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 percent and 75 percent 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 36,000 pounds per square inch. In comparison, ASTM A441 has a yield strength of 40,000 to 50,000 pounds per square inch, and ASTM A572 has a yield strength of 42,000 to 65,000 pounds 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 percent of an alloy's yield limit. So, ASTM A36 has an allowable bending stress of 22,000 pounds per square inch, ASTM A441 has an allowable bending stress of 24,000 to 30,000 pounds, and ASTM A572 has an allowable bending stress of 25,200 to 39,000 pounds.
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 36,000 pounds per square inch and an allowable bending stress of 22,000 pounds per square inch, is slowly being replaced by ASTM A572 Grade 50, which has a yield strength of 65,000 pounds per square inch and an allowable bending stress of 39,000 pounds per square inch--77 percent stronger than ASTM A36. No doubt, future alloys will surpass new alloys such as ASTM A572 Grade 50.
- "Fundamentals of Building Construction;" Allen, Edward; 1999.
- Photo Credit steel sheets image by Mikhail Tischenko from Fotolia.com
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