Post-tensioned concrete has been used in building construction since the late 's. Today post-tensioning is becoming increasingly popular for many different use categories including parking structures, high-rise office and residential, hospitality, and podium slabs for mixed-use projects. (To find out more on podiums, check out everything you need to know about building with concrete podium slabs). Whether you are an architect seeking the best way to design a space or an engineer who is relatively new to post-tensioning, this post will shed light on the top 10 false assumptions about post-tensioned structures and uncover the truth.
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Reality: Post-Tensioned structures can be modified just like any other building construction method as long as future modification is considered in the initial design. Tendon layouts can be designed to allow for future cores, knockouts, and openings. A 2-way banded layout can be employed to leave huge areas for future openings. Designers of office buildings, hospitals, and other medical facilities are often reticent to use post-tensioning because of this misconception. This is unfortunate because these buildings would benefit most from the cost efficiencies inherent in post-tensioned structures (see Myth 9). Planning and collaboration between the architect and the engineer will result in a smart, flexible design solution.
Reality: It's important to remember that the overall budget of a structure is more than just the initial construction cost. While upfront costs for a post-tensioned parking structure can be higher than for precast, maintenance and repair costs are considerably lower. Over the life cycle of the building, precast is often the more expensive approach. If durability, minimal maintenance, an open layout, reduced vibration, and better distribution of light in the structure are of the highest priority, post-tensioning is truly the superior building choice.
Reality: Finite element (FE) software is used to provide structural analysis to design buildings with complex geometries. When the user doesn't fully understand the limitations of the program or if they are inexperienced in post-tensioning, the FE program will likely deliver weak or incorrect designs. PT structures can be designed by hand although this is very time-consuming and impractical. Although FE design programs can be a powerful tool, most buildings can be simplified into 2-D strip models that are easily designed using common equivalent frame design software. And in our experience, using 2-D frame software usually results in the more efficient and safe designs.
Reality: This is actually the responsibility of the engineer of record. The American Concrete Institute's building code requirements for structural concrete (ACI 318) describes the anchorage zone as 2 distinct zones: local zone and general zone. ACI 318 requires that the contract documents assign responsibility for the design of these 2 zones. The local zone is the prism immediately surrounding the anchorage device and the design of this zone is strongly influenced by the characteristics of the anchorage device. It follows that the design of the local zone should be performed by the post-tensioning supplier. The general zone encompasses a larger zone around the local zone and is inclusive of the local zone. The design of the general zone is influenced by not only the local zone but also by the overall geometry of the member and the reinforcement that exists in the member. Since the engineer of record has the most complete understanding of the geometry and reinforcement within the member, it follows that the engineer of record should be responsible for the design of the general zone.
Reality: All concrete will experience shortening. It is inherent in the curing process of the material. When concrete cures it undergoes volume change and shrinks. Contractors must control the geometry of the building by setting forms to account for the shrinking. Post-tensioned slabs experience additional elastic shortening due to prestress; however, this additional shortening is a fraction of the overall shortening of the slab. So, while post-tensioned buildings experience more shortening than non-prestressed buildings, the overall increase is insignificant.
Reality: Below grade applications are structures like parking garages that are built below buildings. There is a common misconception that there is not enough room to stress the PT in a below-grade situation because of the confines of the surrounding walls. There is also the fear that the surrounding walls will result in restraint to shortening cracking. There are several strategies that can be used to achieve the benefits of post-tensioning but they require the expertise of a PT specialist. Benefits of PT below-grade include reduced cracking, enhanced durability, thinner slabs, and reduced excavation.
Reality: ACI 318 code states that the size of the drop panel has to be ' of the span in each direction for conventionally reinforced concrete only. This size limitation does not apply to PT slabs. The engineer has tremendous flexibility to size relatively small and efficient drop panels that are effective in shear and flexure.
Reality: Post-Tensioning is not a cure-all that can compensate for sloppy design. Unlike adding more rebar to conventional reinforced concrete, more PT can have detrimental effects. Post-Tensioning is an active force on the concrete. Too much PT can create an overbalancing effect. Overbalancing usually manifests itself during construction causing deflection problems, cracking, or severe blowouts. The repair can be complicated and expensive.
Reality: Okay, this one is true. However, the use of post-tensioning also results in many other cost savings to the owner that are often overlooked. The following is a list of additional cost-savings to the owner:
Overall building height is reduced. This can be used to add more leasable space within the same building height, it can be used to increase ceiling height, or it can be a cost savings.
Because the building height is reduced, there is a reduction in all building services that are tied to building height such as mechanical, electrical, plumbing, elevators, etc.
Because the building height is reduced, there is a savings in the cost of the building façade.
Because the building height is reduced, operating costs such as heating and cooling costs are reduced for the life of the building.
When used in below-grade applications, excavation costs are reduced.
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Formwork and reshoring costs are reduced.
