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More than ten billion tons of concrete are used annually in construction. However, the biggest deficiency with concrete is that it can only transfer very small tension loads. That’s why we have rebar. And while rebar is commonly used, what isn’t commonly used is high strength rebar, the kind you find in some of the most expensive and high-end buildings in the USA, but seldom used in Canada.
Today, the requirements for new buildings are becoming more difficult. Buildings are striving to reach higher green standards, architects continue to push the edge in their designs, and developers want taller buildings.
And yet in Canada, engineers appear not to be fully utilizing the latest in structural materials.
The application of high strength steel has supported global breakthroughs in the economics and the elegance of steel constructions (improving strength to bear higher load and stress capacities, cost reductions and design improvements). Think of the Empire State Building, Sydney Harbour Bridge, Taipei 101 Tower and many more examples to the testament of high-grade steel. Today, there are better systems, yet many builders continue to use basic rebar materials.
Buildings that assume the use of lower grade steel have many negative impacts such as structural integrity, quality of build, but most importantly, time and costs. We will explore the introduction of high strength reinforcement systems (HSRS®) and why high-rise buildings should be using them.
Advantages
To understand the benefits of using HSRS®; a comparison of two grades of steel is presented. BSt 500, the common reinforcement grade and SAS 670, the high strength reinforcement.
Notice that the shape patterns are different. The ribs of the bars make SAS 670 continuously threadable. The relative rib area is higher which improves the bond behaviour of the steel.
In addition, HSR steel is available in a diameter ranging from 18 mm up to 75 mm. The combination of three advancements—high strength material, larger diameter sizes, and threadable reinforcing bars allows for the production of prefabricated cages for columns, shear walls and beams. [1]
This shows an example of how reinforcement ratio or cross sections can be reduced significantly. All three columns have the same load capacity. The integrity of the column is strengthened while at the same time decreasing the overall area size as well. Essentially, effective floor space increases with Figure c). This gives more freedom in creative design for architects due to higher loading capacity with the same or smaller dimensions with the same capacity. [3]
Congestion Relief
A recent 31-storey condominium project proved benefits of 100 ksi reinforcing steel in such a situation. “Typical spacing of the confinement steel in the columns is 4 to 5 inches using standard Grade 60 #4 bars. “This creates severe congestion issues at the intersection areas. By switching to #5 bars made of Grade 100 steel, the spacing increased to between 8 to 12 inches, allowing faster construction and easier placement of the concrete.” [2]
Applications
Below, some examples highlight the benefits of the initial construction method in globally recognized buildings utilizing HSR steel.
High-strength reinforcement systems are reshaping the real estate development space. Providing innovation, reducing building costs and awareness of environment efforts. MANA bar mill has been North America's leading steel supplier. Read more to discover the great advantages of using high-grade steel.
Large bar diameters of high strength reinforcement can be a very cost-effective option in foundation work. It can be used as the main reinforcing system or as an alternative to embedded steel members in concrete sections for deep foundations. The idea that using high strength steel bars to decrease the bar diameter and further to reduce the borehole diameter in deep foundation elements should be applied to structural members in high rise buildings. A smaller borehole increases the drilling speed and reduces costs. Which in turn, decreases the cross sections of concrete structural members by increasing the material strength of the reinforcements. [1]
The use of threaded bars and mechanically coupled connections can eliminate complicated welded splices. Also, the bar threading has been proven to yield a high level of bond strength with the surrounding concrete. This application presents benefits in multiple aspects such as required labour, assembly times and inspection work for welding. [4]
Conclusion
With the sprawl of urban living, high-rise buildings are in constant demand. Engineers should continue the exploration of the positive structural and economic effect of high-grade steel.
By using innovative building methods with HSRS® compared to common reinforced building practices, the advantages include reduced material and member cross sections quantity, increased effective (sellable/rentable) floor space and significant fabrication and installation time savings. High strength reinforcement systems are proving to be a cost-efficient alternative for projects with a wide range of applications. The combination of high strength concrete, HSRS®, and innovative design tools has brought great alternatives to structural steel construction.
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References
[1] F. Hude and J. Silva, "PREFABRICATED HIGH STRENGTH REINFORCEMENT CAGES FOR COLUMNS - A BOOST FOR HIGH RISE STRUCTURES," Stahlwerk Annahuette, Max Aicher GmbH, Stressteel Engineering Systems LLC, Ainring, Fairfield, 2017.
[2] M. A. Noor and A. U. Ahmed, "Study on Grade 75 and 60 Reinforcement in RC design," BUET (Bangladesh University of Engineering and Technology), Bangladesh, 2008.
[3] M. Scheibe and F. Hude, "HIGH STRENGTH REINFORCEMENT SAS670/800 POSSIBLE APPLICATIONS AND EXAMPLES," Stahlwerk Annahutte, Ainring, 2011.
[4] F. Hude and J. Silva, "HIGH STRENGTH REINFORCEMENT (HSR) - THE SMART SOLUTION FOR A SUSTAINABLE FUTURE," Building Tomorrow's Society, Fredericton, 2018.
[5] D. Jungwirth, T. Voss, M. Scheibe and F. Hude, "High strength reinforcement S 670 - Applications and examples with high rise buildings," Max-Aicher GmbH, Freilassing, 2017.