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Our 1951 house has 2x4" sill plates atop of concrete. We are in seismic zone D.

How frequently should the sill plate be bolted to the concrete?

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Depends on the load (weight and height above sill) acting parallel to the wall and sill plate.

We’ve learned that a load on the wall will just unzip it from the sill plate as the load gets transferred along the wall without a shear anchor bolt to stop the load in the first foot or so. That is to say that a bolt installed in the first foot of the wall will stop the load from traveling along the wall and just unzipping the wall from the sill plate.

Remember, the holddown must be installed in a 8” thick footing wall for it to be effective.

Generally in Zone D a 5/8” dia. x 24” deep rod with hook will be acceptable in a one story building with a roof that transfers the load to the sill using solid sheathing and nailed at 6” oc staggered.

I’d use something like this at each corner both ways:

https://www.google.com/search?q=simpson+hdu8-sds2.5&rlz=1CDGOYI_enUS900US900&oq=simpson+hdu&aqs=chrome.2.69i57j0i512l5.13468j0j7&hl=en-US&sourceid=chrome-mobile&ie=UTF-8#imgrc=XrxmqTzahUeAiM

Also, I’d use a 5/8” x 10” anchor bolt with hook at 24” oc along the wall and nail the sheathing 4” oc into sill.

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  • Questions: (1) What do you mean by "[...] and nailed at 6” oc staggered"? Are you referring to the nailing schedule (6", 4", 2") around the plywood sheathing, or something else? (2) Also, this is existing concrete, so we won't get a hook at the end. Do we need to drill down 24" and epoxy? Or something else?
    – KJ7LNW
    Commented Oct 18, 2021 at 23:46
  • Also what did you mean by "[...] at each corner both ways" ? I understood that to mean put a bolt at each end of each wall when the walls meet at a corner, so a total of 2 bolts at each corner; is this correct?
    – KJ7LNW
    Commented Oct 18, 2021 at 23:55
  • Yes, add two holddown bolts at each corner (one for each direction of resistance).
    – Lee Sam
    Commented Oct 19, 2021 at 0:24
  • Thanks! Can you also answer the 2 questions in my other comment?
    – KJ7LNW
    Commented Oct 19, 2021 at 0:26
  • Installing nails to secure the plywood (structural sheathing) to the sill by staggering the nails into the sole plate will help keep the sole plate from splitting out and being useless. If concrete wall is already poured, there are many “retrofit” anchors (try Simpson on Google) that can be used by drilling into the concrete wall and installing an “expansion anchor”.
    – Lee Sam
    Commented Oct 19, 2021 at 0:31

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