Seismic Retrofit of Foundations, Cripple Walls, and Soft Story

Seismic Retrofit of your Home Structure
The 1994 Northridge Earthquake and 1995 Kobe Earthquake demonstrated that earthquake damage to
buildings is influenced by construction materials, anchor bolts not effectively connected to the concrete
foundations, unbraced cripple walls of the crawl space or basement, soft story in the first floor due to large
openings without effective bracing, year built, and number of stories. Seismic retrofitting would ensure the
structural integrity of your home to withstand earthquakes with no or little structural damage, which will
ensure life safety and help in returning your family quickly back to their normal life style.

Earthquake Awareness and Preparedness

Retrofitting the Soft Story

		One important trait of typical home configurations in North America, Japan and elsewhere is to have
		more and larger windows and doors on the first floor than on other floors. This results in what is called a soft story, where earthquake damage to the house is concentrated on the first floor.
		Numerous buildings - regardless construction materials - were severely damaged, leaned, or
		collapsed as a result of the soft story of the first floor during the 1994 Northridge Earthquake and 1995 Kobe Earthquake, as shown in earthquake pictures 1 and 2.
		Houses with a soft story lacking interior supporting walls are particularly vulnerable to earthquake
		damage, especially if there is living space above them. Examples are buildings with large openings in the external walls such as garage doors and windows, or with garage and parking spots on the ground floor.
		The narrow walls on either side of a large opening must be able to support the horizontal seismic
		load that is transferred from the roof and the additional living space above into the concrete foundation below. In order for these narrow walls to support the load, they must be properly braced or strengthened.
		The garage door opening in sketch A is in line with the rest of the house where additional bracing
		may not be required. On the other hand in sketch B, bracing is most likely required.
		As shown in Figure 5, bracing the walls can be done by using steel bracing or specially-detailed
		plywood panels as recommended by a registered professional engineer or a qualified building contractor specialized in seismic retrofit. The cost is $5,000 to $25,000 according to the retrofitting measure.

Retrofitting the Foundations

Upgrading the Anchor Bolts or Steel Plates

		Go down into the crawl space (the area between the first floor and the foundation) to find
		out if your house is bolted or plated to its foundation. Look for the heads of anchor bolts that fasten the sill plate (the wooden board that sits directly on top of the foundation, which is called the mud sill) securely to the foundation, or for steel plates that are sometimes used instead of anchor bolts, as shown in Figures 1 and 2.

Figure 1 - Foundation with Anchor Bolts

Figure 2 - Foundation with Steel Plates

Retrofitting Post-and-Pier Foundations

		In such foundation type, the outside of the house is supported by wood posts resting on
		unconnected concrete piers. Sidings are often nailed to the outside of the posts, but are not strong enough to brace the post-and-pier foundation. During an earthquake these posts may fail and the house would shift and collapse.
		Consult a registered professional engineer or a licensed foundation contractor for a suitable retrofit
		which may include bracing the posts, or adding new reinforced concrete foundation and plywood walls in the crawl space. The cost is $1,000 to $25,000 according to the retrofit measure.

Retrofitting Unreinforced Masonry Foundations

		These are brick, concrete block, or stone foundations that cannot resist earthquake shaking and
		need to be replaced with reinforced concrete foundations.
		Consult a registered professional engineer to design the new foundation. The cost is $15,000 to
		$50,000 and can only be done by a licensed foundation contractor.

