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Post-tensioning is a method of strengthening concrete with high-strength steel strands, usually referred to as tendons. Post-tensioning applications include office and apartment buildings, multi-storey car parks and bridges. In many cases, post-tensioning allows construction that would otherwise be impossible due to either site restrictions or architectural requirements.
Four massive concrete cantilever beams projecting 15 feet into thin air helped hold up Fallingwater's cantilevered terraces and living room. But these continuous loads were so great that, over time these cantilever beams deflected, cracking the beams and resulting in the steel reinforcing bars embedded in the concrete to stretch. Structural engineers concluded that unless action was taken, the rebar would continue stretching till eventually the cantilevers would collapse.
The solution was "post-tensioning," a technique that has been used to rectify sagging concrete bridges. 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 get underway, temporary steel bracing had to be placed in the streambed of the waterfall to support the ends of the beams. Then, in November 2001, construction workers carefully numbered and removed each of the 600 stone flags on the living room floor and pulled up the redwood subfloor, revealing the grid work of concrete beams and joists that lay beneath.
The post-tensioning contractor, VSL, proceeded to thread a bundle of thirteen ½-inch steel cables along each side of the drooping cantilever beams. One end of each bundle was secured in a concrete anchor block that was fixed 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 continuing through the terrace wall. Near the middle, the cables passed over a concrete 'deviator' block, which was anchored to the beam above the furthest end. By pulling on the exposed ends of the cables with sufficient force, it would raise the beam and transfer its stresses back to the foundation.
Figure Post-tensioning Equipment Illustration
(Image from www. tecpresa.com)
Over a three-day period, the VSL construction team carefully put the above theory into practice. Standing on scaffolding positioned above the creek, they connected powerful hydraulic jacks to the exposed cable ends and slowly pulled. The old concrete held, and the beam ends rose slightly as planned. Each of the now extra taut cable strands were then fixed to a metal cone and grouted into the anchor blocks which were on the beam ends. VSL used this same technique to reinforce a number of the concrete joists, which ran perpendicular to the beams that supported the living room terraces.
Figure Hydraulic Jack
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.
Benefits of Post-tensioning
Since the introduction of post-tensioning to buildings, a great deal of experience has been gained as to which type of building has floors most suited to this method of construction. Current architecture continues to place emphasis on the necessity of providing large uninterrupted floor space, flexibility of internal layout, versatility of use and freedom of movement. All of these are facilitated by the use of post-tensioning in the construction of concrete floor slabs, giving large clear spans, fewer columns and supports, and reduced floor thickness.
Post-tensioned concrete slabs in buildings have many advantages over reinforced concrete slabs and other structural systems for both single and multi-level structures. Some of the main advantages are described below.
Longer spans - Longer spans can be used reducing the number of columns. This results in larger, column free floor areas which greatly increase the flexibility of use for the structure and can result in higher rental returns.
Overall structural cost - The total cost of materials, labour and formwork required to construct a floor is reduced for spans greater than 7 metres, thereby providing superior economy.
Reduced floor to floor height - For the same imposed load, thinner slabs can be used. The reduced section depths allow minimum building height with resultant savings in facade costs. Alternatively, for taller buildings it can allow more floors to be constructed within the original building envelope.
Deflection free slabs - Undesirable deflections under service loads can be virtually eliminated.
Waterproof slabs - Post-tensioned slabs can be designed to be designed to be crack free and therefore waterproof slabs are possible. Achievement of this objective depends upon careful design, detailing and construction. The choice of concrete mix and curing methods along with quality workmanship also play a key role.
Early formwork stripping - The earlier stripping of formwork and propping requirements enable faster construction cycles and quick re-use of formwork, resulting in a more economical technology.
Materials handling - The reduced material quantities in concrete and reinforcement greatly benefit on-site carnage requirements. The strength of post-tensioning strand is approximately 4 times that of conventional reinforcement. Therefore the total weight of reinforcing material is greatly reduced.
Column and footing design - The reduced floor dead loads may be utilised in more economical design of the reinforced concrete columns and footings. In multi-storey buildings, reduced column sizes may increase the floor net lettable area.
