Engineering Rome

Relieving Arches of Roman Structures

sarafoxx sarafoxx Sep 16, 2013

Relieving Arches of Roman Structures

Ancient Rome was defined by its incredible buildings, reaching into the sky taller than ever before. These buildings tended to be huge and sometimes overwhelming to those looking up at them. With enough power, influence, and money, it was possible for structures like the Palaces of Palatine Hill, the Baths of Caracalla, the Colosseum, and the Pantheon to be built in ancient Rome, when the building techniques were only beginning to be defined. For the people, it was a magnificent marvel. However, from a structural point of view there was the new problem of how to support all the weight from the never-before perceived buildings. Through some Before-Christ-era trial and error, the arch proved to be worth the great Roman people’s attention. Arches were loved by the Romans because of their self-supporting structure. As the buildings and monuments got bigger, taller, and heavier, the arch was put to more and more use, but sometimes more than one arch was required to hold up the weight of what was above it. These arches built into the wall above other arches or door frames are called relieving arches, and they helped to distribute the weight above down to the supporting columns or walls holding the doors or windows in place, sometimes using two or three reliving arches above a single opening. Relieving arches were also used to protect thinner sections of walls in a structure or to allow faster construction by resisting creep, or the slow flattening of concrete over time. All throughout Rome relieving arches were built into the walls to create bigger and more awe-inspiring structures and monuments, either for public use or for the praise of an emperor.

Palatine Hill/Roman Forum


Palatine Hill is the center-most hill out of the seven of Rome, and was where the emperors of old would build their huge houses (fig. 1). From this place is where much of the history of Rome begins.

Fig. 1: Map of Palatine Hill (source: Google Images)

It is said that the Luperacal Cave is situated on the hill, where the she-wolf cared for the twins Romulus and Remus. These twins then decided to build a city on the hill, and after a fiery argument with his brother Romulus killed his brother and named the city after himself. There have been archaeological finds on the hill of the remains of settlement dating as far back as Romulus, the first king of Rome, and are called Romulus’ Huts. Therefore, the Roman Empire has its roots in this central hill. There have been archaeological finds of remains of settlement dating as far back as the first king of Rome, Romulus Many emperors had their homes built on this hill, such as Augustus, Cicero, and Marc Antony, although eventually the hill was covered with royal domains and monuments devoted to the gods. This is where the word ‘palace’ comes from, since Palatine Hill was home to so many extravagant houses. In the Middle Ages churches and convents were built on the hill next to the crumbling imperial houses, and at some point a large botanical garden was created on top of the ruins (Encyclopedia Britannica).


With such a commanding view of the land below, as well as history mixed with legend soaked into the ground, it is no surprise that those with the money and power to do so built extravagant houses, temples, and monuments on the central hill. These large palaces would sometimes reach up to seven stories tall, presenting engineers of the time a challenging problem to solve: how to hold up the enormous weight of the building without countless pillars. The answer was in relieving arches. These semi-circles with a bar underneath them would be built into the wall above openings that had large masses situated above them, like doors or windows (fig. 2).

Fig. 2: Relieving arches above doors and windows of the semi-circular building behind instructor Steve

In order to deal with these large weights relieving arches would be built on top of each other, lining up vertically so the redirected weight would flow smoothly to the designated structures that reached the ground. Even so, sometimes these arches would not match up horizontally with other walls although relieving arches are usually integrated into the design of a building . In the Domitiani Vestibule of Palatine, it is easy to see how the relieving arches don’t match up on connecting walls, so they must have been used to strengthen the walls and controlling settlement. With this design they could allow mortar to cure and construction could continue upward with very little ‘creep’, or a deformation of material over time (Lancaster). The relieving arches would also keep the creeping of the walls to a minimum while directing the heavier loads to the thicker parts of the wall between niches that reached the ground.

From Palatine Hill there is a wonderful view of all of Rome, and the hill itself spills gracefully into the Roman Forum (fig. 3), next to the Colosseum and built by Caesar for the people. While this Roman Grand Bazaar is now just a pile of ruins, it used to contain temples to gods, countless shops, and areas for business operations. When in use, the Roman Forum was the true heart of Rome, beginning as a market place and over the years expanding into the economic, political, and religious center of the city. Sadly, because of the extent of damage to the Roman Forum, it is hard to tell just how tall the buildings in the Forum could have been. The buildings with upper levels no doubt employed relieving arches for the massive structures, but evidence suggests that nearly all the structures in the Forum were held up by Roman columns.