Retrofitting the Cripple Walls

		In wooden houses, wood floors and stud walls are built on top of an exterior foundation (short wood
		frame walls called cripple walls) to support the house and create a crawl space. These walls may collapse if they are not braced to resist the horizontal movement of an earthquake resulting in shift and collapse of the house.
		Go under the house through the crawl space to see if there are any cripple walls. If the cripple walls
		are covered on the exterior with only stucco (cement plaster) or wood siding, they are not strong enough to resist earthquakes.
		Older homes may have diagonal wood sheathing on the inside surface of the crawl space
		extending from the sill plate to the base of the floor joist. Inspect the conditions of the wood. If not good, retrofit according to the next paragraph. If the wood conditions are good, make sure that the diagonal sheathing is adequately nailed to each stud as shown on Figure 3. Inspect the conditions and spacing of the nails, and add nails if necessary.
		Wooden houses with cripple walls should have a minimum of 3/8 inch structural grade plywood
		panels on the inside surface of the crawl space extending from the sill plate to the base of the floor joist and adequately nailed to the studs. Metal brackets should also be connecting the rim joists to the top plates as shown on Figure 4.
		You (if experienced in home maintenance) or a building contractor can strengthen the cripple walls
		relatively inexpensively, as it costs $500 to $2,500. On the other hand, it may cost up to $25,000 to repair after an earthquake.

Figure 3 - Diagonal Wood Sheathing

Figure 4 - Retrofitting Cripple Walls using Plywood Panels

Figure 4 - Isometric

Figure 4 - Side View

Figure 4 - Elevation

Figure 5 - Soft Story Retrofit

Sketch A

Sketch B

Steps of Seismic Retrofitting Measures
The components of your home shall be secured to each other, so that they function as a single unit during
an earthquake, transferring the forces from the upper stories through a continuous load path to the walls or
braces then to the foundation. As discussed in the previous retrofitting sections, the earthquake damage
will most likely be concentrated on the first floor because of the foundations, cripple wall, or soft story
issues. Therefore, the best return on the investment of retrofit efforts is to initially address issues of the
lower portions of the house then upper portions as follows:

		Step 1: Foundations: Add epoxy anchors or steel plates, brace/replace the post-and-pier foundation,
		or strengthen/replace the unreinforced masonry foundation.
		Step 2: Brace the cripple walls.
		Step 3: Strengthen or brace the soft story.
		Step 4: Anchor or strengthen the connections of walls to the floors and roof.
		Step 5: Brace the chimney.
		Step 6: Retrofit inadequate major renovations.

Photo 1 - Building Leaned due to
Partial Failure of the Soft First Story

Photo 2 - Collapse of the Soft First Story

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Retrofitting the Masonry Chimney

		During Northridge Earthquake, about 60,000 masonry chimneys collapsed and caused serious
		damage and injury. In the mid 1990’s, building codes did not require houses with masonry chimneys to be reinforced or braced to the main structural components.
		Inspect the chimney for signs of significant cracking (greater than the width of a dime) along the
		mortar joints and ensure that mortar is in good condition and does not easily crumble when scraped with a screwdriver.
		Chimney repair can be achieved by adequately placing few metal straps secured to the structural
		members (floor, ceiling and roof joists) that would help in bracing the chimney
		Check with a qualified building contractor to brace the chimney, which costs $2,000 to $12,000.

Retrofitting Inadequate Major Renovations
If you did major renovations to your home in the past, or if you decide to do at any time in the near future, contact a registered professional engineer to ensure that all building codes requirements are being
met, and to determine that the new structure will have the ability to withstand earthquake forces. Such
renovations include:

		Replacing large portions of walls with windows or doors.
		Adding large skylights or additional stories.
		Opening large portions of existing floors, such as creating a two-story foyer.
		Additions that create an L-shape configuration.

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What are Seismic Retrofitting Measures?
Seismic retrofitting are measures taken for older homes to meet the current building codes by
improving their structural integrity to withstand moderate earthquakes with no-to-minor
structural damage and major earthquakes with minor-to-moderate structural damage. Seismic
retrofitting the structural system of your home is your own decision at your own risk according to
your financial situation as it may be costly. The retrofit process can be complex and typically
costs $3,000 to $10,000 or more according to the size and number of floors, and if the home
has a basement, a room over the garage, or built on steep hillsides. However, retrofitting
measures may be addressed one step at a time. The best return on the investment in
retrofitting efforts is to initially secure the home foundations followed by bracing the cripple
walls, then bracing or strengthening the walls of the soft story, and finally anchoring the
connections of the walls and the masonry chimney to the structural members of the floors and
roof.

