Buildings
Post-tensioned concrete has proven to be a preferred method of construction for commercial and office buildings, residential apartments, high-rise condominiums, parking structures, and mixed-use facilities such as hotels and casinos. Developers and owners who select post-tensioning benefit from longer, thinner slabs, which results in greater design flexibility as the number of columns is reduced. Further, post-tensioning requires less reinforcing steel to achieve the same strength as well as smaller shear walls and column sizes. This results in more durable, lighter structures with long, clear spans.
The benefits don’t stop there. High early-strength concrete allows for faster floor construction cycles and the use of standard design details for post-tensioned elements, minimum congestion of prestressed and non-prestressed reinforcement, and earlier stripping of formwork after tendon stressing can also significantly reduce the floor construction cycle. Greater span-to-depth ratios are allowed for post-tensioned members as compared to non-prestressed members. This results in a lighter structure and a reduction in floor-to-floor height while maintaining the required headroom.
Post-tensioning also provides superior performance of diaphragm action at building irregularities, resisting tensile forces resulting from separation of “wings” at reentrant building corners. Further, post-tensioned slabs span farther than non-prestressed slabs, allowing for wider column spacing and fewer columns. Cast-in-place post-tensioned concrete also greatly reduces the floor-to-floor height when compared to a structural steel option, which also results in significant savings in the façade, HVAC, electrical, plumbing, and vertical transportation systems.
For today’s commercial building owner, post-tensioning offers the following benefits:
• Significant reduction in the amount of concrete and reinforcing steel required.
· Thinner structural members as compared to non-prestressed concrete, resulting in lower overall building heights and reduced foundation loads.
· Aesthetically pleasing structures that harness the benefits of cast-in-place structures with curved geometries, and longer, slender members with large spaces between supports.
• Superior structural integrity as compared to precast concrete construction because of continuous framing and tendon continuity.
· Monolithic connections between slabs, beams, and columns that can eliminate troublesome joints between elements.
· Profiled tendons that result in balanced gravity loads (typically a portion of dead load only), significantly reducing total deflection.
· Better crack control, which results from permanent compressive forces applied to the structure during prestressing.
• Reduction in overall building mass, which is important in zones of high seismicity.
As compared to steel, non-prestressed concrete and precast construction offer faster floor construction cycles, lower floor weight, lower floor-to-floor height, larger spans between columns and reduced foundations.
Floor Framing Systems
The floor system is typically the costliest structural element in any given building and for low-rise buildings with few floors, the floor system represents the majority of the structural cost. As the number of floors increase, the cost of foundations, vertical elements carrying gravity loads, and lateral load-resisting systems become a larger percentage of the total structural cost. However, even in tall buildings, the cost of the floor system often dominates the economics for the structure. With post-tensioned concrete, commonly used floor systems include flat plates, flat slabs, beam and slab systems, waffle slabs, and joist and beam systems. Post-tensioned tendons are used as primary tensile reinforcement in slabs, beams, girders and joists.
Post-tensioned tendons in floor systems can be either unbonded or bonded. In the United States, the majority of post-tensioning used in buildings is unbonded. The wide usage of unbonded post-tensioned tendons has been influenced by economics, performance, and extensive laboratory testing programs performed at various universities for more than 40 years.
