If you look at the top of the stairs relative to the floor of the house, you can see this house fell six feet when the cripple walls collapsed and the house had to be torn down. The premiums paid for three years of earthquake insurance would have paid for a complete retrofit and saved the house. Familiarize Yourself with the Concepts
The fact that there is no building code for seismic retrofit work means seismic retrofit companies and earthquake retrofitting consultants have no guidelines to follow. This is true for East Bay and all Bay Area seismic retrofit companies. It is expected that this situation will remain the same until the California Earthquake Authority, "CEA", The California Department of Insurance, and the Association of Bay Area Governments "ABAG" fund the development of a retrofit building code.
The purpose of this part of the website is to describe the technical aspects of how earthquake protection for your home, and ultimately earthquake safety for you family, can be achieved. In this way, you will be able to decide for yourself what your home needs based on the most up-to-date structural engineering principles.
The concepts of how a retrofit works are simple. The basic idea is to keep the house from sliding off its foundation with adequate cripple wall bracing. Cripple wall bracing is achieved by retrofitting the cripple walls and turning them into retrofit shear walls. Here we will be discussing the three components of retrofit shear walls: foundation bolts, plywood and shear transfer ties. For an in-depth study of shear wall construction, see the book written on seismic engineering and shear wall construction at www.shearwalls.com.
Earthquake engineering principles are easily understood, though their application is often quite complex. In order to retrofit a house properly one must know how homes were damaged in previous earthquakes, be familiar with old building codes that explain how older homes were built, the earthquake resistance of no longer available building materials, the "National Design Specification"; which is the bible of retrofit structural engineering, metallurgy, old concrete and familiarity with test results from various steel and wood research laboratories. Above all, one must know how to apply this information to a house in a way that is cost-effective.
As a rule, structural engineers rarely understand how to implement retrofit engineering principles in a practical way that weighs cost versus benefit. This is because they are not familiar with tools and labor costs. It is not uncommon for a homeowner to pay $4,000 to a structural engineer for a design that is too expensive to implement.
Contractors on the other hand are familiar with tools and labor costs, but rarely have the understanding of earthquake engineering principals to do an effective retrofit. This often results in work that provides little or no benefit. See "False Security".
If you are concerned with the seismic safety of your home, it is best for you to take the time to understand the seismic engineering principles and get some sense of how such earthquake engineering applies to your house. Earthquake protection for you and your family is in your hands.
Basic Terminology
This first illustration shows a cripple wall under a house before it has been retrofitted. It is important to be able to identify the structural components of a cripple wall so that any proposals you receive from seismic companies can be better understood.
1. The Cripple Wall –the short wall between the floor and the foundation of the house. Not all houses have cripple walls. If your home was built before 1940 or built on a hillside you probably have cripple walls. Un-reinforced cripple walls readily collapse in earthquakes.
This is what your crawl space looks like under your house. The retrofit contractor does all of his work in the crawl space. Notice the un-braced cripple wall at the back.
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2. Floor - This is the area you walk on.
3. Foundation - This is the concrete found on the perimeter of the house that the cripple walls sit on top of. The cripple walls in turn supports the outside edges of the floor.
4. Mud Sill - This is a piece of old growth redwood that rests on top of the concrete foundation. All the earthquake force is ultimately transferred into the mudsill, which is why it must be bolted into the foundation.
5. Cripple Wall Framing - This is the 2 x 4 cripple wall framing upon which plywood is nailed and forms part of a shear wall. If the cripple wall framing collapses, the house will collapse with it. Retrofit shear walls are built to make sure that this force is transferred out of the floor, into the foundation, and then into the ground.
6. Floor Joist - The floor joists are underneath the floor and support the floor you walk on.
7. Top Plate - This is made of two horizontal 2x4's, an upper top plate and a lower top plate. The top plates are the upper part of the cripple wall framing on top of which the floor can slide.
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A cripple wall as seen from outside the house. This is the weakest element
in an unretrofitted house. Photo courtesy of Paul Rude
The photo below shows what can happen to a house if the cripple walls have not been retrofitted into shear walls.
Notice where the top of the steps is in relation to the floor of the house.
The illustration above shows a cripple wall that has been retrofitted into a shear wall.
