Homeowner’s Guide to Seismic Retrofitting
The Homeowner’s Guide to Seismic Retrofitting is an unabridged version of the article, “Seismic Retrofitting of Cripple Walls,” written by Bay Area Retrofit for the April 2006 edition of the Journal of Light Construction, a national trade magazine. The Homeowner’s Guide to Seismic Retrofitting explores the function of house bolting and other matters regarding seismic retrofitting. This article discusses how to bolt a house and convert the cripple walls into shear walls, the essence of any seismic retrofit. It is intended to help you determine whether your house needs new or additional seismic retrofitting, or has been retrofitted properly. Every house is different, though the earthquake engineering principles are always the same for every seismic retrofit.
Seismic retrofitting a house to the foundation costs thousands of dollars and researching this topic is the best way to make sure your money well spent when it comes to retrofitting your house. If your home has not yet been seismically retrofitted to the foundation, this guide will help you determine how best to do it. If it has already been seismically retrofitted, this guide will help you evaluate the effectiveness of the retrofit and whether it was done properly. House bolting is only one part of a seismic retrofit and here you will discover there are many other things that comprise a seismic retrofit.
The earthquake retrofitting principles discussed here only deal with the area under the floor. This is because most commonly observed earthquake damage is caused by inadequate lateral bracing under the floor where the house is bolted to the foundation. In older houses, you will not find an attachment of the floor to the foundation. Making this attachment is the purpose of a seismic retrofit.
What Happens after an Earthquake?
Above the first floor, interior finishes on the walls and partitions such as plaster, though not designed to resist earthquakes, do in fact provide a lot of earthquake resistance. Therefore, this part of the house above the crawl space does not need further bolting. The failures always occur in the crawl space under your floor. The house needs to be bolted to the foundation, and cripple walls need to be converted into plywood shear walls.
After an earthquake, wall and partition finishes may be cracked, doors and windows may be racked, and costly repairs may be required to restore livability to common standards. Damage always occurs, whether the house has been retrofitted or not. But damage above the floor is much less likely to result in a hazardous condition if the house was seismically retrofitted.
Most Bay Area homes were built before the building code required house bolting or had other earthquake resistant provisions. Un-retrofitted older homes with un-braced cripple walls need to be seismically retrofitted to avoid collapse in an earthquake. California does not have a seismic retrofit building code to guide cities and seismic retrofit contractors in the proper way to bolt and seismically retrofit your house. Nor is there special licensing for seismic retrofit contractors. Although your city may issue permits for seismic retrofit work and bolting houses, the state and your City has no code by which to evaluate the work. This puts the responsibility on you, the homeowner. If you take the time to understand the basic principles of seismic retrofitting, you can make sure your retrofit is done properly.
Most houses need to be seismically retrofitted in three ways:
1. The cripple walls of the house need to be braced with plywood.
2. The house needs to be bolted to the foundation.
3. The floor of the house needs to be attached to the braced cripple walls.
The following illustrations explain these important three steps. Without all three, your un-retrofitted house can fall off of its foundation.
1. Bracing the Cripple Walls with Plywood
Figure 1 shows what can happen to a house if it is not seismically retrofitted with plywood on the cripple walls. History has shown that un-braced cripple walls are the first thing to fail in an earthquake and their failure usually makes the home uninhabitable. A seismic retrofit of the cripple walls is a must.
2. Bolting the Homes Cripple Walls to the Foundation
Once the house has its cripple walls with plywood, the house now needs to be bolted to the foundation to keep it from sliding off of the foundation.
3. Bolting the Floor of the House to the Braced Cripple Walls
Bolting the floor of the house to the braced cripple walls is the third component in any effective seismic retrofit. In Figure 3, the house has been retrofitted so the cripple walls are braced with plywood to prevent collapse, and it is bolted to the foundation, but the floor is not bolted to the cripple wall. Bolting the floor of the house to the cripple wall is done with hardware called shear transfer ties. Once this is done, our seismic retrofit is complete.
Diagram 1 below shows what a cripple wall looks like, viewed from the crawl space before a seismic retrofit. You should be able to look under your house and identify these components.
Diagram 2 below:
This is a drawing of a house that has had a good seismic retrofit. The plywood shear walls keep the 2×4 studs of the cripple wall from falling over. The bolts keep the mudsill from sliding off the foundation. The shear transfer ties bolt the floor joists of the house to the cripple walls. Unfortunately, most seismically-retrofitted homes are missing at least one of these components.
