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General Background: To escape the hustle and bustle of city life in downtown Pittsburgh, Edgar and Lillian Kaufmann, owners of the Kaufmann department store chain, decided to build a summer retreat in rural area of Southwest Pennsylvania known as the Laurel Highlands. The grandiose setting of a waterfall on Bear Run was chosen as the site of the house. Due to the prestige of the Kaufmann family, and the beauty of the natural setting, Frank Lloyd Wright took interest and control of the designing of the house called Fallingwater. Construction began in 1936. In 1963, Edgar Kaufmann, Jr. donated the house to the Western Pennsylvania Conservancy.
The Problem: In his design, Wright utilized multiple cantilevered terraces, to match the theme of the waterfalls. These terraces were designed of reinforced concrete. Wright had previously exploited the use of reinforced concrete, but had not undertaken such an extensive endeavor using this material as he would in Fallingwater.
Upon casting the lower terrace that cantilevered over the creek and the waterfall, large deflections were instantly discovered. Some of this initial deflection was due to the neglect to consider the wet weight of concrete by setting the forms level, instead of cambering them upward. The majority of the remaining deflection was caused by a lack of tensile reinforcement in the main inverted tee-beams that supported the lower terrace.
The lack of tensile reinforcement was not overlooked. In fact, the contractor added seven reinforcing bars to the beams after noticing the mistake. This infuriated Wright who would later say that the excessive weight of the added bars actually led to the excessive deflections. Regardless, even the addition of the extra rebar was not enough to control the nearly 8” of deflections of the end of the lower terrace.
Though the deflection is a great cause of concern, the cracks caused by the deflection also is of interest. There are a large number of cracks on the main beams of the lower terrace, as well as on the supporting concrete floor joists, and the upper terrace’s parapets, clearly evident in Figure 2.
Another interesting aspect of Fallingwater concerns the window mullions - the vertical steel tee-sections of the window frames that are embedded in the lower terrace parapet and extend into the upper terrace. These mullions were most likely considered as a non-structural element. However, finite element analysis has shown that the mullions act as structural elements and transfer loads from the upper terrace to the lower terrace. The mullions are shown in Figure 3.
Because of the structural deficiencies in Fallingwater, the Western Pennsylvania
Conservancy decided to hire the engineering firm, Robert Silman and Associates
of New York to investigate the house and to propose a solution to mitigate
the problems. The house is now temporarily shored with steel to assure
the safety of the structure. The steel shoring can be seen clearly in
Figure 4.
The Solution: Due to the historic nature of the house, many otherwise suitable methods of rehabilitation could not be considered due to the fact that the rehabilitation would alter the structure’s original appearance. In other words, the option that least changes the original nature of the house is the most optimal. Because of this, the beams are being externally post-tensioned. Post-tensioning is a construction/design method that utilizes high-strength steel cables or strands. The strands are typically bundled in a duct and anchored at one end of a span and pulled to a specified tension. Afterwards, the strands at the other end of the span are anchored. This pre-compresses the concrete, thus reducing tension. This is highly beneficial since concrete can handle compression very well and tension in concrete leads to cracking.
External post-tensioning generally specifies that the duct is not cast throughout its length inside the concrete member, but rather runs through anchor blocks and deviators attached to the sides of the beam. The deviators provide the necessary moment-arm to counteract some of the beams loads. External post-tensioning is favorable because after the stone floor is replaced, there will be little evidence that any work even took place. When the strand will be tensioned, the lower terrace will rise upward. Though it is very possible to tension the strands enough so as the lower terrace will rise the 8 inches required to be level again, it is highly unfavorable. This is because doing so will crack non-structural materials, like windows, since the non-structural materials have had many years to deflect along with the terrace. Therefore, the terrace will only be raised about an inch – enough to make the house safe.
Penn State's Contribution: One of Penn State’s main objectives in the Fallingwater rehabilitation project is to successfully monitor the house through a variety of different instruments throughout the many different stages of the construction. One important consideration is not to permanently alter the house by installing the instruments. Because this was such an important consideration, all of Penn State’s instruments were chosen such that they would do little to no harm. Gauges are either clamped or bonded with epoxy. The epoxy marks can be sanded and repainted after the gauges are removed. Penn State can be shown instrumenting Fallingwater in Figure 5.
Penn State is monitoring multiple effects during the entire construction process. First the deflection of the terrace throughout the construction process is monitored with dial gauges. The window mullions (vertical components of the window frames), though they are non-structural items actually are taking some of the loads. Because of this, the strains in these mullions are being monitored through all stages of construction with use of vibrating wire gauges and Demec gauges. Strains are also being monitored with vibrating wire gauges on the three beams that support the lower cantilevered terrace during the post-tensioning process only. Lastly, the widths of the cracks in the cantilever beams are being monitored, as the cracks will close when the beams are compressed. It is also of interesting note that the company performing the post-tensioning operation is also taking deflection measurements during the tendon jacking through the use of the linear variable differential transducer (LVDT). Deflection results will then be compared.
Instrumentation Types used by Penn State: Dial Gauges (Deflection) – See Figure 6
Vibrating Wire Gauges (Strain) – See Figure 7
Demec Strain Gauges (Strain) – See Figure 7 |
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