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Re: Screw Pullout Values

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Michael L. Hemstad wrote:

Gary,
I designed a small stick-framed building a short time ago, and wanted to
use lag screws in withdrawal.  I could have gone to the NDS as Bill
Polhemus suggests, but in my youth I had a number of bad experiences
with hardware store lag screws repeatedly breaking during installation,
regardless of pilot holes, thread lubricant, and chanting over an
incense holder.  Thinking to step up in quality, I went to Simpson's
catalog and looked at their screws.  Nowhere in their catalog do they
give a withdrawal value.  Mind you, I'm talking about side-grain
withdrawal; they feel it's too unpredictable to tabulate.

Needless to say, end grain withdrawal is just that much less
trustworthy.

Now, when I'm not worrying about my structures breaking their fasteners,
I do some woodworking as a hobby.  I just finished a set of oak
bookshelves where I used sheetrock screws in end grain withdrawal to
hold the shelves in dadoes in the sides.  Those things took up like they
were in rock.  In fact, I overdrove one and broke it, but it didn't
strip (seems to be a theme with me, breaking screws).


If you think that's fun, try driving trim-head stainless into Ipe (Brazilian walnut/ironwood(r)). The stuff has a sp.gr>1 in some cases. You can't even sink a trimhead Robertson flush without stripping the recess unless you predrill _and_ csink. The NDS, iirc, has factors of safety in the 4-6 range for values, based in the variability of wood and the change in conditions from installation to service. If you've picked up PT SPine at a lumberyard, you know what I mean - 19% moisture is a joke. I should be able to pull a 10d sinker straight out, based the the NDS values. Even Oak timbers can have a lot of moisture unless you pay for them to be dry. And they can get pretty dry with RF kilns, aka microwaving, but you can add about 300% to the BF cost.

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