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Post-tensioning, which is a form of prestressing, has several advantages over standard reinforcing steel (rebars):
Common Applications for PT
Post-tensioning, or PT, has become increasingly popular over the past 30 years or so as the technology has been perfected. At one time there were problems with corrosion of the cables, especially in deicing-salt-laden parking structures, but better materials and construction methods (plus good training and certification programs) have eliminated most problems.
External post-tensioning is a good method for strengthening or retrofitting concrete structures. www.vsl.net
The applications for post-tensioning are almost endless, but what holds it back is the lack of experience among designers and builders. In an article in Structure magazine (PDF), Gerard Feldman noted that "Most engineers have only a cursory overview of post tensioning in college...this unfamiliarity creates a bias towards conventional reinforced concrete structures."
Some of the more common applications are:
Water tanks can be post-tensioned. DYK in El Cajon, CA
Post-tensioned concrete construction in buildings has seen extraordinary growth in North America since it was first introduced in the &#;s. By some estimates, more than 60,000 post-tensioned concrete buildings have been built in the United States covering upwards of 6 billion ft2 of floor area.1,2 Prestressed concrete and, more specifically, post-tensioned concrete is perhaps the most significant development in the design and construction of concrete buildings over the past century.
Most structural engineers and architects are familiar with the commonly known benefits of post-tensioned concrete, such as allowing for thinner members, longer spans, reduced deflections, and reduced costs. These benefits are well established, but did you know that post-tensioned concrete results in a more sustainable design solution when compared to other types of construction? This article looks at 6 of the &#;unexpected&#; benefits of using post-tensioning for building construction.
Most of us know that post-tensioning saves construction costs in terms of concrete and reinforcing steel quantities when compared to conventional reinforced concrete construction. It may surprise some to learn of the cascading cost savings that occur in many components of the total building cost. Below is a listing of building components that will experience cost savings when compared to any other building construction type:
Since PT floors are thinner, there is generally an overall savings of 20% in the dead load of the building compared to conventional concrete. Due to this decrease in dead load, the amount of reinforcing steel is reduced in columns and foundations.
When seismic effects govern the lateral load resisting systems, the cost of shear walls and foundations can be reduced by 20% or more due to the reduced structural mass.
Since PT floors are thinner than conventional concrete floors and significantly thinner than composite concrete over metal deck systems, floor-to-floor heights are reduced. The typical structural depth for a steel office building floor is 30 inches. Similarly, the typical structural depth for a conventional concrete office building floor is 24 &#; 26 inches. The structural depth for a post-tensioned concrete office floor is 18 inches or less. This saves more than 6 inches per floor compared to a conventional concrete building, and over 1 foot per floor compared to a steel building. Therefore, the costs for all building systems tied to building height are reduced proportional to the reduction in building height. For example, the cost of the building façade will be reduced by 8 percent compared to a structural steel office building. These savings also apply to MEP risers, elevators, and more.
Another benefit of post-tensioning is speedy construction. The reduction in materials used and the simplicity of the formwork ensure that construction of a floor can be completed faster. Post-tensioning saves you time in these areas:
One of the most time-consuming activities in construction is placing steel, and since PT designs reduce the quantity of steel needed, slabs will be ready to pour at an earlier date. Unlike conventional concrete slabs, which requires approximately 5 to 7 lb./ft2 of steel, post-tensioned flat slabs only need about 2 to 3 lb./ft2 of steel.
PT slabs are ready for stressing once the concrete reaches a strength of psi. It is quite common to stress the tendons 2 to 3 days after the pour. Once stressing is complete, formwork can be stripped immediately and moved to a subsequent pour.
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Thinner slabs and beams require fewer levels of reshoring below the formwork. This allows follow-on trades such as construction of interior partitions, façade work, and MEP to start their work earlier. This significantly reduces the overall project schedule.
