You may like to read a much-simplified article we wrote on seismic retrofitting we wrote for the Journal of Light Construction. It contains the essential information you will need. Please go to this article on retrofitting homes to read the article or to the original publication found in the Journal of Light Construction.
Seismic Behavior of Level and Stepped Cripple Walls, a publication of the Consortium of Universities in Earthquake Engineering (CUREE), contains the following statement regarding damage from the Northridge Earthquake: “More than half of the $40 billion dollar property losses were due to failures of wood frame construction, primarily as a result of the damage or collapse of residential, single-family homes, multi-family apartments or condominiums.”(10) The San Francisco Bay Area is riddled with earthquake faults, some of which will rupture with an intensity far greater than that experienced at Northridge. The Association of Bay Area Governments (ABAG) expects that over 155,000 housing units in the Bay Area will be lost and 467,000 people displaced when just one of these, the Hayward Fault, ruptures with an expected magnitude of 7.2 or greater. The number of dead and injured could run into the thousands.
Homeowner Preparation for the Inevitable
Homeowners are not unaware of this fact. In response to homeowner demands to protect their homes from earthquake damage, many contracting firms in the Bay Area now specialize in residential wood frame seismic retrofit work. As an aid to homeowners the original article can be downloaded at Homeowner’s Guide to Seismic Retrofitting. This article and this web page discuss how to bolt a house and convert the cripple walls into shear walls. Shear walls are what resists earthquakes. This guide is intended to help you evaluate your seismic retrofit needs, determine if your house needs seismic retrofitting, has been retrofitted properly, or needs for you to retrofit your house even further. Every house is different, and no house matches exactly what you will see here, 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 is well spent. 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.
Looking at the Nuts and Bolts
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. There is a companion Video to this article that explains why under floor house bolting and cripple wall bracing is the number one priority in any seismic retrofit. Another video reviews these same seismic retrofitting principles in case you already know a fair amount about seismic retrofitting a house. 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 and does not need further bolting. The failures always occur in the crawl space, which is why the house needs to be bolted to the foundation here as well as convert the cripple walls into shear walls.
After an earthquake, wall and partition finishes may be cracked, doors and windows may be damaged, and costly repairs may be required to restore livability to common standards, but damage above the floor is much less likely to result in a hazardous condition than would be found in a house that is seismically retrofitted to the foundation. Damage always occurs if the house has been retrofitted or not. Attaching the house to the foundation with bolts and cripple wall bracing with plywood (these are all the components used in seismic retrofitting), should keep your house from falling from its foundation. Many homes on the West Coast were built at a time when the floor of the house was elevated off of the foundation with something called cripple walls and were not bolted to the foundation when they were built.
Most of these homes were built before the building code required house bolting or had other earthquake resistant provisions. Adding these provisions is a seismic retrofit. Un-retrofitted older homes with un-braced cripple walls need to be seismically retrofitted to avoid collapse and thereby add the earthquake provisions that are in modern building codes. It is important to remember 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 who specialize in house bolting and seismic retrofitting. Although your city may issue permits for seismic retrofit work and bolting houses, the state and your city have 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 and you get what you pay for.
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.
These three steps serve to seismic retrofit and bolt a house to the foundation by converting the cripple walls into shear walls. The following illustrations explain what these three steps are and why they are important when you do a seismic retrofit on your house. If any one of these three areas is not made earthquake resistant with a seismic retrofit, 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, and it is bolted to the foundation, but the floor is not bolted to the cripple wall. Doing this is the last part of our seismic retrofit. 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 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 the components shown in Diagram 1. The purpose of a seismic retrofit is to make sure all these components of a house are bolted together.
Shows the same cripple wall after the components of the cripple wall have been bolted together with a seismic retrofit. The plywood shear walls are part of the seismic retrofit because it will help bolt the house to the foundation by keep the 2×4 studs of the cripple wall from falling over, the bolts keep the mudsill from sliding off the foundation, and the shear transfer ties bolt the floor joists of the house together to keep them from sliding off the top of the cripple walls. When you crawl under your house you should see something that looks similar to Diagram 2. This is a drawing of a house that has had a good seismic retrofit. Unfortunately, most seismically retrofitted homes are missing at least one of these components, and all of them are required to seismic retrofit a house properly.
Different Ways to Seismically Retrofit Shear Walls
Plywood is used in a seismic retrofit in four different ways. The following is a description of how seismic retrofitting uses these four ways to seismically retrofit a house.
The plywood is then nailed into the 2×4 blocks at the bottom of the plywood which have in turn bolted the house to the foundation. The point of concern when using the nailed blocking method is that the blocks can split which can make a seismic retrofit ineffective.
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 plywood blocks to the foundation as shown in Diagram 5. This way of retrofitting a house is partly because of a report published by the American Plywood Association 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. For this reason, this is a good option for your seismic retrofit.
3. Reverse Blocking Method
Diagram 3 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. 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 with a special saw. The lower edge of the plywood is then nailed directly into the redwood mudsill as shown in Diagram 7. This method of building a retrofit shear wall is the best way to seismic retrofit a house. This is because this retrofit method is the method that most resembles the shear walls that have been tested in laboratories by the American Plywood Association. As far as possible, every retrofit should use tested methods.
The International Code Council asked the leading authority on shear wall testing to evaluate these four methods of bracing cripple wall and bolting a house it its foundation for creating shear walls. 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.
I hope you find this information useful and if you have any questions, or would like to discuss this further, please don’t hesitate to contact me.
It is very important to connect 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 toenails 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 diagrams above 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.
Seismic Retrofit Design Principles
You will have to use these engineering calculations.
Below is a drawing of what it probably looks like under your house if you do not have a cripple wall. The mudsill needs to be attached to the foundation and the end and rim joists attached to the mudsill. This house does not have bolts.
Below is an example of what can happen when the floor framing is not attached to the mudsill even when the house is bolted. The mudsill on this house happened to be bolted. This house slid off its foundation because the floor joists and mudsill were not bolted to the foundation 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 a lot 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.
Lateral and Overturning Forces acting of a Shear Wall
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 forces also creates overturning forces 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 turn over or overturn the shear wall. This is resisted by hardware called hold-downs and are a 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, depending on which side of the shear wall is being attacked. Our seismic retrofit must resist this force that is trying to overturn the shear wall.
Here is an example of a shear wall overturning. This drawing is exaggerated in order to illustrate what happens. 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 and a seismic retrofit must prevent this from happening.
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 and are part of a seismic retrofit. 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 hold-down, 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 corrected by putting concrete under the hold downs. This is done 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.
Concrete has been poured and the plywood installed.
To the left is the hole for the concrete under where the hold down will go. To the right is the same hole filled with concrete. An all-thread rod with a nut at its lower end has been cast into each block of concrete to anchor the hold-downs to the new concrete.