What’s a Steel Joist?

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Steel joists are used in many architectural and structural engineering applications. They are commonly used to support floors and ceilings in large structures, such as commercial buildings and bridges. A steel joist must be designed to withstand forces and strains while minimizing weight, space requirements, and material costs. Improperly designed joists can fail prematurely and have catastrophic effects.

Designing a steel joist requires an understanding of the forces that will be applied, the span length between supports, the spacing between the joists, the joist material, and how the joist is connected to other structural members. Steel joists are designed to withstand different types of loads and forces, depending on the application. Some examples of the types of forces a steel joist may be exposed to include weight loading, wind lift, and vibration.

Different types of steel joists are available based on loading and mounting configurations. The most commonly used type is the standard open weave (OWSJ) steel joist. This design consists of two parallel members, known as chords, with a repeating triangular mesh structure located between the chords. There are many other designs of steel joists, such as sloped, curved or barrel top chord, gable and scissor. More complex joist designs are used for special applications and are more expensive than a standard open mesh joist.

The construction of a joist will affect how much force it can withstand and how it will bend, or deflect, under load. The deflection of a steel joist depends on its size, how it is supported, the material, and where the forces are being applied. The depth of a joist, which is dictated by the spacing between the parallel chords, is a primary factor in how much it will deflect under load. Steel is most commonly used for joists used in structural and architectural applications, but joists can also be fabricated using other materials, such as aluminum.

Steel joists are usually designed with a factor of safety in mind. This can create oversized joists created to support the expected load or to address other factors that cannot be anticipated during the design of the beam. Too large a safety factor will result in a larger-than-necessary joist design, requiring more space to install, possibly causing weight issues and resulting in higher manufacturing costs. Until recently, steel joists were designed using force diagrams and a series of complex mathematical equations. Today, engineering software is usually used to analyze and design steel beams to meet performance criteria.

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