Seismic retrofitting an old house

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mhylmz

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Our house was built in 1956. We live close to San Andreas fault line, it is within 10 miles distance to the house, in San Francisco Bay Area.

The house has no cripple walls and it is a single story, ranch style house. It has a crawlspace, and a foundation wall of about 2 ft high. The floor system directly sits on the foundation wall. There is a 4x6 lumber on top of the 2x6 mudsill and the floor is nailed to 4x6 on the perimeter. Floor joists are also 4x6, spaced every 4 ft OC. Sill plate is bolted to foundation every 6 ft.

Due to a small renovation, I removed the plaster walls from some exterior walls. Now that the walls are open, I wonder what can be done for seismic retrofitting those walls. I will add foundation plates and framing angles below the flooring where it is needed. I will also add a plywood under drywall and use shear wall nailing pattern to increase lateral strength. But I don't see a good way to add a steel holdown in the open walls. It is too far from foundation and I need to drill through 10" of wood to get to foundation.

So, my questions are:
- Is a holdown needed in this case?
- Can a holdown be attached to the 4x6 lumber using 1/2" structural screws?
- What else can be done to strengthen the walls?

Reference: http://www.strongtie.com/ftp/fliers/F-SEISRETRGD12R.pdf
 
Welcome to the site, it great that you are looking at this. If you ever want to sell the house, having an engineers report might be asset. So I would have you get an engineer involved but if not Simpson makes a whole pile of different tie downs that could be used. If you can get the the tie down bolts that are there you could add a joiner nut and ready rod up to a tie down that you nail to the studs.
 
Our house was built in 1956. We live close to San Andreas fault line, it is within 10 miles distance to the house, in San Francisco Bay Area.

The house has no cripple walls and it is a single story, ranch style house. It has a crawlspace, and a foundation wall of about 2 ft high. The floor system directly sits on the foundation wall. There is a 4x6 lumber on top of the 2x6 mudsill and the floor is nailed to 4x6 on the perimeter. Floor joists are also 4x6, spaced every 4 ft OC. Sill plate is bolted to foundation every 6 ft.

Due to a small renovation, I removed the plaster walls from some exterior walls. Now that the walls are open, I wonder what can be done for seismic retrofitting those walls. I will add foundation plates and framing angles below the flooring where it is needed. I will also add a plywood under drywall and use shear wall nailing pattern to increase lateral strength. But I don't see a good way to add a steel holdown in the open walls. It is too far from foundation and I need to drill through 10" of wood to get to foundation.

So, my questions are:
- Is a holdown needed in this case?
- Can a holdown be attached to the 4x6 lumber using 1/2" structural screws?
- What else can be done to strengthen the walls?

Reference: http://www.strongtie.com/ftp/fliers/F- SEISRETRGD12R.pdf

If you are east of the fault, load up on Waste Management stock, because when the fault eventually becomes active, west side will become the trash recepticle.

An engineers report would be a waste of money.

There are retrofit sil tie-downs and tutorials, that can be spaced between the existing, however, more is not always better, because you can also cause the foundation to fail at their connection, if the spacing is too frequent. A common installation is a heavy "L" bracket that either bolts to the ledger and the foundation or just to the foundation.

There are also specialty CO. who specialize in this.

Because your floor system is 2-4-1 plywood nailed with 16D's, you can expect little or no diaphragm displacement.

1/2" OSB is the current recommended underlayment, case 1 layout and shear nailed, for stucco, which affords you the greatest shear resistance of the exterior finishes.

For the interior you can go as far as multiple layers of 5/8's drywall shear nailed, however the standard is 1/2" standard nailing pattern or sometimes 5/8 shear nailed with 6D.
 
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