Wood/Lag screw allowed tension values

[Ron247]

Does anyone have a reference or source for the allowed tension for Lag screws and Wood screws? I can easily find shear values and pullout values based on inches of thread into the wood but I have never seen any reference for just tension across the area. The other value that is hard to find is pulling the head through the wood. By my thinking, the pullout per inch of thread into wood is somewhat useless without having an allowed net tension and an allowed value for pulling the fastener head through the wood.

I cannot find any allowed stresses or loads for just tension. I also do not see any material specifications that list these values. I do not see anything in the NDS about this. Any help would be appreciated.

 

[JAE]

I would guess that the tension on the screw itself is higher than what the wood can resist for most lengths of screws/lags...so perhaps it never controls.
The NDS commentary doesn't say this though.

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[Celt83]

for head pull thru that was recently added to the NDS check out table 12.2F in the 2018 NDS.

Allowable tension values for the screw your best bet is to get some of the ESR reports from the typical suppliers.

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[JAE]

From

http://www.woodweb.com/Resources/wood_eng_handbook/Ch07.pdf

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[JAE]

A specialized screw ESR report. There might be other things like this "out there".

https://www.fastenmaster.com/tl_files/fastenmaster/download/ESR-1078.pdf

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[jayrod12]

I can't imagine a scenario where you would exceed the steel capacity of a lag screw before it pulls out of the wood. The longest thread length you can get on lag screws is 6" typically, if you checked the steel for the wood capacity using 6" of embedment, I'd bet you are no where in the vicinity of the steel capacity. You could check it as a standard low grade bolt for the steel checks.

 

[BridgeSmith]

The capacity of the screw itself should be fairly straightforward for any particular screw (cross-section area X tensile strength), but impossible to generalize due to the endless variety of cross sections and strengths of fasteners.

Personal experience has taught me that steel failure not a limit state that can be ignored. For a typical lag screw, maybe, but I've seen timber fasteners fail in tensile fracture. Heads pulling through is definitely a possible failure mode as well.

 

[Ron247]

Thanks everyone for the input.

Celt83, thanks for the heads up on the table in the new code, I never noticed it but only recently got the new NDS.

Jayrod12, To my knowledge, the only time tension of the net area controlled in one of my designs was fastening a metal bracket to wood. A panhead type wood screw can fail in tension before it pulls out of the wood because the wood pull-thru no longer controls. Also, you can special order fully threaded lags I think. For the most part, pull-through in wood appears to be one of the more likely failure modes to occur.

The following is a brief example of how these numbers compare:
A 1/4" diameter shank has about a 1/2" head across the long dimension of the head. Allowed pull through for 1/2" diameter head in 1.5" of wood is 339 lbs. Pull through of 1" of wood is 271 lbs.
Withdrawal per inch is 225 lbs per inch for a G of .5

With 20,000 psi allowed tension (same as A307), you can get a tension of 538 lbs based on root diameter and 932 lbs based on shank diameter. I use the root diameter for tension load since the start of the threads still has the full tension load on the area. I know bolts used for steel connections use full diameter, but I do not see using that for lags and wood screws unless the allowed stress was already "factored" to account for the difference. The smaller diameter lags have about a factor of 2 difference between full diameter and root diameter loads. Any opinions of which is the correct diameter to use for tension, shank or root?

You can buy a 1/4"x11" that has 6" threaded. Total pullout would be 1350 lbs at that thread length. The pullout is 1350/528 = 2.5 times the safe tensile load. It is even worse (4) for the pull-through.

 

[OldBldgGuy]

The current version of the Canadian Wood Council's Wood Design Manual includes screws and lags; previous version only included lags (>1/4"). It is recognized by AHJs in Canada; I would guess that the American Wood Council would have a comparable document. It has fastener capacities in any direction that you desire through wood or steel plates.