Different Ways to Seismically Retrofit Shear Walls
Plywood is used in a seismic retrofit in four different ways.
2. Stapled Blocking Method
This method of bolting the house to the foundation as part of a seismic retrofit is identical to the nailed blocking method except staples are used to attach the blocks to the foundation, as shown above in Diagram 5. Staples are recommended by the American Plywood Association in a report that states: “Staples provide a method for developing high design shear values, while still using 2 inch nominal framing. The small diameter of the staple legs is not as apt to cause splitting of the framing as are large diameter nails.”
Diagram 6 shows a cripple wall that has been seismically retrofitted into a shear wall made with reverse blocking. The reverse blocking method is quite effective in homes that need a seismic retrofit and which have wide mudsills. In this method of retrofitting and bolting a house to the foundation, the 2×4 reverse block is nailed to the plywood before the plywood and 2x4s are installed on the cripple wall and mudsill. Please note that the house bolts and the shear transfer ties are not shown in the drawings below.
With this method, the mudsill is first bolted to the foundation and is then cut flush with the 2×4 upright studs, using a special saw. The lower edge of the plywood is then nailed directly into the redwood mudsill as shown in Diagram 7. This is the best method. The resulting shear wall most resembles the shear walls that have been tested in laboratories by the American Plywood Association. As far as possible, every retrofit should used tested methods.
The International Code Council asked the leading authority on shear wall testing to evaluate these four methods. I do not have permission to use this institution’s name because of liability issues.
To: The International Code Council
Dear Council Members,
Based on my professional opinion, I would judge the retrofit strategies in the following order, from most preferred to least preferred.
1.) Flush-cut mudsill method
2.) Reverse block method
3.) Stapled blocking method
4.) Nailed blocking method-Plan Set A
I have chosen to order the retrofit strategies based on several reasons. In the past 8 years, there has been an unprecedented amount of cyclic testing on shear walls by APA and other organizations. The results from these various programs would be more similar to either the flush-cut mudsill or the reverse block method; hence I have a great deal of confidence in either of these methods. I believe the flush cut method would be more practical for most retrofits, but the reverse block method would be an acceptable alternative.
In my experience of personally working with small blocks of wood in the laboratory as well as small building projects of my own, I believe that multiple nails through the face of the small blocks greatly increase the splitting potential of the small wood blocks. Obviously if the blocks split for either the nailed or stapled blocking method, the structural integrity of the retrofit will be compromised. Nails tend to split wood worse than staples. Therefore, I believe the stapled block method is preferred over the nailed blocking method.
In summary, on paper, all of the retrofit strategies are acceptable. Since APA has not, and has no plans to conduct testing of these retrofit strategies, engineering judgment based on experience can be used to rank the different methods. I am of the opinion that my itemized list above is a reasonable ranking of the four methods.
Seismic Retrofit Design Principles
You will have to do a bit of arithmetic and use a very simple formula known as the base shear formula to determine exactly how many bolts, how much plywood, and how many shear transfer ties your house will need in order to consider it fully bolted and seismically retrofitted.
Geologists are able to calculate an “anticipated” amount of force that will be generated by a major earthquake in a specific geographical region. Knowing that “anticipated” force, seismic retrofitting uses what is called the base shear formula to calculate the amount of shear force (earthquake force) that will hit the base of a specific house and how many bolts and other hardware are needed for the seismic retrofit. If the three potential areas of failure needed to seismic retrofit a house are made strong enough to resist the forces determined by the base shear formula, the house is considered seismically retrofitted and should survive a major earthquake.
V = 0.2 (W)
V represents the shear force that will be generated at the base of a building.
0.2 represents anticipated force of ground acceleration from a major earthquake. This number varies from region to region and is based upon proximity to known earthquake faults.
W represents the weight of the building. Single story homes weigh approximately 50 pounds per square foot. Two story homes weigh 80 pounds per square foot of the first floor area.
We have a two-story house with a first story that is 25 feet by 40 feet. The first story is thus 1,000 square feet (25 x 40 = 1,000). When we multiply this by 80 pounds, we determine that the building weighs 80,000 pounds. Using this information and the base shear formula we can determine the amount of earthquake force expected to strike this building. We will want to design a retrofit that will resist this amount of force.
So, the base shear formula tells us the anticipated earthquake force equals 0.2 times the weight of the house being retrofitted.
