The Whole House Does Not Need a Seismic Retrofit
There is a misconception that a seismic retrofit involves all parts of the house, including the living area above the first floor that you walk on. This is not the case. As you can see in the photograph below the entire house fell off its foundation several feet but otherwise remained intact. Notice how the windows are not even broken. Houses in themselves are VERY strong. However, connections of the floor to the foundation can be very weak. The whole purpose of a seismic retrofit is to keep the house on its foundation. If someone had retrofitted this house to keep it on its foundation, it would have remained more or less intact.
Why so little earthquake damage?
The honeycomb of cross-walls in hallways, bedrooms, bathrooms, kitchens etc prevent collapse of the living area. The house itself has walls, a ceiling, and a floor which are all attached to each other. The house and the rooms are all cubes and cubes are a very strong geometric shape. The resistance of cubes to collapse prevent significant damage to the main house.
While working for FEMA the author inspected this house and it held together so well that the windows did not even crack.
Same House: Earthquake Damage on the Inside
Even if the house holds together well, if it falls off the foundation, it can still sustain damage to interior walls, plumbing, and electrical systems which can be catastrophic.
The author was the FEMA inspector who evaluated the beautiful house below after the 1989 San Francisco Earthquake. Two weeks after the evaluation I drove by and saw an empty lot.
This is actually a 2 story house. The main house was above and below there was living area. If people had been in the first story at the time of the quake there could have been serious injuries.
The Basics of a Home Seismic Retrofit
The description below of the three steps of a retrofit covers the same material in the video above. It uses the design principles behind Standard Plan A. We suggest that you watch this video on designing a retrofit and read the material to help you understand your retrofit.
The term “seismic retrofit” refers to attaching the floor to the foundation in such a way that it does not slide off its foundation when the earthquake shakes it back and forth. For photos of what happens when you don’t do this, visit this page.
This article discusses the seismic retrofit of typical homes found in the older parts of the San Francisco Bay Area such as Oakland and Berkeley. At the same time, cities such as San Jose or Fremont rarely need cripple wall retrofits because when these homes were built cripple walls were no longer the fashion. If you live in in one of these newer cities you probably, need a no cripple wall seismic retrofits
This amazing story from the 1989 Bay Area Loma Prieta is proof a seismic retrofit will do this.
Retrofit a House in Three Simple Steps
The image below illustrates all the components of a cripple wall. Each component connects to another component much like the links in a chain. Just like with a real chain, if any of the links are weak and fail the cripple wall will collapse, with truly catastrophic consequences for your home as well as your personal life. For this reason all attempts are made to strengthen the connections that are a possible failure point. Before spending a lot of money on a retrofit, make sure your contractor can explain how he/she plans to do this.
The Mechanics of all Seismic Retrofits
Step 1: Plywood
This is how plywood prevents cripple walls from collapsing. There are many type of plywood but there is only one that is suitable to resist earthquakes. Other factors that determine how well the plywood will perform are the size and spacing of the nails and the manner in which it is connected to the bolts and floor. Structural plywood, made specifically to prevent cripple wall collapse has an almost miraculous ability to prevent damage like this.
Below is a photograph of a shear wall where vertical bolt drilling equipment would not fit to install standard bolts. In these cases foundation anchors are use to bolt the base of the shear wall to the foundation from the side. Plywood and shear transfer tie installations remain the same.
This is how bolts provided a counter force to an earthquake
Type of Bolts
The Importance of Plate Washers
Wood has a tremendous amount of strength when resisting compression forces as shown by the blue arrows. When put in a state called “Cross Grain Bending” is has practically no value. Plate Washers prevent this.
As the plywood lifted up, which is something that happens to all shear walls, it pulled up on the mudsill and put it in a state of cross grain bending.
DAMAGE CAUSED IN THE BOLT TO WOOD MUDSILL CONNECTION CAUSED THE DEVELOPMENT OF THIS HARDWARE
How Plywood Does its Job
The bottom edge of the plywood is attached to foundation bolts. The earthquake force pushes against the plywood but the bottom of the plywood is bolted to the foundation so the plywood does not move which keeps the cripple wall from collapsing.
All plywood is rated by the building code in terms of how much earthquake force it can resist measured in pounds of force. For example, if earthquake force of 10,000# is anticipated to strike a house, and the plywood is rated for 1000# of earthquake force, we will need 10 linear feet of plywood.
