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Post Tensioning


Designers and engineers typically will choose post-tensioned concrete because of its advantages to other materials, which include low material costs and availability, flexible column spacing and structural depth, and demanding construction schedules. The post-tensioning system is also an efficient choice when issues regarding reduced sound and vibration, as well as future flexibility arise, as seen in residential, office and mixed-use facilities. However, the choice to use post-tensioned concrete is more commonly used when these building issues become a concern to the designer of the structure.                                                                                                                            

During the construction phase, post-tensioning allows longer 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 allows 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 exterior costs.

There are post-tensioning applications in almost all applications of construction. Areas where there are expansive clays or soils with low bearing capacity, post-tensioned slabs on-ground and mat foundations reduce problems with cracking and settlement. Post-tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations. In stadiums, post-tensioning allows long clear spans and very creative architecture. Post-tensioning permits extremely long-spans 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. Post-tensioning also allows bridges to be built to very demanding geometric requirements, including complex curves, variable elevations and considerable changes in grade.

Post-tensioned rock and soil anchors are used in tunneling, slope stabilization and as tie-backs for excavations. Post-tensioning is also the preferred reinforcing system when it comes to construction of water tight structures such as water-tanks.

With the high cost of imported structural steel, along with and the increased demand for structural steel in Asia, steel is at a premium in today's building market. The high cost of steel is also partially due to a decrease in the production of American steel. Due to the ever changing market price of steel, concrete is becoming an increasingly more efficient building material.

With consistent testing and development,  by engineers, they are breaking new ground regarding concrete building systems. Such systems as post-tensioned ductile frames and flat-plate column slab joint systems, allow building designs to meet demanding building codes, while providing a flexible, safe building. Post-tensioned concrete building systems are addressing many of the design constraints presented by today's building designs.

The depth of the structural members depends on the column spacing and on the weight of the applied loads. Post-tensioned concrete is typically used for moderate to long spans with moderate floor loads, as seen in parking and residential buildings. In these situations, post-tensioned concrete framing may result in shallower depths than steel framing.

Construction schedule is also a major driving force behind a project. Post-tensioned concrete buildings can normally be constructed  very quickly, due to the quick strengthening properties of concrete. Form work in relatively simple successive floors can be built within a week of concrete placement. In conventional reinforced concrete, construction of successive floors must often be delayed until the concrete has gained enough strength to support its own weight, often 14 to 28 days. Steel buildings, well known for their fast speed of erection are currently penalized by extremely long delays in steel delivery. When all factors are considered, post-tensioned concrete can yield the quickest overall construction schedule.