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Earthquake mitigation for a slab-on-grade wood frame house

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Wood frame construction is common in North America, and is quite resistant to earthquake damage. However, the house must be bolted to the foundations to resist sideways forces during an earthquake.
Slab-on-grade is a common construction technique in areas with a high water table (so basements are not feasible). The house is basically built on top of a flat concrete slab with footings around the edge.
In new construction, threaded rods are cast into  the concrete footings to secure the sill plates, but in older construction sometimes only a few nails are used and the house is essentially held in place by friction.

This instructable describes a process of retrofitting anchor bolts to the sill plates, with minimal disruption to the household
 
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Step 1:

Picture of
Tools required:
  • Circular saw
  • Claw Hammer
  • Nail gun (optional)
  • Nail Puller
  • Hammer Drill
  • Wood bits
  • Wood Auger
  • Concrete bits
  • Levers or crowbar, blunt chisel
  • Wrench
  • Ear defenders
  • Safety glasses
  • Stapler
Notes on Hammer Drills

I used a consumer grade hammer drill. I tried several models and returned them to the store for various problems. On one, the motor brushes wore out after drilling a few holes. Another overheated. On a third, the electronic speed control failed; I suspect the vibration sheared off a component lead. It may have been better to rent or buy a professional grade tool. However, I don't consider that drilling a total of maybe 50 holes over a period of many weeks should overtax a consumer quality tool.

Notes on Masonry Bits

I used hammer grade twist drills with a spade-shaped carbide tip. After a period of use, the carbide tip wore down and became rounded over at the edge. I was able to refinish the edge using a diamond cutting wheel as a grinder.
It is possible that using a cutting lubricant such as flowing  water would have extended the life, however I wished to use resin and did not want to saturate the concrete with water.
The bits become hot while working. It is important not to allow them to become too hot which will affect the temper of the steel. I cooled them occasionally by dipping them in a pot of water

Materials Required:
  • Expanding concrete bolts
  • Threaded steel rod, washers, nuts
  • Steel brackets
  • Nails
  • Epoxy resin
  • Tuck tape
Consult your local building codes for specifications for nails, size and length of anchor bolts, bolt spacing etc.
For example, a code may specify 1/2" bolts every 4 feet extending 6" into the foundations, using either expanding anchors or chemical fastening (epoxy)
fzumrk2 years ago
I highly recommend that anyone attempting to implement this type of retrofit consult with a local engineer before proceeding. Building code requirements will vary with your locality. The design of seismic and wind load resisting systems for residential construction is a fairly complicated topic. The proper design of anchor bolts embedded in concrete is also a complicated topic.

Some additional comments:

Step 3: Note that the plywood sheathing carries most of the lateral load, not the stud to bottom plate connection. Reinforcing the stud to bottom plate connection for shear is not really necessary.

Step 4: The type of bracket you show in the photo is designed to resist an uplift force, not a horizontal shear force. The manufacturer representatives with Simpson Strong Tie and/or USP can assist you in selecting appropriate connector products for a given application.

When using epoxy anchors, you want to use products that are tested and approved for this purpose. The major manufacturers of these products include Hilti, Simpson Strong Tie, Epcon, and Powers. Again, I recommend talking to the manufacturer's representatives to select appropriate products. The strength of adhesive anchors can vary greatly depending on the type of epoxy, anchor diameter, spacing between anchors, proximity of the anchor to the free edge of the concrete, depth of embedment, and strength of the concrete.

Step 9: When replacing the plywood sheathing, I would recommend installing 2x4 blocking behind the plywood between the studs. The blocking would be set with the wide face vertical. The existing sheathing above and new sheathing below can then be continuously nailed to the blocking. Size and spacing of nails should meet local code requirements.

Although you can buy those nailing plates at home centers, they are not load rated for retrofit applications. They are commonly used in factory built truss fabrication, but in this case they are installed under controlled conditions with special presses. Simpson Strong Tie and USP make load rated connectors for this type of application, but you will need to talk to them to select an appropriate product.

Step 10: The angle bracket shown is probably not a good choice for this application. Again, the manufacturer's reps should be consulted for appropriate product choice.
kelseymh2 years ago
This is a great instructable for an experienced do-it-yourselfer! And it doesn't just apply to the West Coast -- as several recent news items remind us, there are epicontinental earthquakes which can affect the midwest and east coast as well.