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Strength Comparison: Steel vs. Concrete in Skyscrapers

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When discussing the structural integrity of skyscrapers, it is often debated whether steel or concrete is the stronger material. However, the question often focuses on the robustness of reinforced concrete as a construction material, especially in structures like the Burj Khalifa and other residential towers. This article aims to clarify the strengths of both materials and provide a detailed comparison focusing on the properties that make reinforced concrete a viable, if not superior, choice for many modern skyscrapers.

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Understanding Tension and Compression

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Tension and compression are the two primary forces that buildings must resist. Understanding these forces is crucial to determining the strength of a structure. Tension is the force that stretches or elongates a material, while compression is the force that squeezes or compresses a material.

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Considering the density of steel at 7850 kg/m3 and concrete at around 2800 to 3300 kg/m3, we can see the differences in weight between these materials. Steel is significantly lighter, which is a significant advantage when designing tall structures that need to support their own weight over long spans.

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Steel vs. Concrete: Strength Characteristics

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Steel is more resilient than concrete and has superior resistance to tensile, compressive, and flexural stress. In tension, steel is about 8 times stronger than concrete, which makes it ideal for structures where tensile strength is critical. Concrete, on the other hand, is more effective in compression, making it suitable for load-bearing walls and foundations.

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The following table summarizes the compressive strengths of standard and high-strength concrete:

" "" "Concrete TypeCompressive Strength (psi)" "Conventional Concrete7000 psi or less" "High-Strength Concrete7000 to 14500 psi" "" "

For further reference, the strength of grade Fe-600 TMT steel is approximately 98625.6 psi, and when combined with concrete, it exhibits a strength of 6.8 times that of concrete in tension and shear.

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Burj Khalifa: A Case Study in Reinforced Concrete

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The Burj Khalifa is a striking example of the effective use of reinforced concrete in architectural design. This structure, currently the tallest building in the world, incorporates a mix of high-strength concrete and steel.

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In the foundation of the Burj Khalifa, self-consolidating C60 concrete is used, while at the core walls, C50 self-consolidating concrete is employed. The lower levels utilize C80 concrete for cylinders, reducing to C80-14 at floor 101. The Young’s modulus of C80 concrete is said to be around 43800 N/mm2. For more details, see the footnote 1 for the full document describing the construction specifications.

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High-strength concrete, with a compression strength of up to 85 MPa, is used in the lower half of the Burj Khalifa. This is complemented by reinforced steel with a yield strength of 520 MPa and a diameter of up to 40 mm.

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Jeddah Tower (Kingdom Tower): An Advanced Construction Project

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The Jeddah Tower, also known as the Kingdom Tower, is the next tallest building in the world. This structure introduces a new benchmark in high-strength concrete and steel usage.

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The lower half of the tower requires concrete with a minimum cylinder strength of 85 MPa at 90 days, while the upper floors and spire specify concrete strengths of 75 MPa and 65 MPa, respectively. The reinforcing bars used are conventional 420 and 520 MPa yield strength material with diameters up to 40 mm. The high-strength concrete used in the tower can achieve a cylinder compression strength of up to 85 MPa. The reinforced steel used is also grade 60 and has a yield strength of 520 MPa.

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Compressive strength is particularly important in concrete structures. Traditional concrete may have a compressive strength of 3000 to 5000 psi, while ultra-high-performance concrete (UHPC) can achieve compressive strengths of up to 35000 psi. In terms of tensile strength, traditional concrete can support up to 400 - 700 psi, while UHPC can support up to about 1400 psi.

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Conclusion

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While steel has significant advantages in tensile strength, concrete excels in compressive strength. Reinforced concrete, which combines the two materials, offers a robust solution for constructing skyscrapers that can withstand both tension and compression forces. Structures like the Burj Khalifa and Jeddah Tower demonstrate the capabilities of reinforced concrete and the push for higher strength and more resilient materials.

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For further reading, please refer to the documents mentioned in the footnotes for detailed specifications and construction insights.

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Footnotes

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[1] KHALIFA-WorldsTallestBuilding.pdf

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[2] Details of Jedda Tower