Written by KEC President, Don Kline
Post-tensioned concrete has been used in building construction since the late 1950’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.
1. Myth: It’s difficult to core holes or cut future openings in post-tensioned slabs.
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.
2. Myth: Post-tensioned parking garages are more expensive than precast parking garages.
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.
3. Myth: You need sophisticated finite element analysis software to design post-tensioned structures.
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.
4. Myth: General zone reinforcement should be designed by the post-tensioning supplier.
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.
5. Myth: Post-tensioned buildings experience significantly more shortening than non-prestressed buildings.
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.
6. Myth: Post-tensioning can’t be used in below-grade applications.
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.
7. Myth: Drop panels need to be 1/6 the span in order to be used in the calculation of section properties for strength calculations.
Reality: ACI 318 code states that the size of the drop panel has to be ⅙ 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.
8. Myth: The more post-tensioning, the better.
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.
9. Myth: Post-tensioning reduces material costs.
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.
Formwork and reshoring costs are reduced.
10. Myth: Cracks in the wedges indicate a post-tensioning system defect.
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, give us a call at 703-331-1284.