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# Re: porches in seismic areas

• To: seaint(--nospam--at)seaint.org
• Subject: Re: porches in seismic areas
• From: Drew Morris <dmorris(--nospam--at)bbfm.com>
• Date: Thu, 15 May 2008 18:25:47 -0800

I seem to remember hearing about these double holdowns wood moment connections and how they failed during the Northridge EQ. I don't remember the details, but I expect the forces in the bolts were greater than designed and there was slippage. Does this seem familiar to any of you who did forensics after that EQ?
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Joseph R. Grill wrote:
```
```Yes, I was thinking lighter loads. Also, only "similar" to a Simpson base.
If the side plates are taller, say 12", then the bolts could have a 9"
spacing.  A 5/8" bolt in a 6x6 DF would give you 1420x1.6=2272 lbs per bolt.
At 9" couple that would give a 1704 ft-lb moment resistance.  At 7' you
could restrain a 240# lateral load. That would be for the bolts only.  At 90
degrees that same 240 lbs applied at the top (I'm being kind of general
here) of two 3" wide plates produces a moment at the bottom of the plates of
1440 in-lbs (if I'm correct). Looks like 2-3/8" plates might work at the 90
degree situation.  Depends on how big the porch and how many posts resisting
the seismic or wind. If deflections at the top of the post are agreeable, if
may be work.
Joe

-----Original Message-----
From: chris.slater(--nospam--at)gmail.com [mailto:chris.slater(--nospam--at)gmail.com] On Behalf Of
Chris Slater
Sent: Thursday, May 15, 2008 4:00 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: porches in seismic areas

I think the loads would have to be incredibly light.

Given a 7' column, with only 500# of lateral load, you get 3500 #-ft.
That's resisted by the couple in the bolts, at 3" apart.  So each bolt
winds up needing to be able to resist 14 kips.  Pretty sure NDS values
for through bolts in double shear are at the most in the 4 kip range.
Maybe less...

Am I missing something?

```
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