Therefore the earthquake force that is anticipated to strike this home at its base (foundation area) during a major earthquake is 16,000 pounds. In order to seismic retrofit this house, we will need to be able to resist this amount of force.
This means a properly designed seismic retrofit must have enough foundation bolts to resist a minimum of 16,000 pounds of force, AND plywood on the cripple walls able to resist a minimum of 16,000 pounds force to keep the cripple walls from collapsing, AND enough shear transfer ties that bolt the floor of the house to the cripple wall to resist a minimum of 16,000 pounds of force where the floor framing sits on top of the cripple wall. Once this is done, you can consider your seismic retrofit complete.
How much Hardware and Plywood does my seismic retrofit need?
All foundation bolts, nails, staples, plywood, and shear transfer ties needed to bolt a house to its foundation are rated in terms of the amount of earthquake force they each can resist. For example, a 1/2-inch foundation bolt with a mudsill plate and plate washer can resist 1,200 pounds of force. If our house must resist 1200 pounds, our seismic retrofit would need one bolt. Each linear foot of plywood bracing using the flush cut method can resist 600 pounds of force. If we need our seismic retrofit to resist 1200 of force at the cripple wall, we would need 2 linear feet of plywood. Good shear transfer ties that bolt the floor to the cripple wall can resist around 600 pounds of force. For our seismic retrofit to be complete, we would need 2 shear transfer ties. When all of these components are used to resist this amount of force, the house can be considered seismically retrofitted.
The house in our example could be attacked by 16,000 pounds of earthquake force in any direction and needs a seismic retrofit to resist that amount of force. We know that a 1/2-inch foundation bolt with a plate washer and mudsill plate provides 1,200 pounds of earthquake resistance. To determine the number of 1/2-inch foundation bolts our seismic retrofit will need, we divide 16,000 by 1,200. The answer is 13.3 foundation bolts. We round this up to 14 foundation bolts, which completes the bolting portion of our seismic retrofit. This means our seismic retrofit needs a total of 14 foundation bolts to protect the house in the north-south direction and 14 foundation bolts to protect it in the east-west direction. Our seismic retrofit contractor will therefore need to install 7 foundation bolts along each foundation wall. Foundation bolts only need to be installed at plywood shear wall locations in seismic retrofit work because practically all of the earthquake force is absorbed by the plywood and transferred to the foundation bolts.
Next our seismic retrofit needs to address the bracing of the cripple walls. Each linear foot of good plywood cripple wall bracing suitable for seismic retrofit can resist 600 pounds of earthquake force for each linear foot of plywood. If we divide 16,000 by 600 we get 26.6. We may round this up to 28 because plywood comes in 2-foot length increments. This means our seismic retrofit will need 28 linear feet of 15/32” structural 1 plywood in the east-west direction and 28 linear feet of plywood in the north-south direction, or 14 feet of plywood on the cripple wall on each side of the house. Once this plywood is on our cripple walls, this portion of our seismic retrofit is complete.
We use a similar method to determine how many shear transfer ties are needed to complete our seismic retrofit. Shear transfer ties are used to bolt the floor framing of the house to top of the cripple wall. Good shear transfer ties can resist 600 pounds of earthquake force. 16,000 divided by 600 equals 26.6.
We round this up to 28 so that our seismic retrofit can have an equal number of shear transfer ties on each side of the house. This means we need 28 shear transfer ties in the east-west direction and 28 in the north-south direction; or 14 shear transfer ties along each side of the house. Installing more would be a waste of money.
Upper Top plate splice
It is very important to splice together any breaks in the upper top plate. This is because the movement of the floor is transferred through the toenails into the floor joists and the toe nails push and pull on the upper top plate. You want to make sure this movement is transferred to the shear wall.
The circled portions and red line of the diagram below illustrate what happens. As the floor moves to the left the movement of the floor is transferred to the toenails. The toenails in turn push on the upper top plate. This force is transferred all along the upper top plate until it reaches a break in the upper top plate at which point it stops, unless the two pieces on either side of the break are spliced together with nails or a steel strap. 12d common nails installed with a nail gun by shooting the nails up though the lower top plate into the upper top plate on each side of the break is the easiest and cheapest way to make this connection. Install a lot of nails, probably 20 on each side of the break in the upper top plate. Spread them out over 2 or 3 stud bays. They are cheap and easy to install.
