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Nails in Pull-Out
3

Nails in Pull-Out

Nails in Pull-Out

(OP)
Does anyone know of any empirical values for allowable nail pull-out forces?
I am in Alberta, Canada, and the design code we use here is the CSA 086.1-94.
There is a section on nails in pull-out, and it says that such a load on a nail can only be used in wind or earthquake design. I don't have a problem with that, since I'm designing for a wood-framed roof in the "uplift" mode.

The problem I have is with the values attributed to the nails are on the order of 25lb. I think this is ridiculously low, having pulled a few nails in my day. Furthermore, the tables do not distinguish between box nails, spiral nails, and the method of installation (hammerred or air-nailed).
I am having trouble getting a cost-effective design with values like these. It seems to me that this area of the code needs to be refined. But in the meantime, if anyone knows of an alternate source for design values please let me know.

RE: Nails in Pull-Out

2
I'm "down under" in the US and the American Forest & Paper Association publishes the NDS (National Design Standard) which provides tables of "withdrawal" values for various nails in wood, depending upon the specific gravity of the species you are using.

The tables are for all nail types (i.e., the nail type doesn't affect the pull out capacity).  For example, a 10d nail (0.128 inch dia. = 3.25 mm dia) in a Douglas Fir North wood (S.G. = 0.49) would yield a pull out capacity (allowable) of 26 lbs (115 N).  This value would have to be multiplied by a duration of load factor (depends on the sustained time of the load) as well as a few other factors pertaining to wet service or temperature, which usually don't control.  

The equation given is
          W = 1380 x G^(5/3) x D
where W is the pull out (withdrawal) capacity (lbs.)
      G is the specific gravity of the wood
      D is the diameter of the nail (inches)

The NDS states that nails and spikes shall not be loaded in withdrawal from end grain of wood.

You're right that the values are small, but with the moisture and temperature shrinkage characteristics of wood, pull-out can have a large variability in capacity, thus the larger than normal safety factor built in to the values.

RE: Nails in Pull-Out

(OP)
Thanks for your post

I suppose I'll have to live with these values and be more creative in my solutions!

I was concerned that perhaps the design resistances were unduly low. But you're right that the conditions for end nailing are more variable than side nailing or toenailing. The low values, while conservative in regular circumstances, are justified.

Thanks again

RE: Nails in Pull-Out

MathMan
Having designed many timber roof trusses in the past I would recommend that you convert your PULL OUT loads to SHEAR loads by using metal straps. Purpose made straps/ties are available from Simson Strong-Tie (www.strongtie.com). The allowable loads for nails in shear is much higher than the pull-out values.

Regards

Mike

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