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Wood connector allowable values

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Nels, yes I'm here. these are all good questions.

first a clarification. let's not mix "shrinkage" issues with "load distribution"
issues. there are wet service adjustment factors for connections, C_M, that
account for the shrinkage issues. group action factors, C_g, deal with load

for group action factor applicability, try to approach these cases from a
different standpoint. how does the load get distributed into the "group of
fasteners"? for a tension splice, where the entire load is transferred into the
"group" of bolts, the group action factor is applicable, because in this case,
there is evidence that loads will not be uniformly distributed to those
individual fasteners. for anchor bolts in a mudsill, however, the loads are
being distributed into the "individual" bolts along the length of the wall. in
this case each bolt is seeing more of a unit load. for your drag strut example,
you should decide whether it is acting more like a tension splice, where all the
load is coming into a "group" or "cluster" of bolts, or whether it is acting
more like a mudsill, where it is picking up unit loads along the length of a

regarding the formulas for calculating design values for connections, we are
working on a mechanics-based model that we hope to put in a technical report in
the next month or so. it should help you have a better "intuitive" feel for the
different yield modes that are represented. I'll also briefly discuss connection
design and those yield modes at the NDS Seminar at the LA Wood solutions fair on

Hope this helps.

Buddy Showalter, PE

From: raranous(--nospam--at)
To: seaint(--nospam--at)
Subject: Re: Wood connector allowable values


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
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. 
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
calculate, but the reductions you are talking about do not make a lot of sense

I firmly believe that the best connection is one that does not end up in
due to shrinkage.  Further, I would tend to use a spacing more on the order of
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) 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