There are post-tensioning applications in almost all facets of construction. In building construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender, dramatic elements. Thinner slabs mean less concrete is required. In addition, it means a lower overall building height for the same floor-to-floor height. Post-tensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors. This reduces the foundation load and can be a major advantage in seismic areas. A lower building height can also translate to considerable savings in mechanical systems and facade costs. Another advantage of post-tensioning is that beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other. Structurally, this is much more efficient than having a beam that just goes from one column to the next.
Post-tensioning is a useful system for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations. Post-tensioned systems can be either stand-alone structures or one or more floors such a multi-storey car park, an office or residential buildings such as apartment blocks. Post-tensioning allows bridges to be built to very demanding geometry requirements, including complex curves, variable super elevation and significant grade changes. Post-tensioning also allows extremely long span bridges to be constructed without the use of temporary intermediate supports. This minimizes the impact on the environment and avoids disruption to water or road traffic below. In stadiums, post-tensioning allows long clear spans and very creative architecture.
An unbounded tendon is one in which the prestressing steel is not actually bonded to the concrete that surrounds it except at the anchorages. The most common unbounded systems are monostrand (single strand) tendons, which are used in slabs and beams for buildings. A monostrand tendon consists of a seven-wire strand that is coated with a corrosion-inhibiting grease and encased in an extruded plastic protective sheathing. The anchorage consists of an iron casting and a conical, two-piece wedge which grips the strand.
In bonded systems, two or more strands are inserted into a metal or plastic duct that is embedded in the concrete. The strands are stressed with a large, multi-strand jack and anchored in a common anchorage device. The duct is then filled with a cementitious grout that provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct. Bonded systems are more commonly used in bridges, both in the superstructure (the roadway) and in cable-stayed bridges, the cable-stays. In buildings, they are typically only used in heavily loaded beams such as transfer girders and landscaped plaza decks where the large number of strands required makes them more economical.
Rock and soil anchors are also bonded systems but the construction sequence is somewhat different. Typically, a cased hole is drilled into the side of the excavation, the hillside or the tunnel wall. A tendon is inserted into the casing and then the casing is grouted. Once the grout has reached sufficient strength, the tendon is stressed. In slope and tunnel wall stabilization, the anchors hold loose soil and rock together; in excavations they hold the wood lagging and steel piles in place.
There are several important factors in a post-tensioning system. In unbounded construction, the plastic sheathing acts as a bond breaker between the concrete and the prestressing strands. It also provides protection against damage by mechanical handling and serves as a barrier that prevents moisture and chemicals from reaching the strand. The strand coating material reduces friction between the strand and the sheathing and provides additional corrosion protection.
Anchorages are another critical element, particularly in unbounded systems. After the concrete has cured and obtained the necessary strength, the wedges are inserted inside the anchor casting and the strand is stressed. When the jack releases the strand, the strand retracts slightly and pulls the wedges into the anchor. This creates a tight lock on the strand. The wedges thus maintain the applied force in the tendon and transfer it to the surrounding concrete. In corrosive environments, the anchorages and exposed strand tails are usually covered with a housing and cap for added protection.
In building and slab-on-ground construction, unbounded tendons are typically prefabricated at a plant and delivered to the construction site, ready to install. The tendons are laid out in the forms in accordance with installation drawings that indicate how they are to be spaced, what their profile (height above the form) should be, and where they are to be stressed.
After the concrete is placed and has reached its required strength, usually between 3000 and 3500 psi (pounds per square inch), the tendons are stressed and anchored. The tendons, like rubber bands, want to return to their original length but are prevented from doing so by the anchorages. The fact the tendons are kept in a permanently stressed (elongated) state causes a compressive force to act on the concrete. The compression that results from the post-tensioning counteracts the tensile forces created by subsequent applied loading (cars, people, and the weight of the beam itself when the shoring is removed). This significantly increases the load-carrying capacity of the concrete.
Since post-tensioned concrete is cast in place at the job site, there is almost no limit to the shapes that can be formed. Curved facades, arches and complicated slab edge layouts are often a trademark of post-tensioned concrete structures. Post-tensioning has been used to an advantage in a number of aesthetically designed bridges. For example, Boyne Bridge, M1 Motorway, Dublin.
