Which type of girder has a floor system that supports the deck?

The two-girder system is characterized by its use of two primary girders to support both the crossbeams and the deck. In this design, the layout typically consists of two main girders that run parallel to each other, which are responsible for carrying the majority of the loads from the bridge deck. The crossbeams, or floor beams, are then placed at intervals between these girders to distribute the load evenly across the deck.

This system is efficient in terms of material usage and provides simplicity in construction, making it a common choice for various types of bridge designs. The connection of the floor system to the two girders ensures enhanced stability and structural integrity, as the weight and dynamic forces encountered by the bridge are effectively managed through this configuration.

Other types such as multi-girder systems, while providing more support through additional girders, are not specifically defined by just two main girders, which is the unique characteristic of the option you identified. Similarly, steel box-girders and steel frame systems involve different structural configurations and load distribution methods that do not emphasize the use of just two horizontal girders as the primary support structure for the deck.