When is a beam required under the floor joist?

A beam is required under the floor joists when the span between the joists exceeds the allowable limits for the materials used, which can lead to structural instability or excessive deflection. Additionally, beams are necessary when there are changes in load direction, such as supporting a load-bearing wall or when the joists need to be reinforced in areas with heavy loads. They help distribute weight more evenly and ensure the overall integrity of the floor structure.