Practicality of the Dome
The reason that the Romans began using the dome is because they recognized the benefit of large spaces that were uninterrupted by columns, walls or any other roof supporting structure. With their technology of unreinforced concrete, flat roofs and columns, this vision was not possible because the unreinforced concrete or stone roof could not span large distances without cracking and failure. Stone and concrete both perform incredibly well in compression but poorly in tension. The difficulty in spanning long distances with flat forms of such materials is that the form will experience bending which creates compression stresses on the top and tensile stresses on the bottom of the form as seen below in Figure 1. These bending stresses cause cracking on the tension side and the stone or concrete form will collapse.
|Figure 1: flat form bending and stresses (“Bending Stress”, n.d.)|
Perhaps the Romans had attempted such designs and seen that they needed to develop a more suitable method of obtaining large open rooms. By building on established knowledge of arches, the dome seems like a logical progression as it is merely an arch spun 360 degrees to create a semicircular three dimensional form. The engineers of the Roman time had already realized the potential of the concrete or stone arch and its use in spanning large distances under heavy loads by compression forces, so it is likely they realized that a well-engineered dome would be similarly effective for large spans when applied to a three dimensional space.
Influence from Other Civilizations
Considering that the Romans had intelligent engineers, it is possible that they developed the idea of a dome on their own. However, evidence of dome structures dated prior to the Roman Empire have been found throughout the middle east and surrounding regions although many are corbelled domes and not true domes. A corbel dome is unlike a true dome in that it does not rely purely on the compressive forces between the stone or concrete components to maintain it’s shape and structural integrity and therefore has a very limited span (Chant & Goodman, 1999). Some archaeological investigations have revealed other ancient civilizations that were building true domes and several have been uncovered in the ruins of the Sumerian city state of Ur (Chant et al., 1999). Chant and Goodman (1999) report that true domes and evidence of their wooden centering have been found in the cemeteries for royalty of ancient Ur and dated to 2500 BC. This would place knowledge of the stone true dome long before the rise of the Roman Empire. Also, although not true domes, evidence of timber domes has been found in the ancient Etruscan area of Italy that date back to the beginning of the rise of Rome (Keinbauer, 1971). Since Rome began with heavy Etruscan influence it is easy to see how the timber dome would naturally become a part of the early engineering ideas in the city and it is likely many were constructed that we have no evidence of. As the city of Rome grew in power and began to expand it is unknown whether or not stone domes were encountered in nearby regions because no conclusive evidence for such structures has been found in these areas. It seems just as likely that the domes of ancient Ur were known to the Romans through expeditions or that the Romans developed the technology on their own from progression of the stone arch. Either way, the dome was not a Roman invention but they were the first civilization to perfect it.
One of the greatest difficulties associated with building a large self supporting curved shape out of stone or concrete is the formwork necessary to support it during construction. This difficulty is increased by the double curvature of a dome because it requires more support and smaller pieces of wood in order to approximate the rounded shape. For arches and domes this formwork is termed “centering” and enabled the Romans to construct the desired shape without danger of the unfinished structure collapsing in on itself before compressive forces are established by inserting the keystone and achieving equilibrium. The Romans eventually developed a method of centering for arches that was accurate, stable and provided a firm support to build the arch on as seen in Figure 2 below.
|Figure 2: Roman arch centering (Lancaster, 2005)|
When concrete dome construction began, the centering system used for arches was a good two dimensional starting point. The engineers needed to develop a new system and the error and progression of their method can be seen in the earlier domes. Two different ways were developed to construct the dome centering, radial and horizontal formwork as seen in Figure 3 below (Lancaster, 2005). With horizontal formwork, the planks had to be relatively short in order to approximate a smooth curvature. The ends of each plank typically would line up with a single meridional line so that they could all be supported with evenly spaced radial frames. In a radial formwork system, the planks could be longer and each plank would typically end on a common circumferential line. This method allowed less radial frames to be used but it required the circumferential line framing to be supported in a curved shape which was difficult. It is difficult to tell which system the Romans used more often because many of the domes constructed did not leave evidence of formwork on the interior (Lancaster, 2005).
|Figure 3: Horizontal and Radial formwork for centering of Roman domes (Lancaster, 2005)|
The way that the arches and domes of the Roman civilization were designed often provided ledges at the base of the curvature to place the centering in order to minimize the amount of ground support that was needed. For arches, often times long timber columns could be avoided through these designs and completely self supported centering was achieved (Lancaster, 2005). This avoided the problem of flimsy and unstable wood columns under heavy loads from the stones that would deflect and result in a sagging arch. Domes typically still required some columns for support of the centering because the three dimensional shape and large spans made it difficult to achieve the same stability of centering without added support from the ground but the Romans also had the technology to build self supporting centering for double curvature (Lancaster, 2005). It took the engineers some time before designing a reliable method of centering for domes that yielded a high accuracy completed shape but by the 2nd century A.D. they had succeeded.
Once the problem of formwork was solved, the stiffness of these large domes came into question. If a dome was too flexible, when the centering was removed the dome apex may have sagged down significantly and this deflection could result in extensive cracking at certain areas. This could have been discovered through observed deflections and failed designs or perhaps it was something that the Roman engineers intuitively knew would happen. They developed different methods of building domes with stiffening elements placed throughout the curved shape in order to avoid such large deflections. One method developed used ribs that were visible on the interior of the structure as those on the vault in Figure 4 below. The Romans used this same technique in many of the domes that they constructed. These ribs not only served to strengthen the structure but also provided a strong visual texture to the otherwise plain interior of a curved shape through the indentations called coffers. The structure could not have been built with a uniform thickness matching that at the deepest point of the coffer but it was also unnecessary to use a uniform thickness matching that of the ribs. By employing this ribbed technique, the dome was able to be stiffened and decorated through one method and, although increasing the difficulty of formwork, was used extensively. Another system of stiffening that is evident in domes is placed within the dome and is not visible from the interior or exterior. This system began being used during the 4th century and employs a type of brick lattice that is then filled in with concrete to create a closed dome as in Figure 5 below (Lancaster, 2005). The vertical lines of bricks essentially form a series of arches that are connected at the top of the dome by a common keystone. These arches would transfer loads through pure axial forces down to the base of the dome and in many domes it can be seen that the Romans placed these brick lattices so that they landed on a main support (Lancaster, 2005). In this manner, they could avoid placing unnecessary stresses on openings and thinner areas of supporting walls.
|Figure 4: Vault in Roman Forum with interior coffers forming stiffening ribs (self)|
|Figure 5: Brick lattice ribbing at the Baths of Agrippa (Lancaster, 2005)|
Additional purposes have been proposed for the use of the brick lattices in domes and they were likely also intended by the Roman engineers. During construction of a dome, the lattices likely served to divide the dome up into manageable sections of construction and also aid in placing formwork for the general shape of the dome. Many domes have horizontal courses of bipedalis bricks at vertical intervals between the brick lattices and these could have been stood on by carpenters so they could place the formwork just ahead of the construction. In this way the formwork for the entire dome would not have to be built before any concrete placement began (Lancaster, 2005). The lattices could also keep the dome stiff while curing of the concrete took place (Lancaster, 2005). With the weight of the concrete, the wooden frames used to hold the formwork during curing would have slightly deflected and the brick lattices could help mitigate this problem and increase quality control. After developing a solution to this challenge of dome flexibility, the Romans had all the necessary technology to confidently build these structures on a large scale.
Next Page: Roman Dome Progression
Back to Article Overview: Evolution of the Roman Dome