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AE Department was pleased to host the annual AE Senior Thesis Kickoff
Seminar on April 25, 2002 at the Nittany Lion Inn. The featured speaker
was Mr. Robert Silman, president of Robert Silman Associates with
offices in New York and Washington, DC. The seminar featured the recent
renovation completed at Fallingwater.
The structure, which is located in Mill Run, PA, is the architectural
masterpiece of renowned architect, Frank Lloyd Wright. It was designed
in 1937 as a weekend home for the family of Pittsburgh department store
owner Edgar J. Kaufmann. (photo left)
The focal point of the house is the waterfall over which the house is
built. Yet, this incomparable structure had a critical flaw. Wright’s
design did not provide enough steel reinforcement for the concrete cantilever
portion of the house that hangs over the Bear Run stream. As a result,
Fallingwater’s famed terraces began to droop as soon as they were built,
causing large cracks to appear in the concrete. What is more, the sagging
gradually increased over the next six decades. In 1995, the Western Pennsylvania
Conservancy, which owns Fallingwater, was concerned enough to hire the
engineering firm, Robert Silman Associates in New York City, to examine
the house’s structural problems. The
results of their investigation indicated that the beams supporting the
house were continuing to deflect, and that the building would eventually
collapse into the stream below if no remedial measures were undertaken.
To determine how to relieve the stresses that were threatening the house,
Silman’s engineers probed the building with radar and ultrasonic pulses,
and then performed a rigorous structural analysis. This analysis involved
the development and installation of an in-service structural monitoring
system, including vibrating wire crackmenters and tiltmeters, with a datalogging
microprocessor to gather data to allow calculation of both short-term
and long-term deflections. A finite element computer model of the
cantilever structure was also developed to evaluate both general structural
behavior and local stress distributions.
The repairs to the major cantilever beams at Fallingwater were completed
in March 2002. The severe overstresses were relieved through the creative
use of post-tensioning. High strength steel cables were rigged on both
sides of each beam, anchored in concrete blocks, and attached to the beam’s
ends. The cables or strands were then tightened from the outside using
a hydraulic jack. The tension in the post-tensioning strands exerts a
positive bending moment on the beam, counteracting the negative moment
caused by the cantilever action. The strengthening of Fallingwater’s cantilever
beams will guarantee the structural stability of the house for years to
come. Moreover, the plan stabilizes the house without the need for permanent
props rising from Bear Run stream. Thanks to state-of-the-art technology,
Robert Silman Associates will preserve the most striking architectural
element of Fallingwater: its cantilevered terraces stretching gracefully
over the rushing stream.
A feature article highlighting details of this historic project appeared
in the September 2000 issue of Scientific American.
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