Foundation Bolts - Foundation bolting attaches the mudsill to the foundation. There are several ways to achieve foundation bolting. Basically foundation bolts are long threaded steel rods that pass through the mudsill and deep into the foundation. Foundation bolting is a specialized field in earthquake engineering. Strength is greatly affected by the type of washer, the size and the type of bolt, as well as how far away the bolt is from the edge of the foundation and mudsill. Foundation repairs are rarely necessary. See The Structural Engineers Report on Old Foundations.
Mud Sill Plates - These are specially designed washers that increase the strength of foundation bolting by 60% and more importantly prevent the mud sill from splitting. They were developed after the Northridge earthquake where splitting of mud sills was very common.
Plywood - This keeps the 2 x 4's upright so they do not collapse. Tests done by the American Plywood Association have determined that the type of nails, nail spacing, the type of plywood used and many other factors affect the strength of the plywood component of a shear wall.
Shear Wall - This is the entire assembly of foundation bolts, plywood and shear transfer ties. These prevent the cripple wall from collapsing. All retrofit shear walls need these three components in order to provide your home earthquake protection.
Shear Transfer Ties - These are specially designed pieces of steel hardware that securely attach the floor framing to the upper top plate.
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Basic Concepts
Here we will further explore seismic retrofit principles in more depth in case you would like more technical information on building retrofit shear walls.
In order to build retrofit shear walls, one must concern oneself with the following:
1) The bracing of the cripple walls with plywood.
2) The bolting of the braced cripple walls to the foundation.
3) The attachment of the floor of the house to the braced cripple walls.
The following are simple illustrations why and how retrofit shear walls are built...
Bracing the Cripple Walls with Plywood Foundation
Figure 2: Damage to a house due to lack of foundation bolts.
The hatched areas in the drawing above represent plywood on the cripple wall. The cripple wall will not collapse because of the plywood, but the cripple wall can still slide on top of the foundation. This is why cripple walls need to be bolted to the foundation. Notice in Fig. 2 that plywood is only on part of the cripple wall. It is not necessary to put plywood on the entire cripple wall.
Figure 1: Damage to a house due to lack of cripple wall bracing
This is a cripple wall that has nearly collapsed and the house is in danger of falling off its foundation.
Bolting the Braced Cripple Walls to the Foundation
Figure 2: Damage to a house due to lack of foundation bolts.
The hatched areas in the drawing above represent plywood on the cripple wall. The cripple wall will not collapse because of the plywood, but the cripple wall can still slide on top of the foundation. This is why cripple walls need to be bolted to the foundation. Notice in Fig. 2 that plywood is only on part of the cripple wall. It is not necessary to put plywood on the entire cripple wall.
Installing a Bolt
A fully installed foundation bolt with a plate washer. The MSP is under the plate washer.
Figure 3: Damage to a house due to no connection of floor to cripple wall.
Attaching the Floor of the House to the Braced Cripple Walls
In figure 3, the cripple wall is now braced with plywood and bolted to the foundation. However, the floor can still slide off the top of the cripple wall along the upper top plate. For this reason it is necessary to attach the floor joists to the braced cripple wall with shear transfer ties. The number of bolts, shear transfer ties and amount of plywood needed is determined by a simple formula that is discussed in this article, "The Base Shear Formula".
The illustration below demonstrates what is known as a load path and explains how earthquake forces transfer from the floor you walk on into the ground. Your contractor should be able explain how this will be done.
Bolting: Attachment Of The Mudsill To The Foundation
If the mudsill of a house (the pink area in the illustration at right) is not bolted, the lateral loads (back and forth motions) of an earthquake can jerk the house off its foundation and cause it to collapse. Attachment of the mudsill to the foundation is accomplished with the use of foundation bolts.
This illustration shows earthquake forces pushing on the mudsill and the placement of bolts that prevent movement.
The following topics are discussed in this article. You can click on a topic to go directly to it or scroll down this page to view them all.
The next portion of this Retrofit Design series is Shear walls - bracing of the cripple walls.
Code requirements for bolting:
The Uniform Building Code (UBC) which is designed for new construction and is not intended for retrofit, specifies that only 5/8 inch bolts with plate washers may be used. They should be 6 ft. o.c. (on center) on single and two story homes. Three story homes require an engineer. Older building codes, 1994 and earlier, required 1/2 inch bolts spaced 6 ft. apart no matter how many stories.
