Architectural scholar Stephen Murray said that, in Gothic architecture, buildings are viewed as a triumphal process, every advance in technology resulting in a bigger building. St. Peter's Cathedral at Beauvais was the last -- and perhaps the greatest -- in a sequence of Gothic cathedrals that were built in France during the 11th and 12th centuries.

After 900 years of weather and controversial restructuring, St. Peter's Cathedral at Beauvais is in serious need of repair. Professors at Columbia University are using a 3-D modeling technique that relies on a portable 3-D laser scanner to identify ways to save the structure.

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   The stone behemoth, which rises to a height of 46 meters -- one meter taller than the Greek Parthenon -- has weathered the ravages of nine centuries, yet not without some concessions to its great age. The cathedral has endured violent windstorms and has settled uneasily on a foundation of bedrock and compressed soil, which is subject to changes in the groundwater table. The roof is also failing, and the wooden joints have begun to rot, causing the roof to slip.
   Murray and his colleagues at Columbia University are using a three-dimensional modeling technique that relies on a portable 3-D laser scanner to calculate the best way to save this architectural feat.
   An intervention was attempted in 1993, when enormous tie braces with 20 tons of resistance were applied to stabilize the structure. This attempt has been the source of some controversy. The concern that the French engineers put too much reinforcement in the cathedral ignited a debate that has sparked a renewed interest in the structure on both sides of the Atlantic.
   "It looks really ugly ... like a shipyard or something," said Gautam Dasgupta, professor of civil engineering and engineering mechanics at Columbia. He is part of a team led by Murray that is trying to bring together two sides of the debate about how the cathedral can best be preserved while maintaining its unique façade.
   One faction says that the building is in imminent danger and that heroic steps should be taken to buttress it. The other side, led by architect Jean-Louis Taupin, contends that all buildings oscillate slightly and that the cathedral can afford to shed some of its chains.
   Murray plans to invite both camps to New York in 2003 to discuss the cathedral. He will animate the behavior of the building based on the 3-D modeling technique using a portable Cyrax 2500 3-D laser scanner from Cyra Technologies to generate topographical images of the structure. The scanner emits a green pulsed laser beam with a 6-mm spot size at 0 to 50 m. The point cloud of data reflected from the structure at a rate of 1000 points per column provides a complete image of the surface geometry of the cathedral.
   Professor Peter K. Allen of Columbia University has pioneered laser scanning in this area. His group generated the geometrical data of the cathedral using the Cyra system, while Dasgupta is responsible for analyzing the data and making renderings of the building. The investigators are in the early stages of performing structural and strain analysis -- measuring displacement and compressive stress -- to identify ways to save the structure.
   Murray said that, based on the data yielded thus far, there are issues that are in obvious conflict. In tradition-al treatment of statics, he explained, you need a counterstructure, or the building will rotate.
   The choir changed in concept several times during its development. Yet the various mechanisms of thrust and counterbuttressing have not taken into account the different stages in the cathedral's architectural evolution. Ideally, using the 3-D technology, the research team will be able to remove or replace structures in a computer simulation and determine whether the building will still be standing when the renovation is completed.
   Murray said that he wants to see the behavior of a building translated into words. He noted that it's very difficult to explain the forces that are at work. Eventually, he would like to get his model onto an Imax platform. "If they can do whales, then why can't they do Gothic cathedrals?"

by Brent D. Johnson

Contact: Peter K. Allen, Columbia University, New York; +1 (212) 939-7093; e-mail: allen@cs.columbia.edu. Stephen Murray, Columbia University; +1 (212) 854-8521; e-mail: sm42@columbia.edu. Gautam Dasgupta, Columbia University; +1 (212) 854-3102; e-mail: dasgupta@columbia.edu