In modern concrete construction, achieving optimal structural performance while maintaining cost-effectiveness is paramount. Banded tendons in post-tensioned concrete systems have emerged as a strategic solution to address these challenges.
This method not only saves money but also maintains the strong performance of the structure, thus giving it increased efficiency through material usage optimization.
Banded tendons, which are traditionally used in tall buildings and parking areas, have now become a part of the modern concrete design, which makes the efficient and economical large-span application possible.
What are Banded Tendons?
Banded tendons in post-tensioned concrete slabs are those prestressing tendons that have been grouped into concentrated bands, usually placed over the column lines of a two-way slab. In simple words, in post-tensioned concrete slabs, banded tendons are groups of steel cables (tendons) that are placed closely together in straight lines, usually along the column lines. Instead of spreading tendons evenly across the whole slab, banding them focuses the strength where it is most needed, mainly above the columns, where bending forces are the highest.
By this configuration, the load distribution is enhanced, material usage is optimized, and slab thickness is reduced. It thus not only makes the slab stronger against bending and shear, but also minimises the deflections that occur. Banded tendon layouts are used in a great number of large-span structures, such as high-rise buildings and parking garages, where the existence of large open spaces is a requirement.
The design is cost-effective, as it not only reduces the use of materials, time, and labour for construction but also enables the application of the latest structural analysis methods, such as finite element analysis (FEA), to ensure structural integrity and performance.
Why Use Banded Tendons?
The reasons why they are so important are,
- Material Efficiency: Tendons are concentrated into bands positioned over the column lines; thus, the amount of prestressing steel needed for the slab is reduced, lowering material costs without sacrificing the structure’s strength.
- Improved Load Distribution: Banded tendons improve load resistance by distributing that force evenly, especially over column locations. This in turn increases the slab’s bending and shear capacity, and, as a result, the structure can develop large spans or be used as a high-rise building.
- Constructability: The major advantage of this method is grouping the tendons to simplify construction, reduce labour, errors, and alignment issues. Additionally, the layout that is implemented reduces congestion, making the placement of tendons more accessible and efficient.
- Future Flexibility: The layout is designed to leave clear spaces in the middle of the spans, making any future changes, such as utility penetrations, easily accessible and minimising disturbance to the prestressing system.
Advantages of Banded Tendons
Common advantages include,
- Concentrated Load Transfer
Banded tendons, which are appropriately placed along column lines, aid in the transfer of concentrated loads from columns to the slab in an efficient manner, thus facilitating load distribution.
- Reduced Tensile Stresses
When tendons are positioned right over the centerlines of columns, the tensile stresses in the slab’s top fibers are minimized, thus making structural performance better as a whole.
- Improved Serviceability
Experiments show that banded tendon configurations positively influence structural performance under normal conditions, followed by better longevity of the structure.
- Simplified Detailing
Compared to distributed tendon layouts, banded tendons streamline the detailing process, simplify the situation of crossing of tendons, and make the design more controllable.
- Economical Layouts
The number of intersections of the tendons is minimal, which in turn lowers the clashing points, thus reducing the placement cost, which simplifies the construction process, leading to a low building process cost.
- Flexibility for Openings
The use of dual-banded tendon layouts is flexible in a way that new openings in the floor can easily be made without interfering with tendons during construction, and hence design modifications can be easily accommodated.
Banded Tendons Layout Strategies
Designing banded tendon layouts for post-tensioned concrete systems is one of the primary steps in achieving optimal structural performance, especially in large-span, two-way slab applications.
- Conventional Banded Layout
The main idea is to group tendons into bands, which are placed along the column lines, with uniform distribution of the tendons in the orthogonal direction. This particular setup results in a reduced number of tendons in the middle spans, thus making the constructability part easier without sacrificing strength. This is the most optimal solution for floor systems in regular buildings, where loads are fairly evenly distributed.
- Dual-Banded Layout
Tendons are concentrated in two bands, which run along both axes of the slab, thus giving the structure the highest possible load resistance and making the flexural deflection to be at a minimum over large spans. This is a perfect layout for such buildings as are subjected to heavy concentrated loads or have irregular shapes, since it can provide the best solution to the issue of cracking and shear by reducing them to a minimum.
- Hybrid Layout
By combining the characteristics of banded and uniformly distributed tendons, the hybrid layout is utilized in projects to locally prestress the areas. This method allows a designer to achieve a variety of load patterns by adjusting the spacing and placement of tendons, thus not only satisfying strength and serviceability requirements but also ensuring that the solution is cost-effective.
Design & Construction Considerations
The main issues are,
1. Tendon Placement
Banded tendons are typically positioned directly over the column lines, while the rest of the tendons are spaced evenly in the orthogonal direction. Such a layout simplifies the distribution of the dead loads and live loads and localizes the bending moments to the areas with the highest stresses of the slab, i.e., most of the time, we are dealing with the slab zone above the columns. Besides span lengths, proper tendon profiles should also be designed such that the most efficient force transfer is realized.
2. Slab Thickness
The slab thickness should be sufficient to house the concentrated tendon forces, e.g., around the column zones. The increase in slab thickness in these areas not only helps in avoiding tendon congestion but also ensures that the anchorage is done properly. At the same time, the applied loads are distributed effectively, and the failure due to shear is eliminated.
3. Reinforcement Detailing
The areas between the tendon bands should be equipped with non-prestressed reinforcement to handle the shear forces as well as prevent excessive deflection of the slab. Local building codes should be followed while designing reinforcement to ensure that the concrete is durable and there is cracking control, especially in the high-stress regions.
4. Stressing Sequence
The tendons are stressed in a controlled sequence to ensure balanced force application, account for slab curvature and potential deflections, and maintain structural stability.
By considering these factors, banded tendon designs improve structural performance, reduce material costs, and enhance overall durability.
Conclusion
Banded tendons are an excellent and affordable solution for post-tensioned concrete structures. The grouped tendons over columns, being the most efficient way of using material, not only lead to better structural performance but also make the construction process easier and faster. Also, as a result of correct design and smooth coordination, the layouts of the banded tendon can be used in different types of projects, giving the benefits of improved flexibility, cost reduction, and construction practice aging for modern building requirements.
Apoorva H P Achar