## Pullout Force Questions

## Pullout Force Questions

(OP)

The first posting here, so please be kind. I'm somewhat out of my area, which is why I'm hoping the experts will help!

I'm trying to calculate something I never expected to be so complex, the pullout force of an 8mm lag bolt 2" into a pine 2x4. I researched this, and found a formula, using .41 as the specific gravity, on this site: https://docs.google.com/viewer?a=v&q=cache:uBG... page 8-12. Using that formula, p=8100*G^1.5*D^0.75*L, I come up with 149 lbs for 2 inches of thread penetration. Or just 75 lbs per inch. Which does seem low in my opinion, but I don't know.

However, on this forum, http://www.eng-tips.com/viewthread.cfm?qid=183649 , the pullout strength for a #10 wood screw into southern yellow pine (SYP) was stated, by CJJS, to be 163 lb/inch. And no negative comment implied toward CJJS, it is just that the formula used to calculate the 163 lb/inch of thread penetration wasn't shown. I guess it may be common knowledge among experts, but I'm not an expert, I'm just hoping to try to learn.

So I'm wondering if somebody here could either show me the formula that CJJS used and/or show me where my formula is wrong.

The help is

I'm trying to calculate something I never expected to be so complex, the pullout force of an 8mm lag bolt 2" into a pine 2x4. I researched this, and found a formula, using .41 as the specific gravity, on this site: https://docs.google.com/viewer?a=v&q=cache:uBG... page 8-12. Using that formula, p=8100*G^1.5*D^0.75*L, I come up with 149 lbs for 2 inches of thread penetration. Or just 75 lbs per inch. Which does seem low in my opinion, but I don't know.

However, on this forum, http://www.eng-tips.com/viewthread.cfm?qid=183649 , the pullout strength for a #10 wood screw into southern yellow pine (SYP) was stated, by CJJS, to be 163 lb/inch. And no negative comment implied toward CJJS, it is just that the formula used to calculate the 163 lb/inch of thread penetration wasn't shown. I guess it may be common knowledge among experts, but I'm not an expert, I'm just hoping to try to learn.

So I'm wondering if somebody here could either show me the formula that CJJS used and/or show me where my formula is wrong.

The help is

**greatly appreciated**.
## RE: Pullout Force Questions

Garth Dreger PE - AZ Phoenix area

As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

## RE: Pullout Force Questions

## RE: Pullout Force Questions

I've been doing more research, and have seen the NDS formula. I was confused by that first formula, is it possible it is a typographical error? It seemed though that the units were in inch-lbs.

Would the pullout force be the same if it was, instead of one 2x4 piece, it was two 1x2 pieces? I suppose the question is more about the repetitive factor for calculating, which with two it isn't, but I didn't know if it was worthy enough for a separate thread.

Thanks again for the help. I do appreciate it, and it is interesting to learn.

## RE: Pullout Force Questions

I didn't see any way to edit the previous posting.

## RE: Pullout Force Questions

## RE: Pullout Force Questions

## RE: Pullout Force Questions

For penetration into the side of the grain:

P=1500*D^(3/4)*G^(3/2) whereby D=diameter lag bolt shank in inch; G= specific gravity of wood species.

For penetration into the end of the wood grain reduce P shown above to 75%.

## RE: Pullout Force Questions

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190... - p8-12

"Penetration of the threaded part to a distance about seven times the shank diameter in the

denser species (specific gravity greater than 0.61) and 10 to 12 times the shank diameter in the less dense species (specific gravity less than 0.42) will develop approximately the ultimate tensile strength of the lag screw. Penetrations at

intermediate densities may be found by straight-line interpolation."