1.2. Multiple Rolled Steel Sections (Built-up Columns)

Used when higher load capacity or longer column length is required.

Built-up columns are specifically designed to accommodate higher loads or greater heights than what single rolled steel sections can handle. When a structure needs to support more weight or reach taller than a single steel column could support safely, built-up columns are used. This design uses multiple steel sections to achieve a greater total strength and stability.

Think of a built-up column like stacking several smaller boxes to create a taller tower. A single box might not be tall enough or strong enough to reach the required height, but when you stack them, you can create a stable, higher structure.

Configuration: Two or more sections (e.g., channels, angles, or beams) spaced apart and connected by lacing or batten plates.

In a built-up column, two or more individual steel sections, such as channels or angles, are spaced apart and joined using lacing or batten plates. This configuration helps distribute the loads evenly and improves the overall structural integrity. By connecting these sections, the built-up column can handle loads from various angles and resist buckling.

Imagine a bridge where the beams are connected by cross-bracing (lacing) to create strength. Just as the bracing helps the bridge resist swaying and bending, the lacing in built-up columns helps them resist twisting and buckling under load.

Design Process: Select Section Arrangement: Symmetrical, to avoid torsion/buckling. Combine Section Properties: Calculate total area, moment of inertia, and radius of gyration about principal axes. Design for Overall Stability: As for single columns, but use combined properties.

The design of built-up columns involves careful consideration of how the sections are arranged. A symmetrical arrangement helps prevent twisting (torsion) or buckling under loads. The designer must calculate the total area, moment of inertia, and radius of gyration of the combined sections to understand how they will perform under load. Finally, overall stability checks are done using the combined properties of the sections, ensuring they can handle the intended loads safely.

Think of designing a bookcase. If you stack the shelves unevenly, the weight of the books could cause it to tip over or buckle. However, if you distribute the weight evenly and choose the right materials for the structure, it will hold up well under pressure. Similarly, proper arrangement and calculation in built-up columns prevent failures under load.

Spacing: Adequate spacing provided to prevent local buckling.

When constructing built-up columns, ensuring adequate spacing between the sections is crucial to prevent local buckling. Local buckling occurs when parts of a column, particularly thin-walled members, bend or buckle under load. Proper spacing reduces the risk of this failure mode by allowing the sections to work together more effectively to carry the load without excessive deformation.

Consider a row of chairs lined up in a café. If they are placed too close together, they might topple over if someone leans on them. However, when spaced appropriately, they can support weight without risk of collapse. The same principle applies to the sections of built-up columns.

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