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What is a Steel Joist?

Dorothy Distefano
Dorothy Distefano

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

Designing a steel joist requires an understanding of the forces that will be applied, span length between supports, spacing between joists, 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. A few examples of the types of forces a steel joist may be exposed to include weight load, wind uplift, and vibration.

Steel joists are commonly used in bridge construction.
Steel joists are commonly used in bridge construction.

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

Steel joists are used in steel construction.
Steel joists are used in steel construction.

The construction of a joist will influence how much force it can withstand and how it will bend, or deflect, while under load. The deflection of a steel joist depends upon its dimensions, how it is supported, the material, and where the forces are 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 that are used in structural and architectural applications, but joists can also be manufactured using other materials, such as aluminum.

Steel joists are typically designed with a safety factor. This can make for oversized joists created to support the predicted load, or to address other factors that cannot be anticipated during the design of the beam. A factor of safety that is too large will drive a joist design that is larger than necessary, requiring more space to install, causing possible weight problems, 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 joists to meet the performance criteria.

Discussion Comments


Our church is in the process of putting up a new building. I don't know much about the construction process, but it has been fascinating to watch this unfold.

After the foundation was poured, they began putting up big, tall pieces of steel. When you think about the weight that these steel joists need to hold, you understand how important they are.

I know they also use steel floor joists to make sure that not only the top, but the bottom is reinforced too.

It is a lot different watching this process as it happens as you have a much better understanding than just showing up when the building has been completed.


My husband works for a company that build bridges and he runs a crew for them.

I am very familiar with what a steel joist is, as I hear him describe each bridge he is working on. When he is getting ready to build a new bridge, he will spend many hours studying the plans ahead of time.

Sometimes his jobs have been postponed because they do not get the steel beams in after they have ordered them.

He knows how much weight can be on each steel truss joist and how many steel joists you need in each span.

Sometimes trying to figure out the plans can be very frustrating. There can be a big difference between how something looks on paper and how it should actually be built to be safe.

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    • Steel joists are commonly used in bridge construction.
      By: andrey_plant
      Steel joists are commonly used in bridge construction.
    • Steel joists are used in steel construction.
      By: Vibe Images
      Steel joists are used in steel construction.