The hemispherical arch has been in use for over 6,000 years. It is generally referred to as the Roman arch, as it was during the Roman Empire that it was used extensively for bridge building. The skilled Etruscan engineers taught the Romans the use of the keystone arch, enabling them to build extremely strong and durable bridges. The idea is quite simple.
Imagine a ring of tapered stone blocks arranged in a circle. If one were to take a rope, wrap it around the ring, and tighten it, all it would do is force the stones more tightly together. Exchange the rope for steel cable and tighten by twisting with a steel bar shoved between the cable and the ring and the circle of stones just becomes stronger! A mighty force would have to be used to destroy a ring constructed like this.
To create a keystone arch, one half of this ring of stones was simply stood up on its end. In a typical Roman arch bridge, these ends rest on piers made of stone blocks mortared together with poz-zolanic cement. Sufficient buttressing or adjoining walls provide the tension, as the rope or cable did for the ring of stone. The weight of the stone and the bridge itself compress the tapered stones together, making the arch an extremely strong struc ture.
10.3: Landscape Arch in Arches National Park, Utah. 10.4: Built with cut sandstone, this Roman arch
This arch measures 306 feet (91.8 m) across and is (or voussoir) in Moab, Utah, celebrates its
106 feet (31.8 m) high. 100th birthday.
Heavy wagons and legions of troops could safely cross a bridge constructed of arches without collapsing the structure. Many of these bridges outlasted the Roman Empire, the Dark Ages, Middle Ages, and on into modern times, serving General George Patton during World War II just as they had served Caesar almost two millennia before (Fig. 10.5).
10.5: Built during the Roman Empire, Pont du Gard typifies the use of the keystone arch in bridge building. The lower span is still used as a roadway, while the upper arches functioned as an aqueduct for several centuries.
half circle or Roman arch
S-point Qothic or Egyptian arch lancet arch
10.6: The placement of the compass point determines where the springline begins and defines the shape of the arch. The further the compass point is placed from the center of the baseline, the taller and steeper the profile becomes.
Was this article helpful?