How Post Tensioning Rescues Buildings From Structural Failure Construction Essay

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In the modern Construction Industry there is an increase in the use of concrete throughout buildings. This increase has resulted in Architects being able to push the boundaries of modern designs. Modern design features have also meant that new technologies have had to be produced to incorporate new innovations.

With concrete being a predominate material being used not only in the present day but in the future and past construction Industry, it is vital that Architects have a greater understanding of concrete as a material and such failures that can be associated to it.

The objective of this dissertation is to obtain a clear understanding of the type of concrete failure which occurred with Frank Lloyd Wright's most famous building Fallingwater, and also the solution method in which was used to rectify the problem and prevent it from destroying the Building.

Reinforced concrete has been used not just on modern structures but also on older buildings too. To be able to carry out work such as renovations etc. to existing buildings it is therefore important to have knowledge about such failures and the solutions available.

Therefore the aim is to construct a technical review of the Post-tensioning system which was implemented for the failure of the cantilevered concrete balconies at Fallingwater. In this dissertation I hope to highlight the technique of Post-tensioning, to investigate if Post-tensioning was a viable solution to Falling water's problem, to evaluate the benefits of the technology and to what extent it made an impact on the design features, construction and subsequent use of the building.

Preface

This dissertation is the result of my ever growing interest in the famous Architect Frank Lloyd Wright. I have always been passionate about Frank Lloyd Wright's work and when I had the fantastic opportunity to design a building based upon Wright's famous designs, I became even more enthusiastic about one of Wright's most famous buildings, Fallingwater.

Fallingwater is a house designed by Architect Frank Lloyd Wright. The house was designed in 1934 and was later completed in 1936. It is situated in Pennsylvania, which is 50 miles southeast of Pittsburgh. The most striking single feature of Fallingwater is the cantilevered reinforced concrete balconies that extend out over the falls. Such cantilevers were a relatively new design element in the 1930s, and reinforced concrete technology itself had not progressed much beyond simple, direct bearing configurations.

Table of Contents

Table of Figures

Chapter 1: Introduction

Introduction and Background

This dissertation takes as its subject the failure of the concrete balconies at Fallingwater and investigates not only the cause of the concrete failure but also the technology that was selected to remedy the problem.

Falling Water

Dissertation Organisation

Chapter 2: An Introduction into the failure of the concrete and Post-tensioning as a practical solution of concrete failure for both older buildings in need of repair and the future developing construction Industry. In addition it provides an opportunity to analyze the case study of Fallingwater where post-tensioning technology was used to prevent the collapse of the cantilevered balconies.

Chapter 3: This chapter of my dissertation outlines the aims and objectives of my research and the method of which to meet these aims and objectives.

Chapter 4: This chapter focuses on what Post-tensioning consists of and the technical aspects of this relatively new technology. It also investigates the viability and future role of this new technology being adopted by new and existing buildings.

Chapter 5: This chapter presents the results of my primary research and includes my conclusions and recommendations for future work.

Chapter 2: Overview of Fallingwater

Fallingwater consists of a series of 15ft concrete cantilever balconies which are 30ft above the waterfall. The technical configuration of the cantilevered slabs at Fallingwater is an upside down version of how such designs are commonly used. Instead of protruding vertical beams with the slab on top, at Fallingwater the slab is on the bottom with the beams on top, forming I shapes above the slab. Wood sleepers were attached to the tops of the beams, or I's, and a plywood subfloor spans the sleepers covered by asphalt roof covering and a bed of sand. The flagstone floor is placed over the sand bed.

Concrete beams supporting a cantilever must be reinforced with steel rods because concrete has a very low tensile strength, and the weight of the slab bending the beams downward while supported only at one end causes significant tensional forces in the top side of the beams and slab.

Figure Fallingwater Cantilevered Balcony

(Image from www.amnesta.net/other/fallingwater)

For the cantilevered floors Wright and his team used integral upside-down beams with the flat slab on the bottom forming the ceiling of the space below. The contractor, Walter Hall, who was also an engineer, produced independent computations and argued for increasing the reinforcement in the first floor's slab. Wright rejected the contractor.

The decision was made to double the amount of reinforcement. The additional steel not only added weight to the slab but was set so close together that the concrete often could not properly fill in between the steel, which weakened the slab. Also the contractor didn't build in a slight upward incline in the formwork for the cantilever to compensate for the settling and deflection of the cantilever once the concrete had cured and the formwork was removed. As a result, the cantilever developed a noticeable sag.

Even with the added reinforcement, the living room dipped nearly 2 inches when workers pulled away the cantilever's temporary timber supports in 1936. Cracks quickly opened in the parapet walls of the master bedroom terrace directly above.

It is thought that they could have underestimated the load of the master bedroom terrace on the cantilever below, which could also be a significant reason for the failure of the balconies.

