A: I used 3" galvanized spiral nails.
A: Depends on where you are going to use them. For deck hangers and such I would suggest nails are they have higher shear strength. However, for deck boards I would use screws.
A: These nails can be part of the process. These nails are for fastening metal joist hangers etc to the structural members. They really aren't for attaching the decking to the joists. Outdoor screws are a good choice for attaching the decking to the joists. There are nails specifically designed for decking as well. Outdoor screws are not a good choice for building the structural framework. Outdoor screws are brittle. They break easily. They can shear off under stress. Look for "structural" screws for assembling the framework. They are made with higher strength. There are building codes that regulate all this in many areas of the United States. Read about whatever product you buy to make sure it is compatible with treated lumber.
A: The shear value will vary depending on what species of wood it is being fastened to, what gauge of steel it is going through, etc. Please refer to the National Design Specification (NDS) Table 12P which has allowable shear loads for nails with steel side members ranging from 20 to 12 gauge thickness. SCN nails are ASTM F1667 compliant, so they can be calculated per the equations in NDS (or customers can use the NDS tables). The complete NDS (and all the fastener tables) is available for free on the American Wood Council’s website: (Link to 2018 NDS) https://www.awc.org/pdf/codes-standards/publications/nds/AWC-NDS2018-ViewOnly-171117.pdf. Thank you!