Primary Support: Unlike a floor joist, which is a repetitive, smaller member that supports the subfloor, the girder is a critical load-bearing member that supports the joists themselves. It acts as the backbone of the floor structure.
Load Transfer: It collects the distributed load from the joists and converts it into a few heavy, concentrated loads that are then transferred to the foundation or basement supports.
Size and Strength: Girders are significantly thicker and stronger than the joists they support, as they must handle the cumulative weight of a large section of the floor, including all live (occupants, furniture) and dead (materials) loads.