October 29, 2011
One of the first things that struck as I looked around this October while we were on our Roman holiday was the state of structures built around first century AD. They’re all in ruins, and nearly all of’em reduced to indistinguishable rubble. When you look at it from this perspective, the Colosseum is staggering.
Hit by numerous earthquakes (notably in 217AD, 223AD, 442AD, 801AD, 1349AD, and in 1703AD), aside from fire, lightning, and later pillaged for construction material (thus dangerously weakening its structural integrity), it is some engineering feat that it still stands remarkably intact. Guts exposed, of course, but it is nothing like the rubble (of the Roman Forum, Circus Maximus, the Palatine Hill, Temples of Elagabalus, Venus and Roma) around it.
Primary cause of partial collapse of the Colosseum’s outer rings, as studies indicate, are attributed to heterogenous ground underneath. Reduced soil stiffness, due to less compact Holocene alluvial deposits it partially sits upon (with the intact parts supported on Pleistocene, volcanic, and sedimentary rocks), led to first differential settlements and by virtue, increased local stresses, which over time, initiated its progressive partial collapse.
What’s insightful about the structure is the way its builders conceived an ingenious new way, using vaults and arches, which not only favored practical construction, but also brought a degree of isolation and shear wall effect against ground movement. They connected supporting sections of two theaters to create a freestanding building, which served the practical necessity of grouping a large audience around an area of limited proportions.
At a fundamental level, they used a technique of inserting square bronze clamps between building blocks with great amounts of lead used for sealing, as an advanced earthquake-proof system of its time, in which lead was used for absorbing seismic shocks. (The large and indiscreet holes, now observed on the walls of the Colosseum, were apparently made in the Middle Ages for extracting bronze clamps that were fixed vertically to blocks beneath them with lead poured into cups carved in the stone.)
Some interesting factoids I learnt along the way:
- The Colosseum’s natural period, as it stands today, is about 0.44s.
- The use of Tufa, the volcanic rock, was recommended by Vitruvius, the world’s first known architect, engineer, and the author of the epic treatise, De Architectura.
- The conservative attitude of Vitruvius towards concrete is reflected at the Colosseum, where it is used mainly for the vaults and foundations, but nowhere as a major load-bearing part of the structure. It may be fascinating to assume that Vitruvius understood concrete’s strength rested in its parts. (His specification for concrete is to use close grained hard stone for the mortar because of the importance of strength, while specifying porous stone for stucco, because lack of shrinkage and cracking is important.)
- The Colosseum could be quickly flooded for mock water battles, the details of which are very sketchy, but at one point, I read about a reference to Ctesibius’s book of inventions, which Vitruvius possessed. (We know that he also worked on hydraulics.)
- Vitruvius set rules for the engineer to follow citing that he should “be a man of letters, an expert draftsman, a mathematician, familiar with scientific thought, a painstaking student of philosophy, acquainted with music, not ignorant of medicine, knowledgeable about the opinions of jurists, and familiar with astronomy and the theory of the heavens.”
As a structural engineer, I am astonished by its sophistication, and awe-inspired by the minds that planned, designed and constructed it to withstand forces of nature and man for 2,000 years. An 8th century epigram, attributed to the Venerable Bede, describes this bloodsport arena best:
While stands the Coliseum, so shall Rome; When falls the Coliseum, so shall Rome; And when Rome falls — the world.