3. Steel Section Properties

Steel is an alloy of iron and carbon. Its properties can be greatly varied by altering the carbon content (always less than 0.5%) or by adding other elements such as silicon, nickel, manganese, and copper.

Steel is primarily made from iron with a small amount of carbon. The carbon content can affect the strength and durability of the steel. By adjusting the carbon levels and adding elements like silicon or nickel, the properties of the steel can be tailored for specific uses.

Think of steel like a recipe for a cake. Changing the amount of flour (carbon) or adding ingredients like chocolate chips (silicon or nickel) can create different types of cakes (steel properties).

Practically all grades of steel have a Young Modulus equal to 29,000 ksi, a density of 490 lb/cu ft, and a coefficient of thermal expansion equal to 0.65 x 10^-5 / deg F.

The Young's modulus determines how much a material will deform under stress; for steel, this is consistently high, illustrating its strength. The density shows how heavy steel is, which is important for calculations on structural loads. The thermal expansion coefficient tells us how much steel expands with heat, which is crucial for stability in temperature changes.

If you have a steel ruler, it stays straight until you apply enough force to bend it. The Young's modulus describes how difficult that is. Just like a balloon expands when heated, steel expands slightly too, which is something engineers must consider when designing structures.

The yield stress of steel can vary from 40 ksi to 250 ksi. Most commonly used structural steels are A36 (σ_yield = 36 ksi) and A572 (σ_yield = 50 ksi).

Yield stress is the stress level at which steel begins to deform permanently. Different grades of steel have different yield stresses, which affects where and how they can be used in construction. A36 and A572 are widely used standards for building due to their favorable balance of strength and ductility.

Consider yield stress like the weight limit on a swing. An A36 steel swing can hold up to 36,000 pounds of push before bending permanently, just like a swing might bend if a heavy child sits on it.

Structural steel can be rolled into a wide variety of shapes and sizes. Sections are designated by the shape of their cross-section, their depth, and their weight. For example W 27 114 is a W section, 27 in. deep weighing 114 lb/ft.

Structural steel is commonly shaped through a process called rolling, which allows it to be formed into beams, channels, and more. These shapes are identified with an alphanumeric system that indicates the type of section, depth, and weight per unit length, making it easier for builders and designers to select the appropriate material.

Think of ordering a sandwich. The name tells you the type of bread (shape), the size (depth), and how much it weighs (weight). This is similar to how engineers specify steel sections for construction.

Common sections are: S (standard), W (wide flange), C (channel), HP (bearing pile), and L (angle) sections, among others.

Each type of steel section has a specific application based on its shape and structural properties. For instance, wide flange sections are used for stronger load-bearing applications, while channel sections are useful for framing and bracing. Knowing these types helps engineers choose the right material for a structural design.

Consider the different tools in a toolbox. Each tool (like a screwdriver or wrench) is designed for a specific task (tightening screws or nuts). Similarly, different steel sections are suited for various structural roles in construction.

Rolled sections have residual stresses which originate during the rolling or fabrication of a member. The area that cools first becomes stiffer and develops compressive stresses, while the remaining regions develop tensile stresses.

Residual stresses are internal stresses locked into a material after it has been formed. When a steel section is rolled, the outer sections cool fast, creating compression, while the inner sections may remain plastic longer and experience tension. These stresses can affect performance and behavior under load, making it critical for engineers to account for them in design.

Imagine a sponge that was squished from both sides. Once released, it might not return to its exact shape. Similarly, residual stresses can affect how steel structures react to loads over time.

Due to those residual stresses, the stress-strain curve of a rolled section exhibits a non-linear segment prior to the theoretical yielding.

The impact of residual stresses means that engineers cannot consider the theoretical yield point as the actual yield point. Instead, they must design to account for the section's unique behavior that deviates from expectations due to these stresses, influencing structural safety and integrity.

Think of it like a toy that bends too easily before it breaks. Just because the label says it will 'hold' something doesn't mean it won't show stress and bend first; engineers have to know how much to allow for safety.

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