincidence that from the 11th century through the Crusades, Europeans came into contact with the architecture of the Arabs and their use of pointed arches, and soon after it began to appear in the Cathedrals and churches of Europe. What was to later become known as the “Gothic” architectural style originates in the Abbey church of Saint-Denis near Paris, France, through the vision of Abbot Suger. The first true Gothic or pointed arch is to be seen in the choir space of this church that was consecrated in 1144. In parts of Europe short of natural stone, brick was its natural substitute and brick-built Gothic cathedrals and churches are to be seen in the Netherlands, Belgium, northern Germany, Scandinavia, the Baltic countries and northern Poland.

The use of the Gothic arch revolutionized architecture because vaults based on the pointed arch, unlike the semicircular, could be utilized to roof rectangular and irregular areas — a triumph over space. Its use, along with the mason’s skills, developed in ever more refined, graceful and decorated forms throughout the Middle Ages. Vaulted openings became wider and taller, the columns of masonry increasingly thinner. Eventually the overall cathedral or church resembled a structural skeleton holding acres of stained glass windows flooding the interior with a kaleidoscope of color. The use of flying buttresses of arched masonry became a necessity on the external masonry that was behind the slender columns taking the thrust between windows, in order to transfer it from the high vaults over and onto carefully positioned piers, and from them to the ground.

In the years that have followed the Second World War, with the advent of more functional architectural designs based on flat planes, hard angles and quick-build fixing techniques, the arch has fallen from favor. Some designers see it as too emblematic of past historical styles, and when it is used it is often in only a nonstructural aesthetic manner. This is a great pity, for the arch has stood the test of time both structurally and aesthetically, and its use should not be ignored or discouraged, but rather encouraged.

Technical aspects of the arch
In theory the arch form greatly reduces or eliminates tensile (pulling and stretching) stresses that are resolved into compressive (pushing) stresses. As a material, brick is weak under tension, yet is strong in resisting compression. Leonardo Da Vinci stated: “An arch consists of two weaknesses that become one strength.” In essence, downward forces are directed into sideways thrust and transferred into the walls, or abutments.

An example of a jack arch.

Although the arch is self-supporting, due to the action of sideways thrust the typical arch is not usually a stable structure alone. It is forcing either side of the base outwards — which must be resisted by sufficient solid masonry, or other material, as a buttress. This is why most arches are placed within a wall rather than at its end, although in a series of arches, or colonnade, such as on a viaduct or bridge, engineers make use of the inclined sides of the valley to serve as the buttresses.

The pointed Gothic, or ogive, arch altered the action of the thrust technologically because through its unique shape all loading was directed onto the piers or columns at steep angles, so that this type of arch holds space in structural dynamic balance rather than the static repose of other arches.

It is always best to remember that all arches are constantly under stress with compressive forces acting upon them helping to hold the structure in balance, an idea summed up in an old proverb, “An arch never sleeps!”

Olana State Historic Site in Hudson, N.Y.

In general, the reason for any arch distorting is due to a loss of compression in it or to point loading, the sliding out at the ends or springing points when the thrust is too great for the abutments to support or something is lubricating the horizontal joints. From a practical constructional point of view an arch will fail if it is built from inaccurate and poorly cut voussoirs, if the voussoirs are not fully jointed, or if there has been a subsequent loss or shrinkage of the mortar beds.

The classification of arches
Arches can be classified in several basic ways: by their geometrical shape (semicircular, segmental, semi-elliptical), their architectural shape (Gothic or pointed, Tudor arch), or by the number of radial points, or “centers,” from which the arch is drawn and the voussoirs radiate (one-centered, two-centered, three-centered). These can also be referred to as single arches, formed with a curve struck from a single radial or striking point, or compound arches, the shape being struck from two or more centers or striking points.

Rough arches
Rough arches are built from standard, or parallel-sided, bricks and the mortar joints are wedge-shaped to enable the curve to be turned. The larger the span of the opening and therefore the radius, the less the joints taper. Normally, so that the joints are not excessively wide, the bricks are laid on edge in a series of what are called “rings,” so these can also be called ringed arches. They are normally confined to semicircular or segmental arches and were used on houses where appearance is of secondary importance. They are commonly seen on railway and canal bridges.

Axed arches
Axed arches are built from bricks that have been cut to follow a wedge-shape template, obtained from a full-size drawing of one half of the arch. Historically this cutting was done with a cutting tool called a brick axe that resembled a double-bladed bolster. More commonly the bricks are either cut with a hammer and bolster or on a bench-mounted disc-cutting machine. Though the bricks are wedge-shaped to follow the curve with parallel joints, their tops and bottoms are not shaped to the curve. Traditionally used where appearance was of some importance, axed arches were laid with joints of 6 mm and some were then tuck-pointed to look like a gauged arch.

Gauged arches
So called because they are set out to very precise measurements, gauged arches are always built from soft textured rubbing bricks or “rubbers” that have a high percentage of sand. This allows them to be rubbed square on bed and face and then placed into a cutting box to be cut exactly to shape, with tops and bottoms following the curve. They are laid with fine joints of 2 mm or less using lime putty with a little silver sand. Gauged arches are always reserved for use on the best elevation.

Dr. Gerard C.J. Lynch, LCG, Cert. Ed. M.A. (Dist.), Ph.D, is a historic brickwork consultant, master bricklayer, educator and author. He will provide a follow-up article in the next issue of Traditional Masonry. Copyright Dr. Gerard C.J. Lynch, with some rights held by Traditional Masonry.

TM