fallingwater by gerard feldmann

7/27/2019 Fallingwater by Gerard Feldmann

1/5STRUCTURE magazine September 2005

FallingwateFallingwateIs No LongeIs No Longe

FallinFallinBy Gerard C. Feldma

Fallingwater, Frank Lloyd Wrights masterpiece design, was a radical departure from the typiresidence of the time. The reinforced concrete construction with long multiple cantileverelements was ahead of its time. The problem was a design that left the main cantileverbeams under-reinforced. This under-reinforcement combined with the high compressstresses and an ongoing creep condition in the concrete had caused deflections as high

7 inches on a 15-foot cantilever. The structure could collapse suddenly under such conditions. Becausethis condition, the structure needed to be shored as the permanent design was started. An external potensioning system was selected to strengthen the cantilevers and mitigate the deflections.

A STRUCTURE article, from July/August 2001 by Robert Silman and John Matteo, described the histof the structure, the monitoring of deflections and the analysis of the tiered cantilevered system. Fallingwais currently owned and maintained by the Western Pennsylvania Conversvancy (WPC). The repair work wdone during the winter of 2001/2002. The structural consultant for the WPC was Robert Silman AssociatP.C. (RSA). The Di Salvo Ericson Group (TDEG) was the post-tensioning consultant for RSA. TDEassisted with the evaluation of the existing conditions and guided the post-tensioning design. The potensioning contractor was VSL and the repair contractor was Structural Preservation Systems, Inc (SPS).

Testing and Analysi

The pre-design non-destructive testinprobing established some of the parafor the preliminary design. The initial pwas limited to non-destructive radar with a small hole and limited access the floor. The floor was opened up furtallow more intensive observation and t(See Figure 1) Cores were taken frobeam to establish a compressive strengthe concrete. This work also establishamount of reinforcement in the bwhich had been a matter of conteduring the original construction. (SeeSilmans sidebar) It was during this prthat the first surprise occurred. A cracdiscovered that effectively created a hinge on the center cantilever beam at thof the pier below. (See Figure 2)

The structure had been previously mto evaluate the original design. The final of the external post-tensioning requiredtinuing refinement of the model to detethe overall effect of the large forces thato be applied to the existing structure anewly discovered hinge. The culminatthis process resulted in the final design.

Figure 1: Probe hole to allowinspection of cantilever beams

Figure 2: Large crack in main beammaximum size approximately3/16



Final Design

The discovery of the hinged beam added tothe already complicated design. The exteriorpost-tensioning system was chosen, but theamount of post-tensioning force to be addedneeded to match the different demands of the

individual cantilever beams. It was decidedearly in the design process that the existingdeflections of the cantilevers could not beremoved. The 60+ years of concrete creepand the suspected yielding of the reinforcingsteel prevented this. The task at hand was toprevent any further deflection. This was to bedone by reducing the dead load stresses in the

Figure 3: Elevation of typical main beam strengthening

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reinforcing steel approximately 10 ksi. Thislevel of force would create enough verticalforces to lift the cantilevers slightly.

The exterior post-tensioning needed tolift both levels of cantilevers, as the upperfloor rested on the lower cantilevers through

structural window mullions. The designforces chosen to reduce the stresses of themain cantilever created a problem with thesemullions. The vertical component of the post-tensioning force at the end of the cantileverswould increase the mullions axial forces tonear buckling. The forces involved in thestrengthening system could not be reduced,

so, after discussion with the WPC oaesthetic impact, the structural mullionbeefed up by welding additional plaincrease the axial capacity.

A bonded, multi-span system was usthe main beams and a monostrand syste

utilized on the joists. The final strengthdesign resulted in the following tendonforces on the three main cantilever bWest and Center: 26 strands, 780East: 10 strands, 300 kips. See Figure 3typical elevation of the strengthening sThe post-tensioning stressing, divertedead-end blocks for the main beams

2002 The Structure Group Comp



Figure 7: Stressing of mainpost-tensioning tendon

Figure 4: Typical stressing end detail showing transverse post-tensioning

also transversely post-tensioned to preventsplitting from the large horizontal forces. SeeFigure 4 for the anchor block detail. Themonostrand force was 43 kips per strand.The bearing ends of the cantilevers were moresecurely anchored to the piers below by coringand grouting rock anchors since the existingreinforcement in that location was found,from historic photos and radar investigation,to be minimal.

Construction

As with all rehabilitation projects, thereare surprises when the structure is openedup. The investigation had observed andnon-destructively tested the main cantileverbeams on the interior of the structure. Thefull removal of the floor system, which was

Figure 6: Beam strengtheend block prior to casting

2002 The Structure Group Comp

Figure 5: Failed cantilever joistdiscovered after floor removal

a time consuming project in itself as eachpiece of stone had to be cataloged to find itsplace again during replacement, uncoveredother structural damage. The east side joistcantilevers were found to have effectivelyfailed with large cracks. (See Figure 5) Thesejoists were beyond repair, and were removedand replaced. Additional external post-tensioning was added in this location as well.

The installation of the main external post-tensioning proceeded. Cores were madethrough the parapets and perpendicularlythrough the main cantilever beams at thestressing and dead ends and at the high-pointtendon deviations. Figure 6shows the typicalreinforcement of the stressing end anchorageprior to casting. Figure 7shows the main post-tensioning in the process of being stressed.

