Engineering of Historic Constructions

Today was the first day back to class after the long weekend so it was nice to have a guest lecture specifically on the engineering behind the ancient structures we have seen on this trip.

Bear with me as there is lots to say on this topic and not too many pretty pictures to go along with it.

The first topic was building materials. The Romans used natural stone which was quarried in the shape and size of its intended use. It was important that the stone was close to the site so it could be easily transported. Clay that was used for bricks had to be malleable, but could become waterproof when fired. The shift from the use of stone to bricks had to do with the sudden increase in unskilled workers which could easily produce bricks in the absence of skilled masons.

The lifting mechanisms the ancient Romans used consisted of the traditional pulley, the winch (has a longer lever arm which lessens the effort needed), the horse or shears (a pulley winch combo), tackle (several pulley system which increases the lifting force), and the treadwheel (large 'hamster wheel' connected to tackle).

The building process was facilitated by a network of scaffolds. The scaffolding could either be mobile, on supports, or in a cantilever supported position where the timber was clamped to the wall itself. In fact,  in some of the buildings and ruins we have visited you can still see the holds where the wood scaffolding used to be.

The Romans used several different types of masonry: large unit walls, small unit walls, different arrangements of header and stretcher units.

Foundations were a very important part of why so many of the structures are still standing today. The weight of the construction had to be distributed on a surface larger than the wall cross section. The foundation must also grant stability and avoid settlements of materials. Often these foundations consisted of timber beams, boards and mortar layers.

There aren't many timber roofs left in Rome. Mostly because they were easily destroyed in any natural disaster or fire. The roof truss seen in the Pantheon is of Greek origin and consists of rafters, tie-beams, king posts, battens, and joists all which contribute to the stability of the truss system.

The next thing we talked about was a  very influential part of Roman structures and architecture: arches. The shape of the arch contributes to the resistance of a structure and reduces the bending moment, leaving mostly axial compressive forces. Semicircular arches were used for openings and relieving arches.

From arches, vaults were built. Their construction involved the use of centering framework, and a series of connected arches being built. These connected arches were held in place by a layer of concrete that was poured over at the end. They were also used in retaining walls by the Romans.

From arches to vaults, and from vaults to domes! The 2 directional behavior of domes as opposed to arches led to thinner domes which was essential for such large concrete domes to be build, like that of the Pantheon.

One of thing features that helped the Pantheon become the largest unreinforced concrete dome in the world is the square coffers that lighten the load of the concrete while retaining the structural integrity of the dome.

** pictures and notes thanks to Luigi Sorrentino. Universita di Roma, Sapienza.