Construction of lighter and longer floor structures can result in increased incidences of vibration and deflection. However, post-tensioning produces two key characteristics in the concrete that significantly reduce vibration and sag. First, the profile of the cables within the concrete produce a load balancing effect. Load balancing from post-tensioning effectively carries a significant portion of the slab weight so that the concrete slab does not feel its own weight. This results in reduced deflections or sag. Second, the slabs are designed to remain uncracked under normal loading. When slabs remain uncracked, they have a much higher stiffness. In technical terms, we are able to use the &#;gross section properties&#; in the calculation of deflection and vibrations. For non-prestressed slabs, we must use the &#;cracked transformed section properties&#; in these calculations.
Conventional concrete slabs require a specific level of thickness to maintain their strength and serviceability. However, with post-tensioned concrete, you can build thinner and longer slabs without compromising structural serviceability and strength. The fewer columns give greater flexibility in floor plans and layouts, especially in office and residential buildings.
PT allows you to design longer unsupported spans that are strong with thinner floors. This makes post-tensioning an ideal technique for creating concrete slabs for expansive structures such as parking garages, auditoriums, roofs, bridges, and customized architectural designs.
Since post-tensioned concrete can be profiled for longer spans, curved floors, and irregular grids, it is ideal for creating extended cantilevered slabs for exquisitely designed balconies and other futuristic architectural features. Post-tensioned slabs can create sleek and exceptional designs since they give way to dynamic and alluring contours. They allow architects and engineers to express creativity in building plans without compromising quality, strength, and stability. In simple terms, PT concrete elements deliver outstanding and efficient architectural features, offering infinite possibilities for structural designs. PT flat plates with unbonded tendons allow for random placement of columns supporting the floor slab. This provides tremendous flexibility for architectural designs.
Post-tensioning can be used in mat foundations offering many benefits to the project. Costs will be reduced due to a reduction in the depth of the foundation system. This will offer savings in materials such as concrete and rebar. Additionally, costs for foundation excavation and support of excavation will be reduced as well.
The prestressing forces and cable profiles in mat foundations provide the ability to actively transfer loads within the foundation system. This gives post-tensioned foundations the unique ability to span over subsurface weak points such as utilities, tunnels, stormwater management vaults, etc. Post-tensioning can also be specifically designed to balance loading going to supporting piles or caissons by moving loads from one group of piles to another.
Expansive clay soils or soft soils can wreak havoc on structures that are built using conventional concrete. However, post-tensioned concrete boasts high-end structural advantages, such as high stability and minimum differential settlement, making it ideal for building foundations with soft or expansive clay soils.
Using post-tensioned concrete significantly reduces concrete and steel quantities in the structure when compared to conventional concrete or steel with composite floor slab construction. This efficiency has a cascading benefit throughout the structure by reducing the overall self-weight of the floors transferred to columns, shear walls, and foundations. Reduction in concrete materials reduce cement consumption which is a contributor to greenhouse gas emissions. As stated earlier, there are reductions in other building components as well, including building façade, MEP, mechanical equipment, and elevators. Thus, the environmental impact is minimized throughout the life cycle of the building through diminished material consumption, faster construction, fewer transportation trips, and fewer disruptions to the local neighborhood during construction. Moreover, due to reduced overall building height, there is a reduction in the volume of interior conditioned space. And since concrete provides superior thermal mass, the costs for heating and cooling will be diminished throughout the life of the building.
Post-tensioned concrete construction is a major contributor to the overall sustainability of the building. Architects, engineers, and owners will reap tremendous long-term benefits in sustainability and resiliency through the use of post-tensioned concrete.
If you need a post-tensioning expert or have any questions as you prepare for your next building project, talk to us.
1. Bondy, K.B., &#;The State of Post-Tensioned Concrete Education&#;, Concrete International, October , pp 32-36
2. Post-Tensioning Institute, &#;Tonnage Report&#;, PTI,
If you need a post-tensioning expert or have any questions as you prepare for your next building project, consult an expert!
Photo credit &#;The Surf Club Four Seasons Residences&#; to miamiluxuryhomes.com
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