How to decide on Seismic Retrofitting?
You should seismic retrofit the structural system of your home if you live within 30 miles from
active fault lines or within regions highly susceptible to liquefaction. You probably have to retrofit
it if you live within 30-50 miles from major faults, especially if your home is constructed before
the 1980’s. Major fault lines in California are identified in the USGS maps including San
Andreas Fault, Hayward Fault, San Jacinto Fault, Elsinore Fault, Calaveras Fault, San Gregorio
Fault, Garlock Fault, and Imperial Fault. The New Madrid Fault System and the Wabash Valley
Seismic Zone are the major faults in the Midwest.

Why Seismic Retrofitting?

		It is cheaper to retrofit now than to repair after an earthquake.
		Recent cost/benefit studies have shown that for every $1 spent in seismic retrofitting
		measures, there is a long-term savings of $3 to $16 according to the Institute for Building and Home Safety (IBHS).
		Observations from recent earthquakes demonstrated that injuries, loss of property, level
		of structural damage, time and efforts required for repair and recovery, and cost of repairs are significantly reduced in homes where retrofitting have been made.
		An appropriate seismic retrofit will reduce earthquake damage and save you money. A
		partial improvement of the weakest areas of your home would also be valuable if you start with safety-related improvements rather than property-protection improvements.
		The earthquake damage may be extensive enough to require demolishing the entire
		home.
		Performing structural repairs to your home after an earthquake would be very expensive
		because of the shortage in the available building contractors and structural engineers in the impacted area, as a result of the sudden high demand for their services.

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		Houses built before the mid 1990’s most likely have ½" diameter sill bolts with small, thin, round
		washers. If this is the case, you need to upgrade the bolt diameter to 5/8" or ¾" depending on the size of your house.
		The bolts shall be placed at closer intervals no more than 6 feet apart in a single story and 4 feet
		apart in a multistory building. The edge bolt shall be placed ¾ to 1 foot to the end of the board.
		Although epoxy anchors cost more than expansion anchors, however, they are recommended in
		case of older houses with weaker and/or cracked concrete foundations.
		Larger, thicker, square washers should also be used as they are required by current building codes
		to anchor the sill from the top by clamping it down more securely to the concrete foundations. Studies estimate that the use of square washers increases the holding strength of the bolts to the mudsill by up to 60%.
		Foundation bolts may deteriorate over time, especially in houses having wet or damp crawl spaces.
		The observation of surface rust at the top of the bolts can be an indication of hidden problems if moisture gets trapped between the wood mudsill and the concrete foundations. In such case, the connection strength of the rusted bolts to the foundations is severely reduced and you should consider bolt upgrading.
		Replacing or adding bolts to unsecured houses is one of the most important steps toward home
		safety against earthquakes. The cost is $250 to $5,000 and can be done by a building contractor or someone skilled at home maintenance. On the other hand, it may cost up to $25,000 to repair the foundation from earthquake damage, as the repair may require lifting the house, then placing it back on its concrete foundation.

Mobile Homes

		Mobile homes are prefabricated housing units that are trucked to the site and placed on isolated
		foundations, sometimes without any positive anchorage. Floors and roofs are constructed with plywood and outside surfaces are covered with sheet metal.
		Look under your mobile home. If you only see a metal or wood “skirt” on the outside with concrete
		blocks, masonry blocks, steel tripods, or jack stands supporting your home, you need to have an “engineered tie-down system” or an “earthquake-resistant bracing system” installed otherwise, your mobile home can be knocked off its foundations and damaged as well as breaking the utility lines.

Rental Buildings

		Although you have less control over the structural integrity of the rental building, but you do control
		which apartment or house you rent.
		Review how the construction materials affect your home safety in order to set your rental preference,
		knowing that staircases and balconies attached to the sides of rental buildings can break off in moderate-to-major earthquakes.
		Ask your landlord what measures have been taken to ensure the seismic safety of the building, and
		if you can secure furniture and water heater to the wall studs.