CONSTRUCTION / DESIGN
Post-tensioned concrete has been successfully used on a number of building, but choosing a structural system can be a arduous process. Engineers must research all of the design issues, calculate the loadings, and conclude which structural system is appropriate for its intended use.
The post-tensioned reinforcing system consists of high strength steel strands or bars that are typically used in horizontal applications like foundations, slabs, beams, and bridges, but there are also bars that are used in vertical applications like walls and columns. A typical steel strand used for post-tensioning has a tensile strength of 270,000 psi. In contrast, a typical non-pre-stressed piece of rebar only has a tensile strength of roughly 60,000 psi . Strands normally have a diameter of ½ in., and are stressed to 33,000 pounds using a hydraulic jack.
Depending on the type of mix, the properties of aggregate, and the quality of the curing, compressive strengths of concrete can be obtained up to 20,000 psi or more. Commercial production of concrete with ordinary aggregate is usually in the range of 4,000 to 12,000 psi, with the most common concrete strengths being in the 6,000 psi level.
The compressive strength ƒc’ is based on standard 6in. by 12in. cylinders cured under standard laboratory conditions and tested at a specified rate of loading at 28 days of age. The standard specifications used in the United States are usually taken from ASTM C-39.
The pre-stressing steel is housed in a sheathing or duct to allow it move as the tensioning force is applied after the concrete cures. The steel stretches as it is tensioned, and it is locked into place using an anchoring component that forms a mechanical connection and keeps the force in the strand for the life of the structure.
In building and slab-on-ground construction, unbonded 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 the spacing, the final profile, and where the stresses are to be applied. After the concrete is placed and has reached its required strength, usually between 3000 and 4500 psi, the tendons are stressed and anchored. The tendons 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 state causes a compressive force to act on the concrete. This pre-compression which results from post-tensioning counterbalances the tensile forces created by subsequent applied loading.
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 advantage in a number of very aesthetically designed bridges.
For many buildings, such as residential, office and mixed-use, sound and vibration issues are common. In general, the ability of a structure to attenuate or deaden sound and floor vibrations is proportional to the mass of the structure. The massive state of concrete buildings allows for a more effective deadening of noise and vibrations. Therefore, concrete building systems generally are the more favorable system to address vibration and acoustical issues.
Another building issue to examine is the frequency of remodel and tenant improvements. When a building is remodeled, walls are moved and holes are cut in the floor slab to accommodate revisions to the wiring and plumbing. New adaptations are made to the building for changing lease arrangements or new architectural functions. For these reasons, post-tensioned concrete buildings are more difficult to remodel because the reinforcing steel is embedded in the concrete. This makes it difficult to locate the reinforcing and creates serious, costly problems if the post-tensioning tendons are cut. However, several things can be done to improve the future flexibility of a post-tensioned building. The location of the post-tensioning tendons can be marked on the under side of the slab, future stair block-outs can be added to the building during the original construction and sleeves for future electrical penetrations can be cast into the floor slab.
Post-tensioned parking garages can have economic lives of over 60 years. When designed for durability, actual facility life can be longer. A post-tensioned floor system in a garage is designed to control the entry of harmful moisture and contaminants. With the appropriate weatherproofing, post-tensioned concrete systems also protect the reinforcing steel. Steel frame parking garages typically require continual maintenance due to exposure to nature's elements.