What is the term for a large principle horizontal member used to support floor joists or wood decks?

The correct term for a large principal horizontal member that is used to support floor joists or wood decks is "girder." Girders serve as the main support structure in a building's framework, collecting the loads from the floor joists and transferring that load to the vertical supports, such as posts or columns. They are critical in ensuring structural integrity and stability in a building.

While beams also play a supporting role, they are typically smaller and can span between supports, while girders are the larger elements that provide foundational support for multiple beams or joists. Understanding the distinction between girders and beams is essential for proper construction practices. Additionally, struts and posts serve different purposes; struts are inclined members used to brace other elements, while posts are vertical supports that carry loads down to the foundation.