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Fig. 3: Ruins at the Roman Forum

Baths of Caracalla


The Baths of Caracalla was the second largest public bath in Rome, second only to the Baths of Diocletian, spanned 33 acres and was completed during the reign of Emperor Caracalla, although the design was drawn up by Septimius Severus who ruled previously. Built from 212-216 AD the project would’ve required nearly 2,000 tons of material brought in every day to be completed on time. The numerous baths were fed by the Marcian Aqueduct, which ran parallel to the structure. Larger-than-life statues like the Farnese Hercules originally decorated the halls, statuettes of nymphs were scattered all over the place, and the floors usually depicted athletes competing or showing off in mosaic chips. It is believed that the statue in fig. 4 was placed at the center of the baths so it could be viewed from every angle, even from the upper stories. The story of the piece is that an unfaithful man left his pregnant wife to have an affair with another woman. The abandoned wife gave birth to twin boys, who later took revenge on their father by tying his mistress to a bull to be smashed against a mountainside.

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Fig. 4: Farnese Bull

This piece, called the Farnese Bull, captures the moment the two sons are taking their revenge on an absent father, the mistress horrified as she realizes her fate, and the wife standing back watching with a twinge of satisfaction at the cruel justice. Complete with a frigidarium, tepidarium, caldarium, nymphaeum, smaller baths, libraries and even shops for services and business, it was an exquisite bath to have access to as a commoner (Ancient Roman Baths).


The Baths of Caracalla was a massive structure, holding a number of baths as well as shops and stores at two levels. The walls stood nearly 1,060 feet tall at the highest point, with domes and groin vaults making up the ceiling above the baths. The caldarium, or the hot bath, was integrated with special window placement in order for the sun to help heat the bath and keep it warm throughout the day. The majority of the work of heating the water was done underneath the baths where there was a space to burn wood for fires, heating the water before sending it through pipes that spanned the whole bath area. Because these pipes were so expansive, there were small hallways built into the walls in order to do maintenance if anything went wrong. Grandly decorated on the inside with travertine, marble, and statues, the Baths of Caracalla proved to be a fairly hefty structure to hold up. Many times there would only be a single door leading into the next room, left to hold up the weight of the wall above it on its own. These Baths are probably one of the places where relieving arches are use most discretely; once the frame of the building was constructed the walls were covered up and decorated to hide the rather ugly and boring brick construction. Fig. 5 shows a slightly curved door frame with a relieving arch above it, and more relieving arches above that one. Because the weight of the walls was so enormous, numerous relieving arches were usually required for this structure. Although the beam topping the doorway doesn’t serve any significant structural process, it may have been used for an aesthetic appeal. Perhaps the builders were instructed that arches were becoming too common in Rome and that they should change it up. Whatever the reason, the beam below the first relieving arch in the doorway is not a structural requirement. These relieving arches are a prime example of showing how they pull force from above away from the openings below them, distributing the load to the walls or columns beside the door.

Fig. 5: Baths of Caracalla – in the center of the picture a door can be found under a towering wall reinforced with relieving arches

The windows also show how the use of relieving arches remove the forces from settling completely on the top of the window. Simply integrating a small arch above a window would redirect the load of tons of stones down to settling points at the ground, though in some cases more than one arch is required. Although windows were mainly used in the caldarium in order to help heat the room via the sun, windows looking out to the grounds of the Baths were also built. Mainly situated on the second level of the Baths, these windows did not normally need as many relieving arches as the ground floor doorways.

