Maximum Lag Screw Size Penetrating into 2x Main Member

[JKW05]

Say we're fastening a nominal 2x member to another nominal 2x member with lag screws.

2015 NDS 12.1.4.6: "The minimum length of lag screw penetration, pmin, not including the length of the tapered tip, E, of the lag screw into the main member of single shear connections ... shall be 4D."

If a lag screw length is selected such that the full length of the tapered tip projects through the far side of the main member, then we would have a full 1-1/2" of penetration into the main member. But based on the 4D requirement it appears that the maximum allowable lag screw size is 3/8" diameter (4 * 3/8 = 1-1/2"). Additionally, a 50% reduction of the values in Table 12J would be required (p/8D = 1.5/(8*.375) = 0.5).

1.) Therefore, 1/2" lag screws are not permitted to be used for this condition. Is this a correct interpretation?

2.) 12.1.4.6 specifically uses D in the 4D requirement. Could a case be made for using the root diameter (Dr) to determine the 4D requirement?

Thanks for your input.

JKW

 

[shacked]

I have run onto this issue before and it does not make any sense to me.

Unless there is something else that I am missing, I see no reason why you couldn't use a 1/2" dia lag screw if it is sticking past the opposite side of the 2x.

I am curious to see the replies and or reasoning.

 

[BridgeSmith]

I assume the 4D requirement is to provide adequate embedment to develop the rated capacity of the lag screw, either in shear or tension. You can probably use a bigger lag screw, I just wouldn't count on it to provide more capacity than the 3/8" one.

 

[oldestguy]

I'd go to the WEB site for US Forest Products Lab and see if there may be some paper on the subject or a way to post a question to those experts.

 

[Ron247]

I do not know the answer this but do remember how many times I chased down why something was required that did not really make sense just to find out that was the parameters used in the testing. In other words, IF someone tested these lags and the conditions were always minimum 4D, they then made that a design requirement.

Appendix I may give some insight.

 

[trustthemodel]

From what I recall those minimum penetration limitations are just minimums to ensure the applicability of the yield limit equations.

That said, I think you'll be surprised to see that assuming a smaller diameter lag screw for the same application may actually give you higher reference design values because of those limitations.