The contractor suggested an alternate mstrand system for the east-west joists thlized a crossover anchorage. This negatneed to core through the parapets andpreserved the original structure. Thetendons were stressed incrementally ovedays to slowly apply the large forces structure. Figure 8 shows the crossovchorage being stressed. The parapet petions were patched by SPS to within a of the surface to allow restoration speto patch the remaining material to matsurrounding finish of stucco and paincompleted repairs are invisible to thFigures 9 and 10 show the restored inand exterior, respectively. (See page Figure 10).

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2002 The Structure Group C

2002 The Structure Group C



Figure 8: Monostrand beingstressed at cross-over anchor

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The structure was monitored for movementduring and after the post-tensioning to fieldadjust the forces and to measure the uplift ofthe cantilevers. The maximum uplift, at theend of the west beam immediately after thepost-tensioning, was 0.30 inches. The deflec-tions of the other beams ranged from 0.08inches to 0.18 inches. Long term monitoringhas not recorded any further deflections.

ConclusionsThe completed repairs did not change

the appearance of this landmark structure.The use of the exterior post-tensioning wasthe most practical and least costly methodto accomplish this task, but its impact onthe whole building, both structurally andaesthetically, needed to be checked thoroughly.

Frank Lloyd Wrights architectural masterpieceis no longer falling and will continue tostand guard for many years over the waterfallfrom which it takes its name.

Figure 9: Restored interiorof main living room

Gerard C. Feldmann, P.E. has 18 years ofexperience designing and investigating post-tensioned concrete structures. He is a SeniorProject Manager and head of the ConstructionPerformance Analysis Group at the Di SalvoEricson Group in Ridgefield, CT. Gerard canbe reached [email protected]

See Mr. Silmans sidebaron next page...

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What Went

WrongAfter much testing and non-destructive

evaluation (NDE) work, our firm foundthat the major cantilever beams at the firstfloor were grossly under-reinforced thereinforcing steel had exceeded its yield

strength and the compressive stress in theconcrete (approximately 4400 psi) wasapproaching the ultimate failure strength(approximately 5000 psi). How could thishave happened? There were two structuralengineers collaborating on the design,Mendel Glickman and Wes Peters, bothhighly competent. The theoretical problemwas not difficult, even for 1936 a simpleconcrete cantilever beam. Here are some ofour theories:

1

The house was designed veryquickly. Mr. Wright had visited thesite just once and then had done

nothing more. One morning he received aphone call from the owner, Edgar Kaufmann,who was visiting in Milwaukee, a three-hour drive from Taliesin, the architecturalstudio operated by Frank Lloyd Wrightand staffed by his apprentices. When heasked to come and see plans of the house,Mr. Wright invited him for lunch. He thengathered the apprentices around him and,in the space of one morning, proceededto draw all of the floor plans. While theywere at lunch, the apprentices finished theelevations. Virtually nothing changed fromthat original set and construction began

soon after.

2We suspect that the engineers werebeing pushed by the contractor toproduce the structural drawings

so that they could build the next piece. Itis very possible that the engineers did notrealize that the load from the second floorwas being transmitted to the tip of thefirst floor cantilevers by means of the fourreinforced mullions.

3Even if these facts were so, thereis still evidence that both thesecond and first floor beams

were inadequately reinforced. It appears

as though there were mistakes made. Onthe day that the formwork was decentered,the cantilevers deflected 1 3/8 inches and asignificant crack opened in the second floorspandrel/parapet beam over the masonrysupporting pier below. They called MendelGlickman who went to check his calculationsand when he returned to the phone he isreported to have exclaimed, Oh my God,I forgot the negative reinforcing! Theoriginally designed reinforcing in the firstfloor cantilevers was also far too skimpy, notenough to even hold the weight of the firstfloor by itself.

4The reinforced concrete contrac-tor, also an engineer, determinedby means of his own independent

calculations that there was insufficient rein-forcing in the first floor cantilever beams.The contractors wrote of their findings toMr. Kaufmann, who passed the letter alongto Mr. Wright. The latter, famously egocen-tric and unwilling to admit that he couldever be wrong, responded with a classic let-ter to his client, forcing him to choose be-tween the veracity of the contractor or thearchitect. Mr. Wrights words went some-thing like this: I have put so much moreinto this house than you or any other clienthas a right to expect that if I havent your con-fidence to hell with the whole thing. Mr.Kaufmann backed down and deferred toMr. Wrights power of persuasion. After acontentious exchange with Mr. Wright, thecontractor sneaked in double the amount

of reinforcing called for on the originasign drawings. This was still inadequathe dead loads of the concrete theyapparently not accounted for the loathe second floor on the tip of the first cantilever either, even though it was cshown on the original drawings.

In summary, we never were abconclude what went wrong. But wrodid go and it remained in that dangstate for 65 years until this elegant rwas completed.

Robert SiRobert Silman Associates,

New Yorksilman@rsapc

Figure 10: Fallingas it looks today

50 STRUCTURE magazine September 2005

fallingwater by gerard feldmann

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