Underneath the Baths of Caracalla was the heating system, and ultimately the most important area of the Baths. Without the heating systems underneath the floors it would have been nearly impossible for the Baths to function. An above-ground heating system would have been less efficient since the heat would have needed to travel a longer distance to the baths, as well as more expensive to build a whole new building to hold the equipment for heating. The rest of the Baths were raised 20 feet in order to accommodate the underground tunnels. However, holding the heating systems underneath the entire structure of the Baths of Caracalla meant that the entire weight of the structure was right above the worker’s heads as they burned wood and heated water (Kington, T., 2012). Recent excavations show that the underground of the Caracalla Baths lie in three tiers of tunnels reaching nearly 20 feet both tall and wide. During restorations an underground temple dedicated to Mithras was found. This was a men-only religion that had gained popularity just before Christianity took over, and temples to the mysterious Mithras religion can be found underground in many sites. The one found in Caracalla’s Baths was a chamber with space for seating and laying down during banquets while in the center was a large pit with a metal grill. It is believed that the cult members would drag a drugged bull on to the grill and butcher it, and a cult member would have to crawl under the grill and be drenched with the bull’s blood in order to be initiated. Another Mithras temple can be found under the San Clemente church, where they have something similar to the Baths of Caracalla temple but on a smaller scale, and only a sacrificial podium. The cruel and exclusionary cult was eventually won out by Christianity, and soon many of the Mithras temples were forgotten only to be rediscovered during excavations of other sites. The tunnels of the Baths were used to keep the slaves out of sight of the bath-goers. The entirety of the underground of the Baths of Caracalla is essentially just rows of barrel vaults on top of each other. Interestingly archeologists have found something like an ancient Roman roundabout that would have helped the flow of traffic underground, where carts would haul in tons of wood each day for the burning of fires.

Like nearly every other building after the fall of Rome, the Baths of Caracalla fell into disrepair and over the years was used like a quarry. The Ostrogoths destroyed the hydraulic, installations in the 6th century, opening the door for more exploitation of the site (Rome Cabs Team, 2012). People would go to the Baths to collect raw and refined material. Nearly all the statues were collected by the Farnese family, and can be viewed in museums around the world. The centuries of neglect and prying off stone and metal from the walls had an effect on the building, causing some walls to crumble completely. The stones and pillars that fell were taken and used all over Rome, some of which can be found in a small church across a fountain in Piazza di Santa Maria in Trastevere. Nature also had a hand in destroying the Baths. Countless earthquakes weakened and felled some structures. Earthquakes are a likely cause for the destruction of the domed ceiling as well, although the destruction of the supports for the dome should be accounted for as well. An earthquake in 2009 is the last accounted for natural disaster that caused severe damage to the Baths.



Originally called Flavian Amphitheater, the Colosseum got its modern name from a ‘colossal’ bronze statue of the emperor Nero, standing 157 feet tall, matching the height of the stadium. The Colosseum is well known to have housed ancient and bloody entertainment for both emperors and plebeians alike from 80 AD to the early medieval era, with gladiator fights ending around 435 AD and animal hunts stopping about 100 years after that. Gladiators would fight animals and each other, there would be exotic animal hunting, and executions would be provided to please the masses. Even now it is a wonder at how the engineers decided on the complex maze of arches and vaults to create seating around the elliptical amphitheater with hallways on the inside to move around between seating sections or to enter and exit. At ground level there were 80 entrances, and it is estimated that filling and emptying the entire stadium would take only 15 minutes each (Hopkins, K., 2011).


Built to hold up to 80,000 spectators the Colosseum is the largest amphitheater in the world and is a structural and engineering wonder of the ancient world of Rome. Constructed from concrete and stone and designed with four distinct levels for spectators, this monumental structure is quite literally supporting itself with a complex arch-on-arch system, in a nearly grid like pattern to fit the shape of the amphitheater it enclosed. In order to construct this colossal project, a frame work of travertine limestone was set up in concentric piers and arches and linked later with walls in a ring-like shape. Vaults were put in place for the hallways to and from the spectating areas. Because the stadium was far too large to support itself with only vaults, relieving arches were introduced above the structure of the vault, called vaulting ribs. Though this was acceptable at higher levels, nearer to the ground another relieving arch was required, this one running above and parallel to the vault underneath it (Milani-Santarpia, G.)

