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Re: 3X Plates and Washers on Intermediate Anchors[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: 3X Plates and Washers on Intermediate Anchors
- From: "Robert Kazanjy" <rkazanjy(--nospam--at)gmail.com>
- Date: Thu, 2 Mar 2006 18:09:51 -0800
IMO your 100% correct in suggesting that intermediate "shear" anchors have no washers & no nuts.
As previously stated in the "Perforated shear wall uplift" thread
Having executed & witnesses 100's of shear wall tests, I believe that adding large square washers under the nuts anchors to resist cross grain bending of the sill is the wrong approach, esp near the HD
Not having (or minimizing) cross grain bending is the better way to address the situation.
The REAL solution is using a hold down with that develops adequate load at very small post uplifts. If the hold down doesn't do its job the sheathing will attempt to.
Even with a VERY sturdy hold own (like the Ben Schmid used for the COLA testing) ~80% of the uplift is resisted by the HD. The remaining 20% goes through the sheathing & into the sill (when anchor at 12" has nut & square washer). Omitting the nut & washer on the shear anchors close to the HD's is a better idea.
Increase the flexibility of the sheathing / sill load path & the HD will take more of the load; as it should.
The other reason in support of a 3x sill is that there's really no a great deal of wood to nail into since 2x's have gotten so thin. When a rough 2x was closer to 2", 2x sills were fine. Now that they're 1.5", they're a little skinny.
So........... a 3x (2.5") isn't really needed but 1.5" won't do. A Real 2x would do the trick.
Now for everyone who thinks that drilled holes get bigger when wood dries out...............you are mistaken.
A number of years ago at the Wood Solutions Fair in Long Beach, a discussion ensued about cause / source of over sized holes in wooden members here's how the sides broke down
Bill Warren, SE , Rawn Nelson, SE & John Coil, SE all believed that the holes drilled in green timber got bigger as the wood dried because "the wood shrank away from the surface of the hole"
John Hall (CalTech) , Tara Huchinson, PE, PhD (UCI) & Bob Kazanjy , PE (UCI) all believed that the green wood shrinks as it dries and the hole shrinks with it.
I was unwilling to bet Bill because of the "stakes" involved :)
but I was pretty certain that my group was correct. We likened it to a thermal expansion / contraction problem
Feeling that "one good test" is worth a hundred expert opinions & to prove the point................
I soaked several chunks of 2x4 for weeks to the point of saturation.
I then hole sawed a 1.5" hole through the face of the 2x4's & shoved a 1.5" rod into the hole , took pictures & removed the rod.
I let the chunks dry for a few weeks & the result.........................
The rod no longer fit!
The hole was now oval; fairly large shrinkage across the face but none (no measurable shrinkage) in the longitudinal direction.
If anyone is interested still have the specimens.
If interested, the experiment could be roughly reproduced using a kitchen sponge except the cellular structure is somewhat different.
So, as I understand it, the requirements for 3X sill plates for shear walls with Fv > 350 PLF (CBC Table 23-II-I-1, footnote 3) and square plate washers (CBC Section 1806.6.1) on intermediate anchors are a result of observations that, when wood shear walls lift (due to lack of restraint at the hold downs), the sill plate will tend to split. The square washer distributes the load better and the thicker 3X sill plate will provide a better section to resist flexure. O.K., I understand that.
Maybe these are two separate issues.
What about another solution? What if the nuts (and washers) were left off entirely? It certainly seems to me that would eliminate the requirement for the square plate washers (if none were required in the first place). I'm not sure if this would be a remedy for the 3X sill plates. I understand the requirement for 3X members at abutting panel edges to reduce splitting, but I'm guessing the 3X sill plate requirement is to reduce flexural failure. I'm sure I will be corrected if I'm mistaken.
All code references are to the 2001 CBC (sorry).
Just a thought. Comments are welcome.
T. William (Bill) Allen, S.E.
Consulting Structural Engineers
V (949) 248-8588 • F(949) 209-2509
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