Need a book? Engineering books recommendations...
RE: Shear Walls and 3x Framing (UBC)[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Shear Walls and 3x Framing (UBC)
- From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
- Date: Thu, 11 Dec 2003 23:07:45 -0800
Title: RE: DC PE needed (drawings review)
I’m a middle of the road pragmatist rather than leaning one way or the other – I guess they say I vote my conscience a party. At any rate, it might just be Southern California and if it is, this is a very sorrowful thing for the lower half of the state to have to admit.
As to you second question – The stud knows its not suppose to split when we explain whether or not we used full compliance or Simplified Static design :>) Not to be funny about it, I think the addition in the code to use 2-1/2” square by 3/16” plate washers and 5/8” diameter anchor bolts helped prevent cross-grain tension failures which is what we are discussing. Increasing the area of the washer reduces the failures that we have seen prior to this emergency measure to use 3x plates set up by the City of Los Angeles after the Northridge Earthquake.
I don’t want to take any glory away from the North, so let me know if SEAONC had the same emergency measure after the Loma Prieta earthquake that we in the south copied. I know that Ben Schmidt was up there reviewing damaged homes, but this led to the discovery of issues related to cripple wall collapse and sliding due to lack of anchorage problems on older homes.
On another point, retrofit of Unreinforced Masonry buildings in the 80’s taught many of us to avoid using 10d Common nails in 2x members because of the higher probability of splitting the 2x member. Subsequently, I never specify 10d nails in 2x members even at 6” centers.
Wood is smarter than you think :>)
Happy Holidays my friend,
Merry Christmas Dennis-
Tom (and others);
If memory serves me, the provision to provide a 3x sill plate (not 3x minimum for nailing closer than 6” – especially when 8d are used in lieu of 10d) was an emergency measure that arose from the damage noted from the Northridge earthquake. This was before plate washers was required for anchor bolts and had more to do with preventing splitting of the mudsill. I don’t believe this predicated a need to provide 3x or wider for studs when nailing was closer than 6”.
The “stupidity” arises when the code allows 3/8” plywood values to be the same as 15/32” because the code is not clear about over-nailing issues. When a 15/32” panel is over-nailed by one ply, the panel becomes the same as a 3/8” (five ply down to four) panel. Yet when you over-nail a 3/8” panel you destroy the shear value and have to resort to the APA’s document on allowable over-nailing.
I’ve found that states with less risk tend to provide greater use of plywood on their residential buildings than California. While traveling by car from California to Chicago, I found that almost all states from Arizona to Illinois panel the majority of the building while California not only attempts to comply with the minimum requirements of the code, but does so using the least expensive materials. I’ve seen this on local developments of homes in golf course communities that are sold by Sunrise Corporation and are in the $750,000.00 plus range.
It doesn’t make much sense to me but I was just confronted by a client who accused me (without really understanding the costs) of adding thousands of dollars to his home by over-designing. In reality, I added $370.00 to a home that is nearly six thousand square feet and had to leave off the redundant sheathing I would normally use with proprietary shear panels.
I don’t see NCSEA doing much to educate the public who, at least in my area loaded with “snowbirds” from the Midwest and East Coast who believe an engineer is a necessary evil whose only purpose is to secure a building permit.
Dennis S. Wish, PE
FWIW, I've been told that the thought process for this provision is to differentiate between standard walls (conventional construction) which are typically nailed at 6" oc and walls that are specifically designed to resist shear, which typically have nail spacing of 4" oc and closer. Although I don't agree with the way the "trigger" is set, it appears that it is meeting it's supposed goal.
15/32" Str. I sheathing 8d nails @ 6" oc = 280 plf
3/8" Str. I sheathing 8d nails @ 4" oc = 360 plf (bumped to 430 if studs @ 16" oc)
15/32" Str. I sheathing 10d nails @ 6" oc = 340 plf
15/32" Str. I sheathing 10d nails @ 4" oc = 430 plf
Now the question I have is why not just write the code provision to state the intended purpose instead of coming up with what most rational people would think is an arbitrary trigger. Why not just state that if nails are closer than 6" oc....?
Just my take, and APA had nothing to do with this code language.
I shear DEMAND is 350 plf AND LESS = 2x framing
Greater = 3x.
This falls under the “Very stupid code” category, like we really know the if the actual shear is going 349 plf or 351 plf…
According to footnote 3 under UBC Table 23-II-I-1, any shear wall with a capacity greater than 350 PLF in seismic zones 3 & 4 need 3x members at abutting panel edges and foundation sill plates (unless the anchor bolts are designed for half…).
According to the table, a shear wall 15/32” CDX w/8d @ 4” has a tabulated capacity of 380 PLF.
If I design a structure such that wherever I have a shear wall meeting the above specification the design load is no more than 349 PLF, do I still need 3x members at abutting panel edges and foundation sill plates when my anchor bolts are not designed for half capacity? The concern I have is that, even though I limit the design force in this shear wall to no more than 349 PLF, it still has a capacity of 380 PLF.
T. William (Bill) Allen, S.E. (CA #2607)
V/F (949) 248-8588
San Juan Capistrano, CA
- Re: Shear Walls and 3x Framing (UBC)
- From: chuckuc
- Re: Shear Walls and 3x Framing (UBC)
- Prev by Subject: Re: Shear Walls and 3x Framing (UBC)
- Next by Subject: Re: Shear Walls and 3x Framing (UBC)
- Previous by thread: Re: Shear Walls and 3x Framing (UBC)
- Next by thread: Re: Shear Walls and 3x Framing (UBC)