The Uniform Code of Building Conservation (UCBC), which is a retrofit code applicable to existing homes, requires 1/2 inch a bolts with plate washers be installed 6 ft. o.c. on one story buildings, 1/2 inch bolts with plate washers installed 4 ft. o.c. on two story buildings, and 5/8 inch bolts with plate washers installed 4 ft. o.c. on three story buildings. The importance of plate washers will be discussed in this article.
Types of bolts:
The most common foundation bolts are either mechanical wedge anchors or epoxy bolts, 1/2 inch or 5/8 inch in diameter. The mechanical wedge anchor is installed by drilling a hole through the mudsill into the concrete, beating the wedge anchor into the hole with a sledge hammer and then tightening the bolt. A "wedge" on the bottom of the bolt expands while it is being tightened which secures the bolt into the concrete. An epoxy bolt is installed in the same manner except that it is glued into the hole with epoxy.
All seismic retrofit codes i.e., The International Conference of Building Officials (ICBO), The Uniform Code of Building Conservation (UCBC), and the Los Angeles Retrofit Code recognize no difference in earthquake resistance between wedge anchors and epoxy bolts as long as they are installed per manufacturer's installation instructions. Generally speaking, epoxy bolts are installed when the concrete is in poor condition or the bolts are being installed in brick. They are also the most common type of bolt installed in the San Francisco Bay Area, while mechanical wedge anchors are the most common type of bolt installed in Southern California. This difference merely reflects a "common construction practice" for these geographic areas. However epoxy bolts are always used when anticipated forces will be trying to pull the bolt upward out of the concrete, such as in the installation of hold-downs.
Rusting of bolts:
A common problem that occurs in areas with high moisture involves rusting of the bolts, regardless of which type is used. If a bolt is rusted to the point that the nut is corroded fast to the bolt and cannot be removed, the bolt should be replaced. When trying to remove the nuts and replace the washers, we have snapped off many rusted bolts. Even though the bolts look sound, they can be rusted away where the bolt and the concrete meet. We have seen bolts where less than 1/8 of an inch of the original bolt was left even though the nut and bolt looked intact. We recommend bolts showing severe rust be replaced with hot-dipped galvanized wedge anchors or epoxy bolts.
Recessed bolts:
Another problem is bolts that are recessed into the mudsill. This occurs because the bolts were not sticking up far enough when the foundation was poured. When the contractor installs his dimensional 2-by mudsill on a foundation that has bolts sticking up only 1-1/2 inches, he has to chisel out the mudsill around the bolts in order to get the nuts on. In this situation it is rarely possible to adequately tighten the nuts and it is impossible to install plate washers and mudsill plates. Recessed bolts should be considered marginally functional and should be replaced.
Bolt penetration:
A primary area of concern when examining the bolting of a building to the foundation is the amount of penetration into the concrete. While the UBC requires a 7 inch embedment into the concrete in new construction, the manufacturers of wedge anchors and other mechanical anchors require much less penetration. Manufacturers of 1/2 inch wedge anchors such as Hilti, Ramset/Red Head, Dualbolt, Rawl, etc. generally only require 3-1/2 inches embedment into the concrete for "minimum embedment" and 5-3/4 inches of embedment into the concrete for "standard embedment" into 2000 Psi concrete. 5/8 inch bolts usually require 4 inch minimum embedment and 6-1/4 inches standard embedment. Ratings in shear strength are higher for standard embedments. For example, according to I.C.B.O. Evaluation
Report 1372 dated February 1, 1997, a 1/2 inch bolt with minimum embedment can withstand 1190 pounds of force in shear while the same bolt with standard embedment can withstand 1810 pounds of force in shear. However, it must be remembered that for a 1/2 inch bolt, the building code allows a 2-by mudsill only 840 pounds in shear resistance irrespective of the bolt embedment. What this means in terms of the strength of the connection of the mudsill to the concrete is that it does not matter if the bolt is embedded 2 inches or 20 inches. The failure will occur when the piece of wood that forms the mudsill splits at 840 pounds of force no matter how deep the bolt extends into the concrete. Note photo at right.
