21.1. Introduction

In a previous chapter we have examined the behavior of laterally supported beams. Under those conditions, the potential modes of failures were either the formation of a plastic hinge (if the section is compact), or local buckling of the flange or the web (partially compact section).

This chunk introduces the concept of laterally supported beams, which are beams that have some form of support along their length that prevents sideways movement. In such beams, two main failure modes can occur: 1) Plastic Hinge Formation: This happens when the beam's cross-section is compact, allowing it to flex and create a hinge where plastic deformation occurs. 2) Local Buckling: This occurs in partially compact sections where the material may buckle locally, meaning that certain sections of the beam give way under load before the whole beam does. Understanding these concepts helps illustrate how beams typically perform under load.

Imagine a long, thin ruler supported on a table at both ends. If you push down in the middle, the ends remain stable, resembling how a laterally supported beam works. However, if you press too hard, the middle may bend too much, creating a hinge at that point, similar to a plastic hinge. If the ruler were weaker in the middle, it could also just buckle without fully bending, which is like local buckling.

Rarely are the compression flange of beams entirely free of all restraint, and in general, there are two types of lateral supports: 1. Continuous lateral support by embedment of the compression flange in a concrete slab. 2. Lateral support at intervals through cross beams, cross frames, ties, or struts.

This chunk discusses the various types of lateral supports that exist for beams: 1) Continuous Lateral Support: This occurs when the beam is embedded in a concrete slab, which prevents lateral movement throughout its entire length. 2) Interval Lateral Support: This type of support is provided at intervals and involves additional beams or structural elements (like cross frames, ties, or struts) that prevent lateral movement but might not cover the entire beam length.

To visualize this, think of a bridge: its beams are continuously supported by the concrete deck (continuous lateral support) or by cross beams that provide stability at certain sections along its length (interval lateral support). This ensures that the beams do not sway sideways too much under load.

Now that the beam is not laterally supported, we ought to consider a third potential mode of failure, lateral torsional buckling.

Here, the author introduces a significant consideration when beams lack lateral support: Lateral Torsional Buckling. This is a form of structural failure that occurs when a beam twists along its length under load due to insufficient lateral support. This is distinct from the previously mentioned failures and highlights the importance of proper beam design considering potential instability.

Imagine holding one end of a long stick while pushing down on the other end. If the stick does not have any support along the sides, it may not just bend downward, but also start to twist, similar to how a mechanical pencil twists in the middle while being pressed. Without lateral support, beams can behave in a similar unstable manner and twist rather than just bending.

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