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wcap 10-18-2009 10:18 AM

How do I retrofit-anchor posts to concrete?
Just bought an 1896-built house with a daylight basement. Inside the basement are several timber posts (8x8 and 6x8, probably Doug fir) that are standing on concrete pier blocks embedded in concrete floor. I searched Simpson ties site but nothing seems clear as a way to anchor posts for seismic resistance. We're in the Pacific Northwest on the coast and waiting for the Big One. Any suggestions please?

CookeCarpentry 10-18-2009 10:39 AM

In my opinion, two ways to look at this. 1) the house has been there since 1896 and it's fine - don't fix something that isn't broke. or 2) you might need to consult a structural engineer, as the posts may require removal, pouring of a proper concrete footing (with approved seismic area anchors) and new structural posts.

Daniel Holzman 10-18-2009 10:42 AM

You have hit on a very complex problem, specifically retrofitting columns for seismic resistance. Check the Simpson section on MAS/MASA/MASAP/MASB/MASP for their comments on seismic connections for mudsills. Also check their anchor bolt section SSTB for discussion on installation of anchor bolts in concrete for seismic resistance. There are numerous other places that seismic design is mentioned in the Simpson 2009 catalog, you can find them by downloading the catalog in PDF format and searching seismic, but I don't think any of the other sections are relevant to a column.

Part of the complexity is that unless you have detailed information about the footing for the column, you are not going to be able to tell where the weak link in your system is. For example, if the footer is inadequately sized, and unreinforced, the footer is likely to crack and fail in an earthquake, which will make even the best connection fail.

If you are really concerned, one reasonable option is to attach a pair of steel angles to the columns using bolts, and install epoxied anchors through the angles. If you used 6x6 angles, you could install a pair of 1/2 inch diameter rods per angle. You would have to drill at least 4 feet through the concrete and into the soil below, then anchor the rods in place using a suitable epoxy. Not a simple job, but at least it would attach the columns to the base, and improve the potential survivability of the house in the event of the big one.

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