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Re: Wood connector allowable values

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Nels,

I have not designed a connection since the addition of all the various formulas
that NDS provided.  Prior to that, my approach was much like yours except, I would
use two plates if I had two lines of bolts.  This significantly decreases the
potential for splitting from wood shrikage.  Your logic makes sense to me.  Longer
spacing should increase bolt values, not decrease them.  I do, somewhat,
understand the concept that a group of bolts are not uniformly loaded as we would
calculate, but the reductions you are talking about do not make a lot of sense to
me.

I firmly believe that the best connection is one that does not end up in distress
due to shrinkage.  Further, I would tend to use a spacing more on the order of 6"
than 3."  I think that we need to consider the stress parallel the grain
seriously, and to my way of thinking, increased bolt spacing minimizies the
additional stress on other bolt holes.

NRoselund(--nospam--at)aol.com wrote:

> I often calculate wood connection allowable values using the formulas of
> Division III of Chapter 23 of the 1994 UBC; I've entered many of the formuals
> for bolts, lags and wood screws into Mathcad worksheets.
>
> I try to make sense of the results before applying them to a project, and find
> that some of the Code formuals defy my attempts to give them intuitive
> meaning.  A function of a variable raised to the power of another variable in
> the denominator gets me completely boggled.  So I hope the folks who developed
> the formula were smarter than me and I just try to at least make some sense of
> the results.
>
> One example is the group action factor Cg of Section 2335.5.6.  One of my
> often-used details is a bolted wood drag strut splice.  Intuitively, I say
> that the greater the spacing for the bolts, the less likely is splitting of a
> drag-strut member when highly loaded in tension.  On that basis, I'm used to
> using long splices and 6" to 8", or even 12" bolt spacings rather than the
> minimum 4 diameter spacings.  The formula for Cg does not confirm my
> intuition.  For example, for 8 3/4" diameter bolts in a line, for example, the
> bolts at 3" spacings have allowable values of 14% and 22% greater than bolts
> at 6" and 8" spacing, respectively.  Would I really be doing a better design
> job with shorter splices?  Or how about clusters of bolts at 3" spacing at
> each end of a long splice?
>
> I have read in the NDS commentary, that the formula is taking into account
> non-uniform loading of connectors in a row, but I'm still skeptical.  I'm
> visualizing, for another example, a mudsill bolted to a foundation with bolts
> at 32", or 48", or whatever spacing is appropriate for the load to be
> transfered into the foundation at a shear wall.  Calculated with Cg, 3/4"
> diameter bolts at 32" have about 60% of the capacity that the Code allows if
> computed as individual bolts.  Does anyone use Cg for calculating mudsill
> anchor bolts?  Should we?
>
> What can someone say to relieve my skepticism?  Buddy Showalter, are you
> there?
>
> Another mudsill matter: several years ago, the SEAOSC Existing Building
> Committee had a subcommittee working on techniques to retrofit mudsill anchors
> in existing weak concrete.  We found that pretty crumby concrete (concrete
> that you can't tighten a wedge-anchor into) can develop the strength of a 1/2"
> diameter anchor bolt in a 2x4 mudsill.  We also found that at high loads, an
> anchor bolt being bent over by a displacing mudsill splits the mudsill by
> cutting into it on the side opposite the load.  We found that a square plate
> washer was very effecting in preventing that kind of damage.  Bill Walker of
> Simpson participated in the tests, and soon after, Simpson began selling their
> BP series of plates.  Now, I always require a BP-type plate on all of the
> bolted wood connections that I design for seismic forces.  I'm sure, based on
> what I saw, that they will have greater capacities than bolts with a standard
> cut washer, but I use the Code values, figuring we're getting better seismic
> response at small extra cost.  But, It would be good to know how to use the
> BP-type plates efficiently.  Are there any values, or any testing going on,
> for bolted-wood connections using steel plates instead of cut washers between
> the head or nut and the wood?
>
> Nels Roselund
> Structural Engineer
>