Figure Boyne Bridge M1 Motorway Dublin
(Image from www.emissionzero.ie/img/bridge.jpg)
The Boyne Bridge is a relatively new motorway crossing just north of Dublin. It crosses over the River Boyne and as the site has special historic interest, working in the river was prohibited. The client's design was to use a post-tensioning system which supports the main deck by twenty eight fanned cables that are connected to the concrete frame pylon which is also anchored by the steel cables to the abutment behind. This method also allowed the deck to be assembled behind the abutment and then launched into final position using the steel strand jacks.
Chapter 5: Discussion and Analysis
The amount of post-tensioning strand sold has almost doubled in the last ten years and the post-tensioning industry is continuing to grow rapidly. To ensure quality construction, the Post-Tensioning Institute (PTI) has implemented both a Plant Certification Program and a Field Personnel Certification Training Course. By specifying that the plant and the installers be PTI certified engineers can ensure the level of quality that the owner will expect. PTI also publishes technical documents and reference manuals covering various aspects of post-tensioned design and construction.
The relative economics of post-tensioning versus other forms of construction vary according to the individual requirements of each case. In any basic comparison between post-tensioned and reinforced concrete one must consider the relative quantities of materials including formwork, concrete, reinforcement and post-tensioning. Other factors such as speed of construction, foundation costs, etc., must also be given consideration.
There is not always sufficient time or budget to carry out comparative feasibility studies for all structural solutions. There are however, some useful guidelines which can be employed when considering post-tensioned alternatives. As can be seen from figure 3 below, post-tensioned should be considered as a possible economic alternative for most structures when spans exceed 7 metres.
Figure Cost comparison - Reinforced vs. Post-tensioned flat slab
The graph illustrates two main points. Firstly, how with increasing span the difference in cost between reinforced and post-tensioned concrete flat slabs also increases. Secondly, using an index of one for a 7m span how the cost will vary for other spans. For example, a post-tensioned 10m span will cost approximately 20% more than a post-tensioned 7m span.
Speed of Construction
Economics and construction speed are heavily linked in today's building construction environment. The speed of construction of a multi-storey building is foremost in achieving economic building construction.
Chapter 6: Conclusion
This dissertation has only touch the surface of Post-tensioning as a method of construction in the modern industry. I hope that through this study I have gave an insight and deeper understanding of how Post-tensioning has and can be used within the construction industry. From rectifying existing problems such as those that were present with the balconies at Fallingwater to modern day bridge construction.
Through my study of post-tensioning I have found that Post-tensioning appears to be a very viable system which can be used for many different applications such as car parks and bridges. Although I do have some doubts of the use of post-tensioning systems that are situated near any coast lines which gain the harshness of the sea air, I still feel that it was a viable solution to stabilize the balconies at Fallingwater. I am not completely convinced that there are enough protective measures incorporated within the design of the post-tensioning system, and that over a period of time the cables etc would not corrode and eventually result in the post-tensioning system's failure.
As post-tensioned systems are exposed to very severe environments at times, this can have an effect on their long-term durability. In order to gain a more detailed assessment to improve the durability design of post-tensioned systems under certain exposure conditions, more research studies should be carried out. Research should also be initiated to develop better corrosion protection and to establish how to protect the post-tensioning systems from corrosion damage.
Even with this, Post-tensioning has many uses in the construction industry and by identifying any durability concerns and developing the existing technology further; we stand to gain great benefit to the construction industry across the world and would allow this technology to be incorporated in building design with greater effects.
There is a definite trend towards large spans in buildings due to the fact that there is now more emphasis on providing large uninterrupted floor space which can result in higher rental returns. Post-tensioning is an economical way of achieving these larger spans. For spans of 7.5 metres and over, post-tensioning will certainly be economic and, as the spans increase, so do the savings.
Although my research has uncovered quite a number of benefits to this technology, I find myself questioning if this system has any future at all in Northern Ireland. It also appears to have very limited use in the south of Ireland due to the fact that Ireland as a whole does not have enough "Big" cities such as those in America and Australia where such technology is readily used. Also Post-tensioning only becomes economical when achieving larger spans of 7.5 metres and over. As a result of this it has very limited use within the construction Industry here and I therefore don't see it as a realistic method of construction here in Ireland.
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