Final answer:
A girder is the large principal horizontal member used to support floor joists, often made of steel, concrete, or wood, and it transfers loads from the floors to the supports below.
Step-by-step explanation:
The large principal horizontal member used to support floor joists is known as a girder. Girders are the main horizontal supports of a structure and are often made of steel, concrete, or wood.
They serve to transfer loads from the floors above to the vertical supports, such as columns or walls below. In residential construction, large wooden beams can also function as girders to support floor joists which span across the shorter distance between walls or foundations.
In construction, a large principal horizontal member used to support floor joists is called a beam. Beams are horizontal structural elements that carry loads and support the weight of the building above them.
In the context of floor construction, beams are typically made of materials such as steel or wood and are placed perpendicular to the floor joists.