erched on a hillside in southwestern Pennsylva- nia, about 72 miles from Pittsburgh, is one of the world’s most famous houses. Fallingwater, the stunning creation of architect Frank Lloyd Wright, has been an American icon since its con- A firsthand account struction in 1937. MoreP than two million tourists have visited the site and stared in by renowned engineer awe at the building’s concrete terraces hanging over a clear, swift-running stream. Architecture critics have extolled Fallingwater as Wright’s greatest achievement. In Robert Silman fact, in 1991 the American Institute of Architects voted it the best work ever pro- duced by an American architect. Yet this incomparable structure has a critical flaw. Wright’s design did not pro- vide enough support for the portion of the house that hangs over the 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 gradual- ly increased over the next six decades. In 1995 the Western Pennsylvania Conser- vancy, which owns Fallingwater, was concerned enough to hire our engineering firm, Robert Silman Associates in New York City, to examine the house’s structur- al problems. The results of our investigation indicated that the beams supporting the house were continuing to bend and that the building would eventually collapse into the stream below if nothing was done. In 1996 the conservancy prudently decided to shore up Fallingwater with tem- porary steel beams and columns. At the same time, our office began to draw up a plan to permanently repair the house. We had previously worked on two other buildings designed by Wright—the Darwin D. Martin House in Buffalo, N.Y., and Wingspread in Racine, Wis.—but Fallingwater posed a unique challenge. To deter- mine how to relieve the stresses that were threatening the house, our engineers probed the building with radar and ultrasonic pulses, then performed a rigorous structural analysis. Along the way we also tried to retrace the thinking of Wright and his apprentices. We now have a plausible theory to explain how the design of Fallingwater went awry. The story of Fallingwater begins with Edgar Kaufmann, Sr., who owned a suc- cessful department store in Pittsburgh in the 1930s. His son, Edgar Kaufmann, jr. (he always spelled “junior” with a lowercase “j”), spent a short time as an ap- prentice in Wright’s studio at Taliesin, the architect’s estate in Spring Green, Wis. Kaufmann, jr., convinced his father to retain Wright to do some work at the store and later to design a weekend house for the family on a site that had formerly been The Plan to Save Fallingwater This breathtaking house designed by Frank Lloyd Wright was in danger of collapse until an engineering firm found a way to stop it from falling down ROBERT P.RUSCHAK/COURTESY OF WESTERN PENNSYLVANIA CONSERVANCY CONSERVANCY ROBERT P.RUSCHAK/COURTESYWESTERN PENNSYLVANIA OF Copyright 2000 Scientific American, Inc. Copyright 2000 Scientific American, Inc. a summer recreation camp for the store’s the house so that the section over Bear is a concrete slab that serves as the fin- employees. Run acts as a cantilever. Like a diving ished underside of the structure. Wright The wooded property features a small board, it has a fixed end and a free end. chose this design to give the house’s ex- stream known as Bear Run that cas- The fixed end consists of four large bol- terior a monolithic look, but it also had cades over a series of rocky ledges. The sters, three of reinforced concrete (that a structural purpose. In engineering Kaufmanns had always assumed that is, concrete with steel bars embedded in terms, a cantilever has a negative bend- their house would be located down- it) and one of stone masonry. These ing moment—the load at the free end of stream from the ledges, at a point bolsters rise from the sandstone ledge the horizontal beam is resisted by ten- where the waterfalls could be viewed to the building’s first floor [see illustra- sion in the beam’s upper side and by from below. But it was Wright’s genius tion on pages 92 and 93]. Each one compression in the lower side. (In con- to site the house above the falls, on top supports a horizontal reinforced-con- trast, a bookshelf has a positive bend- of a large sandstone ledge that over- crete beam that extends some 4.42 me- ing moment—the weight of the books is looks the stream. The building was de- ters (14.5 feet) beyond the bolster, jut- resisted by compression in the shelf’s signed in 1935, and construction start- ting southward over the stream. The upper side and by tension in the lower ed in 1936. The design work was con- beams are connected to one another by