1.1. Steel

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 by combining iron with carbon in small amounts, which significantly enhances its properties. The carbon content is typically less than 0.5%, and varying this amount can modify the steel's characteristics. Adding other elements like silicon, nickel, manganese, and copper can further fine-tune these properties.

Think of steel as a customized recipe for a cake. The ingredients (like iron and carbon) can be adjusted depending on the desired taste and texture (properties), allowing bakers (or manufacturers) to create different types of cakes (or steel), each suited for a specific purpose.

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

The Young Modulus represents the stiffness of steel, indicating how much it will deform under stress. The density tells us how heavy the steel is relative to its volume, important for weight considerations in construction. The coefficient of thermal expansion indicates how much steel will expand or contract with temperature changes.

Imagine a rubber band compared to a steel rod. The steel rod doesn't stretch much (high stiffness) when you pull it (high Young Modulus) while the rubber band stretches a lot. Likewise, when temperatures rise, materials expand; this is similar to how a balloon might stretch when heated.

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

Yield stress is the stress beyond which steel begins to deform permanently. Different grades of steel have different yield strengths; commonly used types like A36 and A572 are standard for structural applications, denoting that they can carry significant loads without permanent deformation.

Think of yield stress as the weight limit of a bridge. If too much weight (stress) is placed on it, it will start to warp (deform). Using a steel grade with a higher yield stress is like using a stronger bridge that can hold more weight without warping.

Structural steel can be rolled into a wide variety of shapes and sizes. Usually, the most desirable members are those that have a large section modulus (S) in proportion to their area (A).

Structural steel is available in various profiles such as beams, columns, and shapes. The section modulus is a measure that indicates how effectively a section can resist bending; the larger the ratio of section modulus to area, the stronger the structural component.

Consider different types of spaghetti strands: some are thin, while others are thicker or shaped. The thicker strands can handle more sauce (load) without breaking, similar to how a beam with a larger section modulus can withstand more weight before bending.

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.

Steel sections are categorized based on their cross-sectional shape, the depth of the section, and the weight per unit length. This classification helps engineers select the appropriate steel for various structural applications.

Think of clothing sizes: just as shirts are labeled Small, Medium, or Large based on their dimensions and fit, steel sections are labeled to indicate their shape and size, making it easier to choose the right 'fit' for construction.

Common sections include S sections, W sections, C sections, HP (bearing pile section), and L (angle sections).

Different types of steel sections serve varied purposes. S sections were among the first rolled in the U.S., W sections with less inner slope are favored today. C sections are often used in framing, L sections have angle profiles for supports, and HP sections are used as piles for foundations.

Imagine a toolbox with different tools: hammers, screwdrivers, and pliers. Just as each tool is designed for a specific task, each type of steel section is engineered to fulfill particular functions in a structure.

Rolled sections, have residual stresses that originate during the rolling or fabrication of a member. The member is hot just after rolling or welding, it cools unevenly because of varying exposure...

Residual stresses are internal stresses locked within the material due to uneven cooling after manufacturing. They can affect the structural performance and safety of steel components, leading to complex deformation behaviors.

Think of a cake taken from the oven that has slightly cooked unevenly; some parts cool faster and become firmer while others remain soft. Similarly, residual stresses in steel affect how it behaves under load when exposed to real-world conditions.

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

The non-linear segment in the stress-strain curve indicates that the behavior of steel under load is not always predictable and can lead to unexpected failure modes, particularly in beams and columns under flexure.

If you think of a rubber band slowly stretching until it snaps, there’s more happening than you can see. Residual stress can be the hidden tension that causes materials to fail unexpectedly, similar to how not all stretched rubber bands can return to their original shape.

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