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Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

Looking through table 9.3A of 1991 NDS, I am finding lag shear values that are significantly higher than the equivalent table, 11J from 2005 NDS.  The 1991 values seem to range somewhere between 20% and 100% higher than the 2005 values.  A similar issue occurs with tables 9.3B and 11k, respectively, and perhaps others as well.

I imagine there is a fairly simple explanation for this discrepancy, but I am not sure what it is.  Can someone help me out?

Thank you,


RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

Just to clarify, the change might have occurred sometime between 1991 and 2005, these just happen to be the two codes I have access to at this time.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

Sometimes NOMINAL values are listed versus allowable, so you need to apply applicable factor of safety values.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS


In this case both tables seem to have the same title and are allowable values.  So I'm not sure if your suggestion is the case.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

I don't have either of those NDS eds. on hand, but take a look at how many times over those years the lumber grades have changed, mostly lowered.  Allowable stressed in every direction have gone down, grading rules have changed, our design methods for bolts, etc. have changed, as have all of the multiplying factors for load duration, stresses, materials, etc.  It's getting tougher and tougher to pinpoint what causes some of these changes.  You would probably have to go directly to NDS code committee member, and I suspect some of them couldn't tell you exactly.   It's all an enigma wrapped in a riddle shrouded in a mystery.  Let us know when you find out.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

Maybe someone did some real testing for once??

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

IsaacStructural, I do not use NDS tables when calculating lag screws or bolts when calculating the tensile and lateral forces of lag bolts; I use Kent's ME handbook table which I am attaching as a JPEG.  I am now wandering how much of a difference do you see between the two references.  Obviously, as dhengr indicated, wood quality has changed.  Also there are parameters influencing the differences in values which you'll note in the attached JPEG.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

dhengr is correct..  They tweak these things all the time and it brings up a very important point: the specification and supplement must be used together. In other words, you cannot use the the specification form 2005 with the supplement from a previous year.

I suspect if one were to go through the numbers and all the adjustment factors for two separate specifications, the results would be relatively similar in most cases.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

If you lower the allowable stresses in the lumber, it will also lower the allowable loads on fasteners as these are related.  The allowable stresses in fasteners in wood are not generally controlled by the fastener strength but by the wood interaction.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

I believe that the change occurred in the 2001 edition of the NDS.  The change stemmed from the increased use of reduced body and fully threaded lag screws.  Using the fully threaded lags, the shear plane would obviously be within the threaded portion of the fastener.  Prior to 2001 the tabulated values were based on the full diameter of the Lag.

I have (hopefully) attached a link to information from AWC regarding the changes in the 2001 NDS.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

Mmillerpe is absolutely correct.  This is how fasteners like Timberlok can claim greater capacity than equivalent diameter lags.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

Watch out if you try to create a spreadsheet to calculate the values and compare them to the NDS tables.  For Zperp, the tables use dowel bearing strengths shown in Table 11.3.2.  Those bearing values are tabulated for full diameter, not reduced.  The lag screw tables use the full diameter dowel bearing strength, but then use the reduced diameter when checking failure modes.

RE: Lag Screw Shear Values, 1991 NDS vs. 2005 NDS

I think these last few posts really got to the issue I was noticing.

Thanks for all the help.

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