Plate washers:
A major area of concern in accessing strength of a bolt is the type of washer used to connect it to the mudsill. As an earthquake tries to jerk the mudsill off the foundation, the foundation bolts do not snap off, rather the mudsill splits in two with the bolts acting
somewhat like the head of an ax. See the photo at right. The failure occurs in this manner because the connection of the bolt to the concrete foundation is much stronger than the connection of the bolt to the soft wooden mudsill. For this reason modern retrofit codes do all they can to strengthen the connection of the bolt to the mudsill by using special washers and by increasing the thickness of the lumber that forms the mudsill. After the Northridge earthquake, a tremendous amount of research was done to strengthen the bolt-to-mudsill connection since so many failures occurred at this location.
The most significant increase in strength in this connection was achieved by installing large square plate washers on the bolts instead of round cut washers. See the photo at left. This one simple change resulted in a 60% increase in strength. That is why the 1997 Uniform Building Code and the Uniform Code of Building Conservation require these washers on all foundation bolts. These plate washers, known as bearing plates in the Simpson catalog, are also commonly referred to as compression washers. The graph below demonstrates the advantages of using plate washers over round-cut washers.
The Y axis of this detail represents force applied to a mudsill measured in pounds. The X axis represents the amount of movement (deflection) of the mudsill in inches. The lower line measures deflection with a standard round-cut washer. The upper line measures deflection with a plate washer.
Oversized bolt holes:
The Uniform Building Code requires that a foundation bolt be installed in a hole whose diameter is no more than 1/16 of an inch larger than the diameter of the bolt. These compression washers go a long way in compensating for holes that are more than 1/16 of an inch larger. The most recent version of the Los Angeles retrofit code actually requires that oversized bolt holes be filled with epoxy. Needless to say, for any type of washer to work, the nuts must be tight. The problem with oversized bolt holes can been readily seen in the illustration below.
Mudsill plates
Harlen Metal Products also came up with a type of washer called a Mudsill Plate that is designed to increase the strength of the wood-to-bolt connection. This hardware mitigates problems caused by oversized holes in the mudsill. This hardware is so effective that the earthquake resistance of a bolt can be more than doubled by installing one of these washers on the top and one on the bottom of the mudsill. The UBC recognizes a 1/2 inch bolt with a standard washer as being able to resist 840 pounds of shear. ICBO report #1148 recognizes that installing one of these washers on top of the mudsill increases that resistance to 1340 pounds, a 59% increase in strength, while installing these washers on both the top and bottom of the mudsill increases the bolt strength to 2040 pounds, a 143% increase in strength. That's pretty good for a fifty-cent piece of hardware. The following chart shows the difference mudsill plates can make.
Bolting Homes With Low-Clearance
On many homes it is not possible to install foundation bolts because there is too little room between the top of the mudsill and the floor of the house. This is because either there is no cripple wall or the cripple wall is extremely short. In these cases special hardware known as "foundatoin anchors" are used to secure the house to the foundation.
There are seven commerically manufactured foundation anchors available. They are all similar in design and cost but vary greatly in strength. The International Conference of Building Officials has tested these foundation anchors to determine how much earthquake force each can resist. The weakest commonly used foundation anchor is the Simpson-FJA. It can resist only 185 pounds of earthquake force which is 15% or 1/8th of the force resisted by a 5/8 inch foundation bolt. The
strongest foundation anchor is the Harlen-RFP which is shown in the photo at the right. It can resist 1915 pounds of force which is 150% stronger than a 5/8 inch foundation bolt and more than ten times stronger than the Simpson-FJA. Each of these two types of foundation anchor is similar in cost and requires the same amount of work to install. This shows the importance of hiring a contractor or engineer who is familiar with the most effective and up-to-date hardware.
There are also homemade foundation anchors made of right angled steel. They have been rejected for seismic retrofit use by all three retrofit building codes and have no I.C.B.O. approval. Properly trained contractors and engineers will never use right angled steel as a foundation anchor.
Homes with low-clearance also need shear transfer ties, but instead of the shear transfer ties attaching the floor of the house to the top of the cripple wall they attach it directly to the mudsill.
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