Below is what our final retrofit would look like.
Retrofitting a house with no cripple walls
Below is a retrofit drawing of a house having no cripple walls. The mudsill needs to be attached to the foundation and the end and rim joists attached to the mudsill.
Below is an example of what can happen when the floor framing is not attached to the mudsill. The mudsill on this house was bolted to the foundation. It slid off its foundation because the floor joists were not bolted to the mudsill with shear transfer ties.
Does retrofitting work?
On that corner, at 214 and 210 Elm Street, were two identical Victorian style homes. The same builder, with identical materials and using the same construction techniques, built them 100 years ago.
O’Hearn started by retrofitting #210 by installing plywood shear panels on the cripple walls and bolting the mudsill to the foundation. Unfortunately, there was no time to retrofit #214 before the 7.1 Loma Prieta earthquake hit on Oct. 17, 1989.
In a sense, 214 Elm Street was the “control element” in this amazing experiment. “The building came apart in four sections,” O’Hearn said: “The one we had retrofitted (210 Elm St.) cost us $5,000 to repair. The other one (214 Elm St.) cost us $260,000 to repair. The whole building had to be jacked up, repaired, and slid back on a new foundation.”
O´Hearn offers this advice, “For homes more than 20 years old located in areas of seismic activity, I strongly urge owners to consider seismic retrofit. It’s alot cheaper to retrofit a house now than to repair it after an earthquake.”
Courtesy: American Plywood Association
Seismic Retrofits and Overturning Forces
Overturning forces act on all shear walls. However, sometimes a shear wall will be subjected to very strong overturning forces that can damage the shear wall even if all the components of the house are bolted together. These forces must be resisted to prevent this damage. In the following pages we will discuss overturning forces and how to resist them.
When the floor of the house pushes along the top of the shear wall, it not only tries to slide the wall along its length but also tries to roll it over. Using the methods described for our sample retrofit above, the lateral forces of an earthquake (called shear forces) acting along the top of a shear wall are resisted by all the components of our seismic retrofit, but this force also creates overturning force, much as a tall chest of drawers will tilt up and overturn if you try to slide it across the floor by pushing it from the top.
Let’s assume an earthquake has attacked this house with 3600 pounds of force. This force is distributed along the top of the shear wall at 300 pounds per linear foot and our seismic retrofit has already dealt with this. That same 300 pounds per linear foot must also be resisted on the sides and bottom and tries to overturn the shear wall. This is resisted by hardware called hold downs, part of any retrofit that has tall shear walls. In an actual earthquake, this force alternates back and forth rapidly against the shear wall as the earth shakes back and forth.
Here is an example of a shear wall overturning. This drawing is exaggerated for illustration purposes. Most of the damage to the shear wall occurs where the plywood lifts up and away from the mudsill where the house has been bolted. Below is a close up illustrating tearing of the plywood at the mudsill. Once this happens the shear wall can no longer transfer shear forces into the bolts. A seismic retrofit must prevent this.
The building on the left used to be two stories. This collapse was caused by overturning of tall narrow shear walls that could not resist the earthquake forces generated by the heavy living area above a garage. Even if this house were bolted, it would not have made any difference unless it had a seismic retrofit that resisted overturning forces.
The hold-down hardware shown at the ends of the shear wall in the figure above is designed to resist overturning forces. Each hold-down is connected to the foundation with long foundation bolts set deep in the concrete and secured with epoxy. These long bolts must be installed or the seismic retrofit shear wall will fail.
As it tries to overturn, the left end of this shear wall pulls up on the holdown, which in turn pulls up on the foundation anchor rod.
Sometimes the overturning forces are so great that an un-reinforced concrete foundation breaks, or an improperly reinforced foundation deforms. A seismic retrofit as described up to now will not protect against this. This can lead to a lot of movement of the floor that results in significant damage to the structure.
Seismic Retrofitting the cripple wall when the Concrete will break
Breaking of the concrete can be prevented by adding concrete under the hold downs to provide additional weight to anchor the hold-downs. This can be done by pouring heavy blocks of concrete beneath the hold downs. One cubic yard of concrete weighs 4000 pounds; on tall narrow walls you often need a full cubic yard of concrete under each hold-down.
To the left is the hole for the concrete, under which the hold down will go. To the right is the same hole filled with concrete. An anchor rod with a nut at its lower end has been cast into each block of concrete to anchor the holdowns to the new concrete.