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Re: "Hold-Down Eccentricity and Capacity of the Vertical Wood Member"

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Terri, Dennis--
I've been wondering why there had been no response to Ron Hamburger's paper
in "Building Standards".  It looked like a well written piece, backed by
both lab and field observations and certainly worth some serious
discussion.  (Posts did snap during Northridge and they do fail prematurely
in lab tests.)

We got into this situation precisely because the shearwall/tiedown assembly
is a "system" and neither party (APA and tiedown manufacturers) wants to
take responsibility for the performance of the "other guy's" component.  So
the tiedowns got tested in steel jigs and the pywood/OSB is tested with
grossly oversized tiedowns.  ICBO should have intervened long ago, but they
operate on a consensus basis too, I guess.  So we now a have "system" whose
performance is less than the sum of its parts.

My criticism of Ron's paper would be that it's a little non conservative to
assume the eccentricity at the post to be half its width.  You could argue
that the full post width minus 3/8" (the way many posts get nailed) is more
correct (plus the tiedown offset of course).

As to Terri's posts (wooden and verbal):
1.  True, the typical post is 8' long, but the big problems (high loads and
eccentricities) occur in multi-story buildings with stacked shearwalls.  The
load in the ground floor post comes mainly from the the tiedown above, not
from the nails in the ground floor shearwall.  It is a little conservative
to assume that the weakest material coincides with the bolt holes, but how
else would you perform the analysis?

2.  You could subtract the 18" below the post but you'd still have 23' of
nailing above you in a 3 story building.  Why bother?  (Notice that Simpson
raised their attachment point in the "Strongwall" to take advantage of this
and some of their tests snapped posts.)

3.  Ahh, the meat of the coconut.  The tie down deflections allow the post
to lift, the deflection must be accommodated by the local nails at the panel
corners, lab test failures almost always initiate at the lower corners due
to this uplift movement combining with the maximum panel rotational
displacement that also occurs at the panel corners.  To assume that these
nails can also "protect" the post is wildly optimistic.
There is a serious need for more testing of assemblies but it needs a
different test setup to address this issue.  The recent tests by USP and
Simpson look to be more obfuscation.  A real test would require the addition
of tensile loading of the post to simulate the typical load/deflection of an
HD10A in a shearwall.

4.  I guess I don't follow your reasoning here.  It is the rotational
restraint of the tie down that creates the resisting moment to counteract
the post/tiedown eccentricity.  (However, there is another phenomenon that
is seldom discussed.  To the extent that the tiedown restrains the post
against its tendency to rotate with the sheathing, an additional bending
moment is generated for the poor post to deal with.)

Your arithmetic ignored the the 1/3 increase for short duration loading
contained in Simpson's values.  But you're right "something is wrong".  The
manufacturer's have lead us to believe that their tiedowns are good for a
lot more than the post/tiedown can actually handle.  If you look carefully
you'll notice some CYA language in the catalogs that make this the
specifying engineer's problem!  Zone 4 has tabulated an extensive table of
values for various posts and eccentricities.

If you have high post loads you should be looking at paired tiedowns, Zone 4
tie downs (they don't eliminate the eccentricity, just reduce it and spread
it between 2 posts), MBR rods, or the other rod system (whose name eludes me
at the moment).

By the way, in looking at USP's web site I finally found out what the
"average ultimate" load values are.  Divide 'em by 2.

Chuck Utzman, PE