Steel is a remarkable material that is extremely strong in both tension and compression. In fact, steel is equally strong in tension and compression--one of the few materials with such properties. There are many different steel alloys, but they all have similar stress versus strain ratios and ultimately fail in the same way. Steel alloys are designated by an American Society for Testing and Materials (ASTM) number, such as ASTM A36.
Modulus of Elasticity
All steel alloys have the same modulus of elasticity. Modulus of elasticity is the stiffness of a material, or the ratio of the material's allowable stress versus strain. Steel's modulus of elasticity is 29 million pounds per square inch, which is much higher than concrete's 5 million and wood's 2 million. The different steel alloys have other different properties, including strength limits and bending stresses.
Each steel alloy has a different yield strength. This is the highest force the material carries before deforming, and it is the limit used in building codes to define the allowable loads a steel structural element can carry. Building codes provide an allowable stress between 33 per cent and 75 per cent of steel alloy's yield strength, depending on its use. A comparison of the yield strengths shows certain steel alloys are better for a certain purpose than others. The most common alloy, carbon steel, or ASTM A36, has a yield strength of 16329 Kilogram per square inch. In comparison, ASTM A441 has a yield strength of 18144 to 22680kg per square inch, and ASTM A572 has a yield strength of 19051 to 29484kg per square inch.
Ultimate Limit and Deformation
Beyond the yield strength or yield limit, steel deforms, stretching and bending, until it begins to attain its ultimate limit. During deformation, the steel loses strength, but when the material reaches its ultimate limit, the strength of the material increases beyond the yield limit and finally breaks. Codes do not allow the design of structural elements relative to steel's ultimate limit because of the danger of a building's inhabitants in the deformation stage. Nevertheless, the material, when overloaded, will deform and give, but not fail, allowing time for inhabitants to exit the structure before a structure's ultimate failure.
Allowable Bending Stress
Steel elements in compression are subject to failure by bending. Codes provide allowable bending stresses that can be used to design structural elements. The allowable bending stress is 60 per cent of an alloy's yield limit. So, ASTM A36 has an allowable bending stress of 9.98 Kilogram per square inch, ASTM A441 has an allowable bending stress of 10886 to 13608kg, and ASTM A572 has an allowable bending stress of 25,200 to 17.7 Kilogram.
The steel industry is continuously improving the quality of steel alloys and creating new alloys that are stronger than those that came before. For example, common carbon steel, ASTM A36, with a yield strength of 16329 Kilogram per square inch and an allowable bending stress of 9979 Kilogram per square inch, is slowly being replaced by ASTM A572 Grade 50, which has a yield strength of 29484 Kilogram per square inch and an allowable bending stress of 17690 Kilogram per square inch--77 per cent stronger than ASTM A36. No doubt, future alloys will surpass new alloys such as ASTM A572 Grade 50.
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