During construction, four thick steel mullions were inserted between the windows in the living room to help hold up the terrace, but these vertical supports also transferred the terrace's massive 50 ton load to the outer ends of the cantilever beams, leaving them dangerously overstressed.

Figure Balcony Post-tensioning

(Image from www.post-gazette.com/lifestyle/)

Fallingwater's structural system includes a series of bold reinforced concrete cantilevered balconies; however, the house had problems from the beginning. Pronounced sagging of the concrete cantilevers was noticed as soon as formwork was removed at the construction stage.

Previous efforts failed to permanently address excessive deflections of the cantilever and repair the cracks. After a thorough design review, the owner and engineer selected an external post-tensioning solution for its durability, aesthetics and structural unobtrusiveness.

The house was given to the Western Pennsylvania Conservancy in 1963. They continued to monitor the progressing deflection problem, and by 1994 the deflection ranged from 4 to 7 inches in the most extreme conditions.

The Western Pennsylvania Conservancy conducted an intensive program to preserve and restore Fallingwater. The study indicated that the original structural design and plan preparation had been rushed and the cantilevers had significantly inadequate reinforcement. The primary purpose of the rehabilitation project was to halt the deflection of the cantilevered terraces, which has caused concern since the house was built, in 1936.

The results of the study indicated that the master terrace could not function as an independent cantilever, and that it was transferring its load to the living room level. Furthermore, the study predicted the ultimate failure of the living room cantilevers if no remedial action was taken. The study recommended that the structure be repaired. In order to stop the deflections and provide a margin of safety, Western Pennsylvania Conservancy installed temporary shoring in 1997. A 1999 peer review approved the proposed plan for the structural repair, which will strengthen the living room using post-tensioning, and waterproofing the building.

Structural failure in reinforced concrete can be progressive, and it typically begins with cracking followed by excessive deflection. This is precisely what occurred at Fallingwater while construction was still underway.

Many design issues contributed to the structural inadequacies. First of all, no reverse camber was cast into the cantilevers. Reverse camber involves raising the forms upward to counteract the natural "creep" of the concrete downward. For example, if a cantilever is curved upwards when it is formed, when the concrete naturally deflects due to creep, it will, by design, ideally move downward to a level position.

Proposed plans to stabilize the cantilevers

RSA proposed a technique called post-tensioning to halt further deflections of the living room and master bedroom cantilevers. No effort was made during stabilization to raise the cantilevers back to their intended position. The reinforcement remained in the floor depth with minimum disturbance to the existing concrete fabric.

In order to post-tension Fallingwater, high strength steel cables will be placed along each side of the 3 major north/south concrete beams in the living room. The strands follow a bent path where they start and end at points near the bottom of the existing concrete beams and rise up to the top of the existing beams at the middle. This path locates the strands at the top and middle of the beam where they are most needed for strength. The geometry of the strands' placement is carefully calculated to ensure that the large post-tensioning forces will always oppose the inner stresses now present in the beams with anchor blocks.

The strands extended outside the face of the south living room parapet and a post-tensioning jack will be installed. The jack will be used to tension each group of strands on either side of the beam until the required 200,000 pound or 100 ton force is reached, which places 400,000 pounds or 200 tons of tension on each beam. The jack will then be removed and the strands will be cut off inside the parapet. The concrete will be patched, leaving little trace of the work performed. In order to achieve proper transfer of forces from the living room to the master terrace, the concrete joist directly above the four steel "T" mullions will be reinforced by bolting a steel channel to each side.

For the first time since original construction, the stone floor was removed to expose the inner workings of Wright's design. To preserve existing building elements and minimize incidental damage, the team gently chipped and drilled openings, while keeping construction debris from falling into stream.

Construction plans called for structural strengthening of three support girders spanning in the north-south direction with multistrand post-tensioning tendons consisting of multiple 0.5" diameter strands. Thirteen strand tendons were placed on each side of two girders. One 10-strand tendon was placed on the western side of the third girder (access on the eastern side of this girder was not available). Eight monostrand post-tensioning tendons, 0.6" diameter, were slated for east-west direction.

The monostrand tendons were stressed in the east-west direction and then the multistrand tendons were stressed in the north-south direction and grouted with a high quality, low-bleed cementitious grout mixture.

Figure Post-tensioning Illustration

(Image from www.waterhistory.org/)

Post-tensioning is a process which involves securing a length of steel cable to both ends of a section of concrete, then pulling the cable taut. The cable acts like a giant clamp, compressing the concrete so it resists further bending.

Post-tensioning is a method of reinforcing concrete with high-strength strands (tendons), anchored at each end of a concrete beam and stretched, or tensioned, after the concrete has reached a specific strength. Fallingwater was repaired with an "unbounded" post-tension design where the tendons are coated with corrosion-inhibiting grease and encased in an extruded plastic protective sheathing. The anchorage at each end consists of an iron casting and a conical, two-piece wedge which grips the tendon. The tendons are stressed with hydraulic jacks on the "live end", and the excess tendon end is removed and the pocket is grouted. Post-tensioned construction has been used throughout the world since the 1950s, and the technology was not available to Wright when Fallingwater was constructed.

