Monthly Mechanics: Steel

Steel is an alloy of iron and carbon, and one of the strongest materials that can be produced economically. The history of steel as a structural material begins in the 18th century when cast iron was first used to build bridges. But iron is very ductile, which is not desirable. Carbon stiffens the molecular structure so the material barely deforms up to a very high stress.

Like most materials, steel gets a little longer when in tension and a little shorter when in compression. The change in length is proportional to the stress applied – if you plot the tension versus percent elongation in a steel member, the graph is a straight line. (See The Elastic Modulus for more information.) Moreover, when the stress is removed the steel member returns to its original shape, like an elastic band. Thus, steel is a linear elastic material.

All this is true up to a point. The linear elastic relationship breaks down above the yield stress. Once the stress applied exceeds the yield stress, the steel continues to deform but doesn’t return to its original shape, like melted plastic. If the stress is higher still, the steel eventually ruptures – complete failure. This is called the ultimate stress.

Steel Stress-Strain

Stress-strain graph for typical steel.

Structural engineers don’t like plastic behavior. It’s OK if a structure deforms a little under stress (skyscrapers bend in the wind; bridges sag with heavy traffic), but only if it returns to its original shape when the stress is removed. For this reason, structural engineers usually specify steel based on its yield stress, and they design to make sure this stress is never exceeded. In the US, the most common steel grade is A36, corresponding to a yield stress of at least 36,000 pounds per square inch. Developers and state highway departments, seeking designs for commercial buildings and major bridges, increasingly require steel with a yield stress of at least 50,000 pounds per square inch.

Steel has been mass produced since the early 19th century, and manufacturers have standardized a huge number of shapes, made by extruding molten steel through sets of rollers. They also produce steel plates of various depths, and custom curved beams. Here are some of the rolled shapes available:

Steel Shapes

Wide-flange beams, labeled with a W, such as W12x16 – the first number indicates the depth; the second number indicates the weight in pounds per foot.

Channels, labeled with a C, such as C10x30 – the first number indicates the depth; the second number indicates the weight in pounds per foot.

Angles, labeled with an L, such as L4x3x3/8 – the first and second number indicate the length of each leg; the third number indicates the thickness.

Hollow structural steel, labeled with an HSS, such as HSS8x4x1/4 – the first and second number indicate the depth and width; the third number indicates the thickness.

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