- Historic Sites
Saving The Statue
After standing in New York Harbor for nearly one hundred years, this thin-skinned but sturdy lady needs a lot of attention. She’s getting it- from a crack team of French and American architects and engineers.
June/july 1984 | Volume 35, Issue 4
The statue’s weight is borne by a central pylon based on a typical Eiffel bridge pylon, consisting of four iron piers rising ninety-seven feet and held together by nine levels of horizontal struts and diagonal cross-bracing. From the pylon a secondary framework of lighter iron trusswork reaches out toward the statue’s skin, often coming within inches of it. The skin, with its hammered-copper sheets riveted together, is backed by a web of horizontal and vertical iron belts that follow its shape but theoretically do not quite touch it. To allow for the different thermal expansions of copper and iron, the belts are connected to the skin through riveted copper saddles, with an insulating material used to keep the two metals apart.
The torch turned out to be at “definite risk of structural failure”—that is, falling off.
Hundreds of single, unbraced, flat bars extend between the skin’s iron armature and the pylon’s secondary framework, and each of these bars is joined to the latter by only a single bolt. Altogether they work as a network of tight springs, firmly yet resiliently holding the statue’s sheath to the trusswork attached to the central pylon. The entire load of the copper sheets is brought back to the pylon every 12.5 feet, and thus the statue is in effect an early example of curtainwall construction, independently invented by Eiffel just before it began to be developed by architects in Chicago for the modern skyscraper. This design allows the statue to expand and contract in the sun, and even to twist slightly, as well as to give a bit in the wind. Marvin Trachtenberg describes Eiffel’s structural system in his study of the monument, The Statue of Liberty , as an “uncanny prophecy of stressed-skin construction that would become crucial in twentieth-century aeronautic engineering (in airplane wings, for example).”
Meanwhile, to measure wind speed, anemometers were placed on the torch and at the foot of the statue. Devices to measure temperature and humidity were installed inside the statue and out. An instrument measuring the concentration of carbon dioxide produced by visitors revealed that dangerous levels are often reached inside the statue on busy days. Another set of instruments measured the displacement of air inside the statue to determine how fast it rises and how it moves. All these environmental readings were transmitted daily by a phone connection to Blaine Cliver’s office, in Boston, and to Paris.
To calculate how the structure behaved, how it reacted to all those environmental forces, 142 stress gauges were installed on different bars and elements, indicating every change in “tension, contraction, stress, and so on,” as Despont puts it. The team hoped to learn how the statue would respond to very strong storm winds, but there never were any during testing, so a computer simulation model was created to extrapolate from available data.
Working principally with CETIM, a large French engineering laboratory located near Paris, the team also measured the thickness of the copper plates, plus the thickness of many of the iron bars, and had a microscopic analysis of the armature bars done to determine their exact composition. They are made of puddled iron. Then a survey was done to determine the condition of the 1,600 pieces of armature and the 1,500 copper saddles.