Alternate Nailing Patterns
Question:
Some truss lumber repair nailing patterns call for 16d common nails. Most nail guns do not support 16d nails, but have an equivalent to a 12/10d nail. Is there a substitution guide or ESR report that could help us?
Answer:
The best thing to do in this situation is to contact the truss manufacturer and ask that they provide a new nail schedule based on the smaller diameter nail. You might also ask that, in the future, they provide nailing schedules in terms of a nail sizes that can be more reasonably expected to be used on site.
The reason there is no easy substitution is that load calculations are based on a number of factors. Shear load values for connections using nails are primarily based on fastener diameter. However, nail length may also be a factor and this too may need to be taken into account, since it is likely that a minimum 10d penetration was assumed in the calculation. Another factor that may have been taken into account in the initial repair calculation is the added effect of clinching the fasteners. When a 16d common nail (0.162 in. diameter x 3.5 in. length) is replaced with a 10d/12d common nail (0.148 in diameters by 3 in. or 3.25 in. length) the capacity originally calculated for each nail is significantly reduced.
Per table 11N of The American Wood Council’s National Design Specification for Wood Construction (NDS) the shear value of a 0.162 diameter nail with a 1.5 in. side member thickness into SPF lumber is 120 pounds and for a 0.148 diameter nail is 100 lb. As a rule of thumb, the number of nails would need to be increased by a factor of 1/0.80 (about 20%). If ten 16d nails were specified, about twelve 10d might be required. One may be able to maintain the same nail spacing but might have to make the lumber repair longer to account for the extra nails, if space allows. And again, note that minimum penetration of 10d must be met.