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Fig. 6: Interior of the Colosseum, with various levels of relieving arches visible Fig. 7: The Colosseum

Usually made from brick, this parallel arch would connect with vaulting ribs. This arch-on-arch structure allowed the weight above the brick arch to disperse on to two vaulting ribs, and from there on down to the ground. Even the windows on the upper level, probably used to frame life-size statues of gods and goddesses from the old religion, are reinforced with relieving arches, as seen in fig. 6.This form was used all throughout the Colosseum, thus allowing the structure to stay standing for nearly two millennia. However, as seen from fig. 7, not all of the Colosseum seems to have made it into present time. The stadium has in fact gone through a number of destroying events, the most notable of which are the fire in 217 that was caused by lightning according to Dio Cassius and the 1349 earthquake that destroyed the southern side, although it had survived other previous earthquakes with some restoration. Taking a step back, one will realize that the ruined side is closest to the river. Although the Romans did not know it when they were constructing the Colosseum, they built their enormous stadium halfway on a sort of extended riverbed, building on soft alluvium soil that had been softened and reshaped by the Tiber River. Of course, since the other half of the Colosseum was built on harder, more packed ground it has survived more intact over the years (Cozzo).



Built during the reign of Augustus the Pantheon was dedicated as a temple to all of the old gods of Rome in 126 AD. Currently still boasting the world’s largest unreinforced concrete dome with the opening at the top, the iconic Pantheon is one of the best preserved monuments of Rome and has been in constant use since ancient times. From the very top of the building is an oculus, or a central opening, that lets light slowly pan across the inside of the Pantheon. The height of the oculus and the diameter of the interior are both 142 feet, showcasing a beautiful delicate symmetry the Romans always integrated into their work. Today’s Pantheon is actually a reconstruction of Agrippa’s original pantheon that stood in the same place. After two fires and two reconstructions, the Pantheon was finally here to stay. Although Hadrian had the Pantheon refinished it is not certain how the building was used after that. The Pantheon was again restored in 202 AD by Septimius Severus and Caracalla, for which is a small inscription reading “with every refinement they restored the Pantheon worn by age.” In the 7th century the Pantheon was used as a Roman Catholic Church and dedicated to “St. Mary and the Martyrs” (MacDonald, William L., 1976).


The Pantheon is held up by a series of relieving arches at three different levels throughout the walls of the rotunda (fig. 8). The lowest level consists of a single ring of bipedale arches while the upper levels are both more substantial and consist of two or three rings of brick. A bipedale is the type brick used for building arches, usually about two feet long, and are the most common choice of building material for arches. When entering the Pantheon there is a barrel vault, or an elongated arch, over the entry way that has a number of visible intrados, showing that it is entirely consisting of bipedalis. Flanking the entry way there are two semidomed cut outs, possibly used to frame statues or designate an area to sell items. How the relieving arches are helping to support the semi-domes is still uncertain, but it is known that these arches are intended to redirect the weight of the massive dome above them to the sides of eight piers between the interior niches (fig. 9). The builders used stone springer blocks to concentrate these loads at particular points, which are used in accordance with the brick relieving arches to redirect weight away from architraves, or the beams resting on the capitals of two columns, over columns of niches. Since there are springer blocks at both moments, it is possible that they were trying to direct these point loads though the large arches in the Pantheon to both redirect weight to the eight designated niches as well as strengthen the walls and protect against creep, much like the Domitianic Vestibule.

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Fig. 8: Exterior of the rotunda of the Pantheon, three levels of relieving arches visible Fig. 9: Bird’s-Eye view of the structure of the Pantheon, showing the eight piers in the rotunda where the loads land

A fairly obvious problem of creating such a large dome other than how to support it was finding the best materials to use for the construction of the monument. As will be discussed later, having too light or too heavy a material for the dome would not have been the best choice. The builders of the Pantheon dome used an uncommon lightweight caementa that allowed them a way to control the mass of the structure by using stones of different weights as caementa in different parts of the building. So in some areas caementa is not even used, it is only the weight of these stones being pushed down and against each other by gravity and other forces around their structure. The heaviest of caementa, travertine, was used at the bottom while the lightest, volcanic scoria and yellow tufa, was used at the top. As stated before, the whole dome was not made entirely of the lightweight caementa. It was only used at the crown of the dome.

