The scope of the structural rehabilitation of Fallingwater, Frank Lloyd Wright's architectural masterpiece in rural western Pennsylvania, has been expanded because of the deteriorated condition of the substructure beneath the floor of the living room.

"I was amazed at the amount of damage we found when the stone flooring was removed," says James Loper, a structural engineer at VSL (VStructural, LLC) of Springfield, Virginia, the post-tensioning contractor. "We believe that some localized damage may have occurred during a flood and on another occasion when a tree fell on the structure several years ago." Extensive crackling and spalling, for example, were found in the 4 in (100 mm) wide concrete joists that run perpendicular to the main cantilevered beams supporting the east terrace.

A combination of additional post-tensioning and carbon fiber wrap may be used to reinforce the substructure, Loper says, and even additional joists may be necessary. The restoration team, led by Robert Silman Associates, of New York City, the structural engineer of record, is also considering adding steel plates to the structural window mullions, which investigations have shown to be supporting the master bedroom terrace.

The primary purpose of the rehabilitation project is to halt the deflection of the cantilevered terraces, which has causes concern since the house was built, in 1936. (See "Restoration to Strengthen Cantilevers at Fallingwater," Civil Engineering, May 1999, page 28.) Post-tensioned tendons, each comprising up to nine 0.6 in. (15 mm) diameter highstrength strands, will be installed on each side of the 3 ft (1 m) wide main girders that support the terraces. The tendons will be tensioned to approximately 350 kips (158,760 kg) each. Single post-tensioning strands will be placed in the transverse direction. The $11-million restoration of the structure is scheduled for completion this spring.