The long-term creep in the concrete had left the cantilevers in a permanent deflected condition that could not be remedied by the post-tensioning. The post-tensioning did not raise the cantilevers back up to level but raised them by about ¾ inch. Raising them any further would have opened new cracks in the long-settled structure.

RSA's investigation used impulse radar and ultrasonic pulse testing, they were able to confirm the size and location of the reinforcing steel and found the concrete in generally good condition. The study also discovered that the master terrace could not function as an independently supported cantilever with its existing reinforcement. They concluded that the problems were due to flaws with design and detailing of the reinforcement, and recommended that the structure be repaired.

Chapter 3: Aims and Objectives

With a focused period of research in preparation for this dissertation drawn both from texts and projects within the field of Architecture and the Built Environment, The final proposal identified the following key research questions:

What caused the failure of the Concrete balconies at Fallingwater and How did Post-tensioning solve the problem?

Objectives

The following sub-questions were subsequently formulated to shape the analysis:

What exactly does Post-tensioning consist of?

Was post-tensioning a viable solution for Fallingwater?

What are the benefits of Post-tensioning?

What extent did post-tensioning make to the design features and construction of the building?

With the above questions arising from my initial research into the failure of the cantilevered balconies at Fallingwater, it was clear that more research was required and that the aims of my dissertation would include the following:

Aims

To identify the cause of the failures which occurred with the cantilevered concrete balconies at Fallingwater.

To highlight the technique of Post-tensioning as a solution to such problems.

To establish if post-tensioning was a viable solution for Fallingwater.

To evaluate the benefits of the technology and to what extent it made an impact on the design features, construction and subsequent use of the building.

Why this study is necessary

In the modern Construction industry there is an increase in the use of concrete throughout buildings.

The concrete must be reinforced correctly or failure will occur. Once failure begins, solution methods such as post-tensioning will have to be implemented.

As shown Reinforced concrete has been used not just on modern structures but also on older buildings too. To be able to carry out work such as renovations etc. to existing buildings it is therefore important to have a knowledge about such failures and the solutions available.

Chapter 4: Post-tensioning

Post-tensioning

For more than 60 years, four massive concrete cantilever beams projecting 15 feet into thin air help up Fallingwater's cantilevered terraces and living room. But the unrelenting loads were so great that, over time, the cantilevers deflected, cracking the beams and causing the steel reinforcing bars embedded in the concrete to stretch. Structural engineers concluded that unless action was taken, the rebar would continue stretching to the point where the cantilevers might collapse.

The solution was "post-tensioning," a technique that has been used to shore up sagging concrete bridges. In essence, the process involves securing a length of steel cable to both ends of a section of concrete, then pulling the cable taut. The cable acts like a giant clamp, compressing the concrete so it resists further bending.

Figure Post-tensioning - Fallingwater Illustration

(Image from www.waterhistory.org/)

Preparing the floor

Before repairs could even begin, temporary steel bracing had to be placed in the streambed to support the ends of the beams. Then, in November 2001, workers carefully numbered and removed each of the 600 waxed flagstones on the living room floor and pulled up the redwood subfloor, revealing the underlying gridwork of concrete beams and joists.

Next, the post-tensioning contractor, VSL, threaded a bundle of thirteen ½-inch steel cables alongside each drooping cantilever beam. One end of each bundle was secured in a concrete anchor block attached to both the beam and the house's foundation pier. At the other, unsupported end of the beam, the cables passed through another anchor block and exited through the terrace wall. Near the middle, the cables passed over a concrete deviator block, anchored to the beam above the furthest end of the pier. In theory at least, pulling on the exposed ends of the cables with sufficient force would raise the beam and transfer its stresses back to the foundation.

Over a three-day period, the VSL crew carefully put theory into practice. Standing on scaffolding set above the creek, they attached powerful hydraulic jacks to the exposed cable ends and slowly pulled. The old concrete held, and the beam ends rose slightly, just as planned. Each of the now ultrataut cable strands was then fixed to a metal cone and grouted into the anchor blocks on the beam ends. VSL used this same technique to stabilize some of the concrete joists, running perpendicular to the beams, that support the living room terraces.

The post-tensioning did not raise the cantilevers back up to level, they were only brought up about ¾ inch. Raising them any higher would have opened new cracks in the long-settled structure.

Figure Hydraulic Jack

(Image from )

A hydraulic jack, attached where the tensioning cables exited the building, gradually tightened the cables until they exerted a static pull of 195 tons on each side of the beam, lifting it about 3/4 inch. Then the cable ends were embedded in the anchor blocks and trimmed, and the holes were patched.

Chapter 5: Findings

Conclusion:

Bibliography

References

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