This combination of stones of varying weights was used previously at other sites, such as the Baths of Trajan and the Basilica Ulpia, and all have shown through a mineral analysis that the scoria used, the lightest of the materials, came from Mount Vesuvius. This product was probably imported by ship because it was found in a building at Ostia, the port leading into Rome, as well. The scoria is one of the only non-decorative stones imported into Rome and was only done so when buildings were sponsored imperially. Vitruvius, famous for his ten surviving writings about nearly every subject of Ancient Rome, wrote near the end of the first century BC about the existence of a sponge-like stone from Vesuvius that he called pumex pompeianus (Vitruvius from Morgan). This is likely the Vesuvian scoria material Rome had imported. Scoria has even been found in the vaults at the Forum of Caesar only as caementa, dating back to the middle of the first century BC. The scoria found in Pompeii was produced in a small volcanic explosion from around the 10th-9th century BC, as the city itself actually built on top of the minerals. The eruption of Mt. Vesuvius in 79 AD covered the quarries that had previously been used for collecting material, so the scoria for the Trajanic buildings and the Pantheon had to be excavated from underneath 79 deposits. The areas more easily accessible to quarry the scoria would have been those away from the coast where the deposits were not as deep; however this option would have been much more expensive for the Romans because of the cost of sea transport, and in order for this to work they would have needed to build a new port at Pompeii that had been destroyed by the eruption.

Despite the awe-inspiring destruction, the scoria of the region was found about 20-25 years afterward, possibly under the direction of Trajan since the topsoil still was not developed enough to sustain any large-scale agriculture. Since the Romans rarely ever imported building materials because they had everything they needed right next to them, the structural qualities of this scoria may have been important enough for the imperial administration to put effort into excavating and shipping the foreign material to Rome. Lancaster, in her short paper The Pantheon and Relieving Arches, decided to test different materials to see how they would affect the thrust line of the Pantheon’s dome, choosing from a range of common materials found in Rome. She altered the composition of the Pantheon while keeping the structure the same, so the weight was still distributed down to the eight piers. One alteration was to make the dome lighter overall by replacing the heavier brick and tuff caementa used in the haunches with the same lightweight caementa used in the crown. Surprisingly the model showed that using lighter dome increased the lateral thrust, though not very significantly. The next adjustment was just the opposite of the first; Lancaster made the whole dome heavier by using the haunch material, substituting it for the lightweight material at the crown of the dome. This model had a much more drastic increase on the thrust line, though the dome still would have held itself up. Comparing these two models and the still surviving Pantheon shows that the builders understood to a certain degree to make the crown as light as possible, while keeping the haunch heavier to act like an anchor in order to counteract the lateral thrusts of the dome. While making the dome either lighter or heavier overall was not the best solution, these tests show that it was not as critical as previously believed to have a lighter material at the crown of the dome despite the effort that was taken to obtain the material.

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Fig. 10: Pantheon with a section cut away to see both the interior and exterior design. Note the step-rings on the exterior of the dome.

Moving to the exterior of the dome one can see that there are step-rings on the outside of the Pantheon (fig. 10). There have been two on-going theories of why these have been implemented into the structure. The first is that they would have made construction easier, allowing the exterior to be built in steps and avoiding forming curved extrados, or the exterior curve of an arch, in the lower parts of the dome. The second is that these step-rings were used as extra weight over the haunch of the vault, and is considered more likely because of evidence at Trajan’s Market. At the north end of the market are two semi-domed rooms, the larger one with a 17 meter internal diameter. The exterior of this dome has a 1.3 meter high step-ring and is noted as the precursor to the Pantheon’s dome and step-ring combination (Lancaster). One side of the larger semi-dome at Trajan’s Market has been cut away to show that the structure had actually been completed before the addition of the step-ring, so it could not have been used to aid the actual construction of the dome. Interestingly, the Trajanic brick stamps on the facing of the step-ring at the Market show that the step-ring was added on shortly after the dome was completed. The addition of this step-ring implies that although it was not needed for construction it was added soon after as a structural precaution to keep the dome standing. On another one of Lancaster’s Pantheon models, he removed the step-rings. The result had a very substantial effect on the thrust line, actually pushing the dome outward but still keeping the structure standing.

Lancaster’s adjustments of the Pantheon show that the Roman builders had a highly developed understanding of the structural behavior of the buildings, without having any concept whatsoever of the internal stresses of the materials or anyway to determine the thrust lines. Despite the awesomeness of the Pantheon, it would not have been realized if it had not been for the century of experiments before it, testing more advanced building methods such as relieving arches and vaulting ribs, lightweight caementa, and step-rings. However, they had never been put together in such an exquisite way before the creation of the Pantheon, still boasting the largest unreinforced concrete dome ever built for nearly two millennia, and that is truly incredible.