side.) Wright’s decision to put the con- ducted at the Taliesin studio, with concrete joists, each 100 millimeters crete slab under the cantilever beams Wright’s apprentices Bob Mosher and (four inches) wide. Together the beams turned them into inverted tee beams— Edgar Tafel participating significantly. and joists create a rectilinear grid. each shaped like an upside-down T— The structural calculations for Falling- Above this grid are wooden two-by- thereby raising their resistance to com- water were done in the same studio by fours and planking, which support the pression and enabling them to support engineers Mendel Glickman and Wil- stone floor of the house’s living room a greater load. liam Wesley Peters. and the first-floor terraces. Fallingwater has more than one can- Wright and his apprentices designed Beneath the joists and cantilever beams tilever, though. Terraces extend from 90 Scientific American September 2000 The Plan to Save Fallingwater Copyright 2000 Scientific American, Inc. INTERIOR VIEW of Fallingwater’s living room ment was not enough, as the builders shows the stone floor that rests on the discovered during Fallingwater’s con- house’s cantilever beams. The windows at struction. When workers removed the the south end of the room are divided by wooden formwork from beneath the four steel mullions that help support the concrete of the first floor, they recorded weight of the second floor. an instantaneous downward movement of 44.5 millimeters. It is not unusual for out than the first floor does, extending a small amount of deflection to occur an additional 1.83 meters (six feet) when the scaffolding is removed from a southward [see left illustration below]. concrete structure, but in this case the Four T-shaped window mullions rise bending was especially pronounced. from the south edge of the living room Mosher, the apprentice on site, tele- to the terrace above. At first glance phoned Glickman at the studio in Tal- these steel mullions appear to be merely iesin. After a quick check of his calcula- ) decorative, but we would eventually tions Glickman is reported to have ex- ight learn that they, too, play a key role in claimed, “Oh my God, I forgot the om r ott b Fallingwater’s structure. negative reinforcement!” ( Concerns about the soundness of Glickman was referring to the rein- ANCY V Wright’s design arose even before con- forcement needed to balance the nega- struction started. Metzger-Richardson, tive bending moment, which causes ONSER ANIA C the Pittsburgh engineering firm that compression in the lower part of each V supplied the steel bars for the rein- cantilever beam and tension in the up- forced concrete, insisted that there were per part. In any beam made of rein- ENNSYL not enough bars in the cantilever beams forced concrete, the concrete resists the below the living room. To make the compression on the beam and the steel WESTERN P beams strong enough to resist bending bars in the concrete resist the tension. under their load, the firm doubled the Fallingwater’s cantilever beams could TESY OF OUR C ); number of one-inch-square bars in each handle the compression caused by the t lef m beam from eight to 16. Wright was fu- negative moment, but there were not o ott rious when he learned about the change. enough steel bars in the upper parts of b He believed that the additional steel the beams to balance the tension. and op bars would increase the weight of the The problem became even more ap- t ANCY ( beams too much and thus weaken the parent after the completion of the sec- V structure. In an angry letter to Kauf- ond floor. Soon after workers removed ONSER mann, Sr., he wrote: “I have put so the formwork from the concrete of the ANIA C much more into this house than you or master bedroom terrace, two cracks ap- V any other client has a right to expect, peared in the terrace’s parapets. In 1937 ENNSYL that if I don’t have your confidence—to Metzger-Richardson conducted load hell with the whole thing.” tests of the structure and calculated that Kaufmann, Sr., appeased his architect the stresses in the cantilever beams were WESTERN P the east and west sides of the first floor, by asserting his confidence in him. But near or even exceeded the margins of TESY OF supported by concrete joists under their Wright was clearly wrong about the safety. The engineering firm recom- OUR C floors and by edge beams in their para- cantilever beams: if Metzger-Richard- mended placing permanent props in the K/ pets. And on the building’s second son had not slipped in the extra steel streambed to support the first floor and USCHA R .