Wednesday, August 19, 2009

The Other Eiffel Tower

After 120 years, the famous landmark's original design gets another look

For pictures of its other look:

By Jason Bardi
Inside Science News Service

A new engineering analysis of the Eiffel Tower in Paris shows what the famous landmark would have looked like if its builder had constructed the tower to take into account more realistic wind profiles.

The design was revolutionary 120 years ago, incorporating engineer Gustave Eiffel's newly patented methods for constructing freestanding structures of great height. His method eliminated the need for diagonal truss elements to resist the bending due to an oncoming wind.

The Eiffel Tower was to be his masterpiece -- a testament to engineering progress and the centerpiece of the 1889 Exposition held in Paris on the centennial of the French Revolution. Eiffel wanted to make sure that the thousands of tons of iron used to erect the tower would never collapse in a strong wind.

Like all scientists and engineers of his generation, Eiffel did not completely understand the physics of turbulent wind flow over rough landscapes like urban Paris. Because of this, he did not properly model the wind shear his tower might experience in a strong storm.

By the time construction on the 1,000-foot-tall tower began in 1887, Eiffel had introduced more liberal factors for safety. In order to err on the side of caution, his final design made the tower wider on its lower half than originally intended. The fact that the tower remains intact over the past 120 years is testament to his insight to include substantial factors of safety.

P. D. Weidman, a mechanical engineer at the University of Colorado, Boulder was the first to find the mathematical equation for the skyline shape of the tower according to Eiffel’s patented method of construction. A few years ago, he showed that the shape of the tower is exponential when one assumes, as did Eiffel, that the oncoming wind is uniform with height.

In a paper published last month in Physics of Fluids, Weidman shows what the skyline tower profile would be if it were designed and constructed for a realistic "turbulent boundary-layer" wind profile over Paris. The breakthroughs that allowed engineers to model realistic wind profiles were not discovered until after 1908 -- two decades after the tower was completed.

The end result is that the tower for a realistic atmospheric wind would be wider and have less curvature than the existing tower. According to Weidman, the existing tower is "far more elegant" than the tower designed specifically for a turbulent wind.

The attractive thing about this study, said Jim Brasseur, a Pennsylvania State University professor who was not involved in the research, is that it combines history, architecture, fluid dynamics and beauty. "What more can you ask for?" he asked.
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