When it’s time to design the diaphragm system of a structure, consisting of open web steel joists, steel deck and structural steel, there are many factors to consider. More specifically, different solutions are available for transferring diaphragm forces from the steel deck to the collector beams depending on the structure and components of the building. Read on to learn about two possible design options that will make this task easier.
Shear connectors are the best option for transferring force to the collector beams perpendicular to the joists. Centered between the seats of the joists, these connectors also serve as a support for the steel deck.
Consider a structure where the steel deck is 38 mm (1½ in) deep and the joists have a standard seat. Using a hollow structural section (HSS) with a minimum width of 152 mm (6 in) is ideal to ensure contact between the bottom portion of the steel deck flute and the connector. It should have the same depth as the standard joist seats, that is 102 mm (4 in).
With this option, the designer must clearly illustrate the fastening pattern for attaching the metal deck to the connector, as well as the appropriate section for the application.
If you don’t opt for shear connectors, you can use perimeter angles installed on the joist seats, directly above the collector beam. With this option, the seats will transfer the forces from the perimeter angle to the collector beam.
However, the capacity of the seats to transfer lateral forces is limited. For a standard seat with a depth of 102 mm (4 in), a maximum factored value of 7 kN (1,500 lb.) can be considered. In order to improve this transfer capacity, modifications to the seats may be considered, but the force to be transferred should not exceed 25 kN (5,600 lb.).
If the building designer chooses this solution, they must be sure to indicate the lateral forces to be passed through the seats on the structural drawings, so that these forces are included in the joist design and seat detailing. Otherwise, the bolt diameter or seat thickness may not be adequate.
For structures where the joist has a top chord extension, placing the perimeter angle that collects the diaphragm forces at the end of this extension is strongly discouraged. The joist cannot support the induced rotational forces (lateral moment, Mry).
If this situation cannot be avoided, the force will have to be channeled to the collector beam using a different method, without using the joists. Again, this should be clearly indicated by the building designer in the structural drawings.
The design stage is critical to the success of a project, which is why it is important to consider all the options available to you. For any questions regarding diaphragm force transfer, floor and roof structures or your construction projects, our competent and experienced team is here to help.
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