Subject: Re: "Hold-Down Eccentricity and Capacity of the Vertical Wood Member"
From: chuckuc <chuckuc(--nospam--at)pacbell.net>
Date: Sat, 02 Jan 1999 21:19:16 -0800
It is at icbo.org, standards online, features and articles, nov. 1999.
Do you have the Simpson test data? UPS said they'd send me what they did
(actually Bob Kazanjy did the testing).
> To be fair to Ron Hamburger, I did not read the article in Building
> Standards. I am not an ICBO member and do not know if the article is
> archived on their web site. Would anyone who has the capability to scan the
> document and send it to me in PDF format (or any other that is readable) I
> would appreciate the help.
> I am very interested in the research and testing that you reference. I don't
> intend to be judgmental on any valid argument on this issue, but there seems
> to be strong opposition from each side (SEAOC and Simpson).
> I would appreciate any help I can get on obtaining the article. Thanks in
> Dennis S. Wish, PE
> -----Original Message-----
> From: chuckuc [mailto:chuckuc(--nospam--at)pacbell.net]
> Sent: Saturday, January 02, 1999 6:06 PM
> To: seaint(--nospam--at)seaint.org; seaint(--nospam--at)seaint.org
> Cc: Norm (E-mail)
> Subject: Re: "Hold-Down Eccentricity and Capacity of the Vertical Wood
> 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