The floor system has two primary functions:
• Collect and transmit gravity loads to the vertical elements.
• Resist lateral loads through diaphragm action by providing a continuous path for transferring lateral loads from the bottom chord of one truss to the top chord of the adjacent truss down through the structure.
Thus, one could use precast concrete planks, long-span composite steel decks, open-web joists, or any other system consistent with the structural and architectural requirements. Precast planks and flat-bottomed steel decks are often used as exposed ceilings requiring a minimum of finish. For spans up to 30 ft (9.15 m), 8-in. (203-mm) thick planks are required, whereas for spans less than 24 ft (7.3 m), 6-in. (152-mm) thick planks
are adequate. In a steel deck system, a 7-V2-in. (190-mm) deep deck is required for spans up to 30 ft (9.15 m), and for spans up to 24 ft (7.3 m), a 6-in. (152-mm) deep steel deck is adequate. When precast planks are used, shear transfer is achieved by using welded plates cast in the planks or by welding shear connectors to the truss chords.
For metal deck systems, generally adequate shear transfer is achieved by welding the steel deck to the trusses. Planks used for erection purposes should have connection weld plates, even when shear connectors are provided. The choice of the floor system
depends on the geographical location as well as local conditions. In cold climates, the cost of grouting between precast planks in winter is increased by the necessity for heating.
The floor system may consist of either a series of simple or continuous spans over the chords of the trusses. Because of the large spacing between the trusses, continuous
spans are usually limited to a maximum of two bays. Generally, one end of each span is supported on the lower chord, while the other end is made continuous by simply running the floor system across the top chord of the trusses.
Since the trusses are staggered at alternate floors, the equivalent lateral load on each truss is equal to the lateral load acting on two bays. Hence, floor panels on each side of the truss must transmit half that load to the adjacent truss in the story immediately below. The floor system is designed as a deep beam to resist both the in-plane shears and bending moments. Gravity load design is identical to that of a conventional system. Because the integrity of the system depends on the ability of the diaphragm to transfer lateral loads from one truss to another, it must be detailed with chord elements at the boundaries to resist axial compression and tension.
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