Throughout all of Rome 11 major aqueducts were built over 600 years, construction ranging from about 300 BC to about 300 AD. These aqueducts were built to carry water from areas outside the city into Rome to serve a number of purposes, such as public fountains, baths and theaters, and private houses. In order to keep the public satiated there were water tanks built into the system to limit the amount of water going where in case of a water shortage. Since the public fountains and baths were so important and integrated into the everyday life of commoners, these were the last facilities to be cut off from water. With beautiful working fountains as common in Rome as Starbucks is in Seattle, they were used as declarations of power for rulers and meeting places for commoners. In order to keep a good front for the city the fountains were seen as essential to keep running.

Ranging from the longest, the Aqua Marcia at 57 miles, to the shortest, the Aqua Appia at 10 miles, all these aqueducts played their part in supplying the city of Rome with a constant flow of water. Commissioned either by emperors or a wealthy censors of Rome, aqueducts took from 2-15 years to complete construction. Marcus Agrippa, who completed the Aqua Virgo, had 240 private slaves he dedicated to maintaining the aqueducts. Eventually this team of slaves was supported by public funds.

Fig. 11: The 0.85 mile standing stretch of the Aqua Claudia, found in Rome’s Aqueduct Park

Since these aqueducts could sometimes span miles above ground engineers were called in to not only make the aqueduct structurally sound and cost efficient, but also to integrate a slope to the channels so as to not keep the water flowing too slowly or quickly. To do so the engineers had to take into account the landscape of the aqueduct’s path to Rome, the building materials that would be available on site during construction, and keeping measurements constant in order to match up at the correct location, to name a few concerns. While a fairly large portion of each aqueduct was able to be forged underground, usually by cutting down and through hills, there were times that stretches of flat land needed to be covered in order for the aqueduct to connect with Rome. There are two approaches to solve this problem. One solution is to build a siphon, which requires a certain amount of head loss, or height loss from one tank to another. This option calls for a structure that curves to a certain degree to the hill or slope it is being built on, and will only work if the receiving tank is lower than the header tank. A viable option, but this took a long time to build and was usually not even considered if the materials were not at hand. Siphons also required lead pipes which were difficult to transport out to the countryside. The other solution was to build a bridge for the aqueduct, cutting a straight line from one point to another. This was the preferred solution compared to the costly siphon. A bridge allowed the constructors a smaller room for error concerning the sloping of the aqueduct, easy access to building materials since they could be obtained from local quarries or the land around them, and a cost effective way to get from point A to point B. As seen in fig. 11 these above-land aqueduct bridges consisted of a long, thin row of arches, and in their heyday they could run as far as the eye could see. The Aqua Claudia still has a standing section running 0.85 miles continuously and can found in the Aqueduct Park of Rome.

From time to time, the arches of the aqueducts would need repairing due to a variety of reasons. Looking at fig. 12 it is clear to see that the two central arches have been added on to since their construction, another arch of bricks built under the original.

Fig. 12: Aqua Claudia with reinforcing arches

Although these newer arches of brick are now supporting the aqueduct, they are not considered relieving arches. These reinforcing arches have a number of differences from relieving arches. Relieving arches are found only in structures because they rely on the walls they are placed in because the walls are the main gravity system of the structure. The walls are what determine where the load meets the ground, while the relieving arches built into the structure overhead simply redirect the weight one way or another. The arch system of the aqueducts uses the arches themselves as the gravity system, allowing the horizontal forces of the arches to cancel each other out, assuming that the arches are the same size and therefore producing the same amount of outward thrust (Muench, S., 2013).


Throughout all of Rome there are marvelous mixtures of both structure and art. By creating bigger and more astonishing monuments, temples, and palaces, the Ancient Romans became specialized in the art of implementing support throughout structures. From the towering walls of Palatine Hill and the Baths of Caracalla, to the smoothly curved amphitheater of the Colosseum and the stunning delicacy of the dome of the Pantheon the Romans proved they knew what they were doing despite not understanding the more complex internal stresses and forces going on in their structures. Although their empire fell these ancient people were made immortal by the profound creativity and stoic strength of the buildings they left behind.


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