Engineering Rome

Roman Dome Progression

The development of the Roman dome can be seen through unique aspects in their designs that were used as the civilization’s history progressed. Three specific examples will be studied as each has represents major improvement that allowed Roman engineers to perfect their dome design.

Temple of Mercury at Baiae

The Temple of Mercury at the Roman resort of Baiae is the earliest surviving large scale concrete dome constructed by the Romans and is most likely one of the first. Archaeologists have dated the construction of the dome to the late Republic or early Imperial era and estimate that it must have been built before the first half of the first century AD because of the “packed tufa rubble” used in construction that is characteristic of the Augustan period (Adam, 1994). The dome has a diameter of approximately 21.5 meters and was actually never a temple but was instead used for the city baths along with two other similarly sized concrete domes (“Baiae”, n.d.).

temple of mercury-exterior.png
Figure 1: Temple of Mercury at Baiae (“Temple of Mercury”, n.d.)

Surveys of this one remaining dome have shown large variations in the building plan when compared with the ideal circular plan that a dome should have. There are six locations in which the footprint of the dome does not remain on a perfectly circular plan and the maximum variation reaches a value of 22 centimeters (Lancaster, 2005). These variations also do not occur at equal spacing around the perimeter of the plan and can be seen in Figure 2 below (Lancaster, 2005). Two individuals have expressed theories for the large variations in the dome footprint. F. Rakob has proposed that the centering was composed of eight equally spaced timber trusses that were supported with a central column and the discrepancies in footprint perimeter occurred because the centering was not well constructed and positioned (Lancaster, 2005). Alternatively, J. J. Rasch argues that the centering was composed of eight timber trusses that were accidentally unequally spaced and there was no center column support which resulted in the footprint error from sagging during construction (Lancaster, 2005). Both of the centering layouts proposed by Rakob and Rasch can be seen in Figure 2 above and the profile of Rakob’s plan can be seen in Figure 3 below.

Figure 2: Temple of Mercury plan variations and proposed Roman centering systems by Rakob and Rasch (Lancaster, 2005)

Figure 3: Rakob’s proposed Roman centering for the Temple of Mercury at Baiae (Lancaster, 2005)

The difficulty in discovering the actual cause of the plan variation is the lack of formwork imprints left on the inside of the dome from construction and no written records of this structure’s building technique (Lancaster, 2005). From the imprecision at the Temple of Mercury, which is uncharacteristic of Roman engineering, it can be seen that the stable centering and stability needed to construct a large scale concrete dome had not been mastered. With the precision evident in other Roman engineered shapes, we can imagine that the Roman’s immediately began developing better methods to deal with this first major difficulty of large dome construction.

Temple of Minerva Medica at Rome

The second problem that the Romans encountered with large domes is the flexibility of such a small thickness to span ratio structure. This undoubtedly was an additional part of the problem with the Temple of Mercury at Baiae and is likely something the engineers realized after the building of that dome. The stiffening method of brick lattice discussed in the Roman Dome Development section was employed in the fourth century dome for the Temple of Minerva Medica in patterns seen below in Figures 4 and 5 (Lancaster, 2005). In the plan view of the Temple, the large lattice ribs land directly on portions of the supporting octagon that continue down to the ground. This seems logical for a load transferring system but there are other placements of lattice ribbing that don’t seem to follow the same logic. The smaller lattice ribs in the plan view of the Temple actually land on portions of the supporting octagon that have openings below as seen in the three dimensional perspective. From a structural point of view for load resistance, this seems like the least logical place to support the weight of the dome. Instead, it has been proposed that the Romans did not use lattice ribbing purely for carrying the loads of the dome but also for distributing the loads

Figure 4: Plan of the Temple of Minerva Medica showing lattice ribbing (Lancaster, 2005)

Figure 5: Three dimensional perspective of the Temple of Minerva Medica showing lattice ribbing (Lancaster, 2005)

Baths of Diocletian at Rome

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Back to Article Overview: Evolution of the Roman Dome

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