I talked to a local Simpson rep. some time ago and he said one of the reasons for the footnote on post design for holdowns is for the post when it is in on compression end of the shear wall, in other words the holdown and post may work for the tension side but not the compression side.
Jason
From: Bill Allen [mailto:t.w.allen@cox.net]
Sent: Monday, January 12, 2009 10:52 AM
To: seaint@seaint.org
Subject: RE: Is it just me?
Hi, Gerard.
But that ICC report is only on the hold down, not on the posts. The catalog clearly states as Thor pointed out that a footnote reads "post design by specifier" or "Post design shall be by Designer."
On the few calculations I've done so far, a 4x post works for most cases using the gross section of the post. Of course, I have more calculations to do.
Quick, what do you think about a HD10A attached to a single 2x stud? That configuration has a capacity in the 2008 catalog of nearly 4,000 pounds! Seriously.
T. William (Bill) Allen, S.E.
Consulting Structural Engineers
V (949) 248-8588 • F(949) 209-2509
-----Original Message-----
From: Gerard Madden, SE [mailto:gmse4603@gmail.com]
Sent: Monday, January 12, 2009 9:39 AM
To: seaint@seaint.org
Subject: Re: Is it just me?
If you do this calc regularly, for most two story structures, you are looking at parallam hold-down posts to get this to work... get ready for a pissed off client when the contractor starts blaming us for the price of those. I based my calc on an example I found in the Zone Four catalog, but I have backed off of this since the simpson hold-downs have ICC reports in dbl 2x's
-gm
On Fri, Jan 9, 2009 at 4:41 PM, Haan, Scott M POA <Scott.M.Haan@usace.army.mil> wrote:
There was an article about this is "Structure" magazine or "Structural
Engineer" magazine about this 8 or 9 years ago. I wrote a spreadsheet for
it. It is hard to make laminated studs work even with composite action. My
spreadsheet assumed each stud just grabs an equal amount of the moment. I
think the reason Simpson says its on the specifier because by code it hardly
ever calcs out.
I wouldn't sleep tonight if I were you knowing all the walls the are going to
crack in half at the holdowns during the next big one.
-----Original Message-----
From: Bill Allen [mailto:t.w.allen@cox.net]
Sent: Friday, January 09, 2009 3:02 PM
To: Seaint
Subject: Is it just me?
When specifying a Simpson PHD hold down, one of the footnotes reads "Post
design by Specifier." In looking at the HDQ8 in 2-2x4s, the capacity is
listed as 5,715 lbs. Based on an eccentricity of 3"(CL=1.5" + 1.5" for one
2x), the weak axis bending moment due to the eccentricity is 1,428 ft.-lbs.
Assuming the 2-2Xs are face nailed adequately to transfer VQ/I stresses, this
moment results in a bending stress of 3,266 psi on the gross section. The
allowable stress on a 2x4 DF-L section is of course quite a bit lower than
this, not even considering combined stresses.
Have I forgotten how to properly draw a free body diagram or is there
something else going on here?
Otherwise, is it misleading to list 2-2Xs with a hold down of this capacity?
Regarding the VQ/I stresses, if the height of the studs are 8 feet, then the
shear on the post is Pe/h = (1428)/(8)= 179 lbs. Then VQ/I =
(179)((3.938)/(7.875)=90 lbs/in. Using 10d FN (capacity = 115 x 1.60 = 184
lbs each), the spacing would be 184/90 = 2" o.c.
This doesn't seem right to me.
If the two studs aren't nailed adequately to transfer VQ/I stresses, then the
bending stress due to the eccentricity is even higher (6,528 psi) since S
reduces to 2x1.313= 2.625 in3 from 5.25 in3.
Maybe I should put away the calculator on Friday afternoons.
If anyone would care to shed some light on the calculations, I would be most
appreciative.
Thanks,
T. William (Bill) Allen, S.E.
ALLEN DESIGNS <http://www.AllenDesigns.com>
Consulting Structural Engineers
V (949) 248-8588 * F(949) 209-2509
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