Bigazzi, Donata

English

Almae Matris Studiorum Campus

casa sulla cascata -wright

FRANK LLOYD WRIGHT'S FALLINGWATER  Designed in 1935 and completed in 1939, Fallingwater is recognized as one of the most unique houses ever built in America. It is not only one of architect Frank Lloyd Wright's most distinctive accomplishments but is also a monument to Wright's concept of organic architecture - harmonizing man with nature so effectively, that the structure and its surrounding environment becom  a single, unified composition.  The house looms over a waterfall on Bear Run in northwest Pennsylvania.  Following the natural pattern established by the stream's rockey ledges, the structure is positioned over the falls in a series of cantilevered concrete “trays” anchored to sandstone masonry walls. Although the structure rises more :than 30 feet above the falls, the predominance of its horizontal lines make the house appear to be hovering, thus creating the sheltering effect. Two outdoor terraces extending prominently from the living room and master bedroom comprise almost as much floor space as the interior oft he house.  Fallingwater was built by the Kaufmann family, owners of the Kaufmann department store enterprise in nearby. Pittsburgh. The family home used the home as a weekend and vacation dwelling until 1963, when the property  was donated to the Western Pennsylvania Conservancy.  Wright planned each face of Fallingwater to blend with the natural surroundings. A series of four large bolsters built into a natural sandstone ledge comprise the structure's foundation. Three of these bolsters are made of reinforced concrete, and one is made of stone masonry. Three-foot wide girders daringly cantilever outward from the bolsters approximately 15 feet over the stream. These girders are the primary support for the main level terrace. Concrete joists (four inches wide) spaced at four feet on center span the girders and support wooden planking. Stone flooring rests on the planking. One of the home's signature features is the soffit slab that is both a structural and architectural asset. This slab works integrally with the girders as a load-carrying T -beam, while providing an aesthetically pleasing smooth finish on the structure's underside.  [5] The master bedroom terrace cantilevers approximately six feet farther out than the main level terrace. Four vertical structural steel T-shaped members span upward from the main level to support the master terrace cantilever. Interestingly, these members are also used as window mullions. This was another strategic move by Wright to seamlessly blend structural elements with architecture.  The problem  Bar reinforcement is generally placed in the top of cantilever members to carry tension stresses created by dead and live loads. Such loads at the master and main terraces include stone flooring, furniture, people, and snow. While the cantilever girders at the main level contain sixteen oneinch-square rebars each, engineers found that the reinforcement was inadequate for the design loads. This deficiency has caused both terraces to sag downward towards the stream. A review of historical deflection data indicated that the vertical movement was active - and progressively worsening. Cantilever deflections of up to seven inches over the l5-foot cantilevers caused tension cracks to appear at the parapet walls at the master terrace. Left unattended, these problems might have eventually caused the structure to fall into the stream below. By the time the house celebrated its 50th anniversary in 1989, growing concerns about the structural integrity of the house - especially deflection and sagging of the cantilevered terraces - led to a comprehensive analysis of the structural deterioration using nondestructive testing methods.  [8] Until the 1990s, tension cracks were cosmetically patched without addressing the underlying structural issues. Cracks reappeared in the patched areas as the terraces continued to deflect. In engineering studies, it was discovered that the master bedroom terrace could not function as an independent cantilever, and was transferring its load to the living room level below. The master terrace is propped up by four steel mullions from the main level.. Calculations indicated that this additional load increased stresses at the main terrace to critical levels. Restorative action was recommended to avoid the eventual collapse of the structure.   Repair Procedures  The Fallingwater renovation plan calls for both strengthening and concrete repair. At the main terrace level, strengthening will include bonded post-tension tendons parallel to the cantilevered girders, and unbonded tendons in the transverse direction. The tendons start at points near the bottom of the existing concrete members" and  gradually rise up to the top of the existing beams at midspan. This will create a lifting effect, or positive bending moment, on the cantilevers when the tendons are stressed. The strengthening plan also calls for .steel channel beams to be bolted to each side of the master level concrete joist directly above the four 'T'-shaped mullions. These channels will ensure that the beam has sufficient strength to achieve the proper transfer of forces :from the living room to the master terrace. Like the post-tension cables at the main level, these channels will be hidden from view in the floor cavity.  [11] Another aspect of the repair procedures will involve repair and strengthening of the master level parapet walls. These walls have experienced significant crancking due to the cantiliver deflections.. The plan calls for three l4-foot-long FRP (fiber-reinforced polymer) bars to be epoxy-grouted into narrow grooves cut into each of two parapet walls. This procedure will keep existing cracks from reopening. In addition to the new bar reinforcement, a significant amount of epoxy crack injection repair will be performed by VSL on the parapet walls.  [12] Once the post-tensioning strands, FRP rods, and steel channels are installed, the concrete wil1 be patched and finished to match existing surfaces, thus leaving no visible trace of the work performed 

http://campus.unibo.it/65492/1/wright_falling_water.pdf

casa sulla cascata -wright

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