Intro: Earthquake Mitigation for a Slab-on-grade Wood Frame House
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
- Circular saw
- Claw Hammer
- Nail gun (optional)
- Nail Puller
- Hammer Drill
- Wood bits
- Wood Auger
- Concrete bits
- Levers or crowbar, blunt chisel
- Ear defenders
- Safety glasses
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
- Expanding concrete bolts
- Threaded steel rod, washers, nuts
- Steel brackets
- Epoxy resin
- Tuck tape
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)
In typical wood frame construction, exterior walls consist of 4x8ft sheets of plywood fastened to 2x4 or 2x6" vertical wooden studs. The studs support the weight of the second storey, while the plywood holds the studs in position and resists shear forces due to wind loading or during an earthquake. The wall is fastened to the foundation by means of a wooden sill plate - a horizontal 2x4 or 2x6" wooden member. The studs and plywood are nailed to the sill plate, which in turn is fastened to the foundation with concrete nails.
These nails are insufficient to withstand extreme sideways forces experienced during an earthquake.
The inside of the wall is finished with a plastic vapour barrier and gypsum wallboard, while the outside is finished with building paper
and wooden or plastic horizontal cladding. This is usually overlapped to prevent water ingress.
Mitigation to resist horizontal forces during a quake consists of fitting anchor bolts to secure the sill plate to the foundation. A building code may require a certain size and spacing of bolts, such as every four feet.
To better transfer sideways force to the wall, a steel bracket may be used to fasten the anchor bolts to the studs. While possibly not required by code, the extra effort and expense in this step is minimal. The lower portion of the plywood secured to the sill plate is often affected by moisture, which may affect its strength and ability to hold nails. Brackets specifically made for this purpose are available commercially, pre-drilled with multiple nailer holes on the vertical section and with a reinforced hole to accept the anchor bolt.
Where the sill plate is accessible on the inside, such as in an unfinished room or garage, the anchor bolts may simply be fitted from the inside of the house.
- Using the bracket as a template, drill through the sill plate with a wood bit of appropriate size
- Drill into the concrete below with a concrete bit using a hammer drill. It may be easier to drill a smaller pilot hole first
- Remove any cement dust from the hole with a vacuum cleaner. Using a "blow" attachment or a compressed air wand is more effective at removing dust at the bottom of the hole.
- Measure the depth of the hole with a probe to determine the bolt insertion depth.
- If using epoxy, pour epoxy into the hole and coat the rod with more resin
- Hammer the bolt into the hole until it is fully inserted. Fit a nut over the end of the bolt first to minimize burring of the bolt head, or to allow any burring to be removed by unthreading the nut.
- Remove the nut, fit the bracket over the bolt, replace the nut and tighten with a wrench.
- Bolt or nail the bracket to the stud. Drill pilot holes somewhat smaller than the nail or lag bolt diameter to minimize splitting of the wood.
Where the wall is finished on the inside, typically with wallboard, it is necessary to cut into the wall to gain access to the sill plate. The choices are either inside, though the wallboard, or outside, through the plywood.
Cutting through wallboard on the inside is very easy. However, it is messy, disruptive, and takes some time and skill to replace in a seamless manner, including repainting.
I did this for a few sill plates on internal walls, gaining access through the back of cupboards where an unsophisticated repair would not be noticed. But for the majority of the work along the outside wall, I elected to gain access through the plywood outside wall. This is easier than one might think.
The first step is to remove such items as gutter downspouts or decorative trim that may be secured to the outside of the siding. Then, remove the siding to a height of some three feet above ground level.
Wood siding is typically secured with finish nails, which are hard to explicitly remove but will pull through the wood. It has a tendency to split lengthways if insufficient care is taken.
If possible, start at one end of the siding plank. Lever the plank gently away from the underlying plywood, opening up an even gap. A straight tool such as a wide screwdriver or wood chisel can be used to start, then as a larger gap is produced, a curved tool such as a tire iron or crowbar may be used. Inserting a block under the tool as a fulcrum is useful to apply more pressure.
If access to the end is not possible, insert a chisel beneath the plank from underneath. Drive the chisel if possible the full width of the plank before levering the plank off, to minimize the risk or splitting. The planks may be pulled off carefully by hand. Some of the nails will remain attached to the plank, while some will pull through the plank and remain in the plywood.
Due to the overlapping design of the planks, it is necessary to slide the plank down from under the one above. It may be easiest to start with the bottom plank, or perhaps the second from bottom. The first one removed is the most difficult. Once a gap is made, it is easier to slide a chisel under the next plank.
Remove the nails from the plywood using a nail puller or claw hammer. Knock those in the planks proud of the surface with a hammer on the protruding nail before using the claw on the outside of the plank. Most nails can be reused.
Sometimes a plank will split despite care being taken. Planks may be repaired from the back using staples and glue without damage to the finish on the front. They are not structural, so the repair does not have to be particularly strong
Once the siding has been removed, fold back the building paper to gains access to the plywood underneath. If working in sections, the building paper may be cut vertically. The paper can be taped to the wall higher up to keep it out of the way.
With the paper out of the way, remove the nails holding any cladding to the plywood using a nail puller. Mine is designed to be driven under a nail head with a hammer. These nails are typically hidden under the siding, so it is less important not to make a mess of them.
Then make a horizontal through the plywood sheet. Sheets are typically 8ft high by 4ft wide, so the sheet may be cut the entire 4ft width. Set the circular saw depth to just more than the thickness of the plywood - it is important not to cut into the underlying studs which would weaken them.
Once the plywood sheet is cut, use the nail puller and claw hammer to remove all the securing nails. Then remove the cut section, exposing the sill plate.
Using the bracket as a template, drill through the sill plate using a wood bit. Then switch to a masonry bit with hammer action and drill the concrete to the required depth. A smaller pilot hole may help. If sufficient depth cannot be drilled, perhaps due to striking rebar or a flint, try a different location. Once the hole is complete, use a vacuum cleaner on suck, then blow, to remove any dust from the hole.
Proceed as for step 4, (Inside Access)
When the anchor bolts have been fitted, replace the plywood sheets. Consult your local building code for required nail size and spacing. The plywood is a shear wall and is intended to withstand shear forces during an earthquake, so it should be securely fastened to the studs. Nail above and below the cut into each stud, optionally using nail plates for extra strength.
If using a nail gun, ensure that the nail heads are not driven significantly into the wood as this impairs their holding ability.
Re-fit the building paper, using tuck tape to seal any cuts.
Nail the plywood to the sill plate. I used a rod-in-pipe nail driver to drive nails immediately adjacent to the metal cladding, which tended to spring back into position.
Re-fit the siding starting with the top plank. Insert the tongue of each plank under the next one up. It may help to drive nails partway to hold one end of the plank while fastening the other - if the position is not correct, they may be easily removed with a claw hammer. Seal any overlaps or holes with caulking.
Securing upper floors
Some building codes may require that upper floor membranes (plywood sheet floor) be secured to the foundations, particularly in multi-storey wood structures. This may be done with a threaded steel rod passed through the ceiling to secure the membrane, connected to anchor bolts in the foundation.
If this is not explicitly required, e.g. in 2-storey construction, it may nevertheless be desirable to secure the membrane by means of the wooden studs. Where an anchor bolt and bracket is fastened to the lower end of a stud, another bracket may be fastened to the upper end and a threaded rod passed through the ceiling joist to secure the membrane with a nut and washer, or metal plate.
Drill a clearance hole using an auger bit to pass the threaded rod.