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.
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.
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.