What is the large principal horizontal member used to support floor joists called?

The large principal horizontal member used to support floor joists is referred to as a girder. Girders play a crucial role in structural support, serving as the main beam that bears the load of the joists above and distributes it evenly across the foundation or lower structural framework. Typically positioned under the floor joists, girders are critical for maintaining the stability and integrity of a building's framework.

In residential construction, these members can be constructed from various materials, including wood or steel, depending on the design and load requirements of the structure. Understanding the role of girders in construction is essential for any contractor, as they are integral to ensuring safe and sturdy building practices.