Reality: In a post-tensioning system that's been calibrated correctly, the steel wedges that anchor the strands can be subject to cracking. What may appear to be a defect to the untrained eye is actually a design safety built into the system. Cracks indicate that the wedges are conforming more tightly and getting a better grip on the steel strands as they hold the prestress force in place. It is common to see cracks in 2-part wedges after stressing. It is less common in 3-part wedges.
Now that we've busted through the myths, you can confidently design and build using post-tensioning.
As you begin the planning process for your next building project consider this, and if you have any questions as you prepare, consult an expert!
When shopping for a new home, it's easy to get caught up in the bright and shiny features and add-ons before anything else. New homebuyers should take the time to consider the less exciting (but equally important!) structural elements of their investment. The strong bones of the house will determine its long-term value. A home with stability and longevity will save money in the long run on repairs and play a part in generations of happy memories to follow.
To increase stability and overall lifespan, a post-tension foundation is an ideal option for single-family homes. It not only functions as a safeguard against cracking in highly-unpredictable soil conditions and wet climates, but it also adds to the home's value.
Because of its unparalleled versatility, a post-tension foundation is compatible with any home type from a sprawling ranch layout to a two-story estate. This type of foundation also uses less steel, which reduces costs. While there are minor drawbacks, the post-tension foundation provides critical support and high-quality structural benefits that any homeowner can enjoy.
Simply put, a post-tension foundation is a specific type of slab-on-grade foundation. The concrete is reinforced with threaded steel cables that are tensioned after the concrete cures. To allow the cables to slide more easily after the concrete hardens, the steel is encased in plastic sheathing to prevent the cables (tendons) from touching the hardened concrete.
The plastic sheathing is stretched post-pour, which tightens the slab and applies intense force to the concrete system to create a heavily-compressed state. A slab without reinforcement holds a very low tensile strength and is prone to cracking as the soil shifts. However, in a highly compressed state, a post-tension foundation can reduce the potential for breakage.
Post-tension foundations are often compared to standard rebar foundations in which a grid of strong steel is set in the poured slab for reinforcement. There are benefits and costs to both options, but the differentiator is how the rebar is tightened.
When considering a post-tension foundation, you will want to consider the qualifies that make it an excellent choice for any new home.
Post-tension foundations are extremely versatile and may be the ideal solution for environments that do not support other types of home builds. Particularly in the Gulf Coast region and tropical areas with largely expansive soils that have a tendency to shift periodically, a post-tension foundation operates as a stiff apparatus to resist cracking. Similar to a boat that rocks back and forth on the waves as one large fixture, the foundation will ride the soil movements as it shrinks and expands with moisture changes.
Compared to the intricate arrangement of rebar that must be laid with precision for a standard conventional slab, the post-tension cables are easier to install. Experts will need to use specific machinery to tighten the cables once the slab hardens.
There are two major benefits to using less steel in the home's foundation. The first one is the structure becomes more lightweight than a conventional rebar foundation; it is also easier for the construction team to pour the concrete due to its placement and extra space for the cables.
Second, there's an economic benefit. In general, a homeowner can expect to spend about $4.50 to $5.00 per square foot on either a post-tension or conventional rebar foundation. However, the total expense relies heavily on steel's current market price. Steel is an expensive material, which translates into savings for the homebuyer.
Fewer cables embedded in the slab than an intricate rebar grid means a simpler and straightforward inspection. Problems can be identified easily and quickly and be repaired before they become significant.
New homes with a post-tension foundation are highly regarded as having inherent strength and structural integrity. Any builder can vouch for the importance of improving the home's stability and longevity with the installation of a post-tension foundation. When it comes time for resale, knowledgeable buyers will recognize the home's long-term value and be willing to pay more for this important structural feature.
While there are many wonderful benefits to consider, there are a few minor challenges to keep in mind when choosing a post-tension foundation.
When the foundation is initially poured, there is a vulnerable stage that occurs briefly in which the slab can experience minor cracking at the surface. Prior to tension and after about a week or so of curing, the slab becomes a thick chunk of concrete without tensioned cables to support the bulk of its weight. As a result, the longer ends may sink and allow the middle to buckle. Long, tiny cracks or webs will appear on the surface but do not cause an issue structurally. They may be an aesthetic issue in areas that remain uncovered by tile or wood surface coverings, such as garage flooring.
When post-tension slabs are set, the builder will often include a stamp that warns the homeowner against cutting or drilling into the foundation. Cutting into this type of foundation can be particularly risky for even the most highly-trained construction expert due to the high level of tension suspended in the concrete. If a jackhammer accidentally strikes a cable, it may cause irreparable damage to the home, cause serious injury, and even death.
As previously stated, cutting into post-tension slab foundations can be particularly dangerous and should be avoided. When the bars rip through the slab, they become nearly impossible to repair. Homebuyers should consider any future plans for remodeling, which may prove to be a challenge if it involves adjustments to the foundation.
When considering options for building a dream house from scratch, homebuyers should consider the fantastic benefits of a post-tension foundation. Its structural integrity, economic pros, and overall quality makes this a great option in any climate for any home.
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