As shown by the table below there is a big difference in how much earthquake force plywood can resist, based on the type of plywood, size of nails, and nail spacing. The best plywood with the thickest nails spaced close together creates the strongest plywood connection.
Types of Retrofit Foundation Bolts
There are a few types of retrofit foundation bolts: epoxy bolts, wedge anchors, and Titens. These all are used to attach the base of the shear wall (the mudsill) to the foundation. Each one is designed for a specific retrofit application and depends on what is found under the house.
Step Three: Shear Transfer Ties (Attach Floor the Cripple Walls)
Figure 3 shows plywood bolted to the foundation. Shear Transfer Ties prevent the floor from sliding on top of the bolted and plywood braced cripple wall. do this.Strong steel shear Transfer Ties made of steel attach the floor to the cripple wall. This completes the final stage of a cripple wall to shear wall conversion.
All seismic retrofit guidelines require the installation of shear transfer ties. Be sure you have the right one. Many shear transfer ties on the market don’t do anything at all.
Earthquake simulation testing on shear transfer ties tell us how much resistance a particular shear transfer ties has. Always check to see that you are using the highest strength shear transfer ties available
Below is another kind of Shear Transfer Ties – there are many others.
Load Path of Earthquakes
RED ARROW – force trying to slide floor joist off of cripple wall.
YELLOW ARROW – sliding of joist prevented by shear transfer ties.
ORANGE ARROW – force pushing on top of cripple wall through shear transfer tie.
BLUE ARROW – movement of top plate prevented by plywood, this image is before plywood installation (plywood prevents cripple wall from collapsing) .
WHITE ARROW – Force trying to slide mudsill off foundation. This will be prevented by bolts attaching the mudsill fo the foundation.
BLACK ARROW – Force absorbed y foundation and transferred into ground.
Review of the Three Steps in a Cripple Wall Retrofit
The Cripple Wall Has Been Converted into a Shear Wall That Has Plywood, Bolts, and Shear Transfer Ties
The Three Steps Work Together
The load path lateral motion of the floor down through the shear wall and into the foundation. Make sure your earthquake retrofit contractor explains how the load path will work on your retrofit before you hire him.
The Shear Transfer Ties, Plywood, and Bolts work together to transfer the side to side Motion of an Earthquake into the Foundation and into the Ground. This is the Load Path. If any Component within the Load Path Chain is missing or weak the Retrofit will Fail.
Addressing Breaks in the Top Plates
The horizontal board on top of the cripple wall is known as the upper top plate and it is also the top of the shear wall where all the force is transferred. The floor needs a continuous connection to the shear walls they rest on and which protect them. The connection of the floor to the rigid shear walls is what restrains the entire floor from moving. If part of the floor is on one section of shear wall and another part of the floor is on another section of cripple wall that has not been converted into a shear wall, these segments for floor can move independently. The way this is addresses is by joining all the cripple wall segments together wherever they are separate from each other. These separations are caused by breaks in the top plate.
For example, if a shear wall holds part of the floor to the foundation at one end of the house, but the other end is not connected to the shear wall because of a top plate break, this end will not be protected. Building codes make sure this does not happen with simple regulations simple regulations.
Cripple wall top plates are actually segments of wall that can move independently of each other, boards that are 8 to 24 feet long butted together. Any point where they butt together is called a top plate break. These breaks create a break in continuity, meaning the pull and push of the earthquake force is not continuous along the cripple wall top plate.
In the diagram above, the cripple wall to the left of the break in the top plate is disconnected from the shear wall to the right of the break in the top plate. If there were no break and the top plate were one piece, then the two would be joined together, and any movement of the cripple wall would be restrained by the shear wall. Hence the top of the cripple wall and the top of the shear wall must be connected together so they cannot move independently.
The break in the top plate must be connected so the floor responds to earthquake forces as one unit.
Steel straps and nails are the two methods by which continuity is restored.
In the image below, a steel strap called a CST strap connects the two sections of the top plate.
Connecting the Plywood to the Mudsill
While on a committee developing Standard Plan A, a regional retrofit guideline, the author had 4 methods of attaching the plywood to the mudsill evaluated by the largest national shear wall research laboratory in the world. These methods are the Nailed Blocking Method, the Stapled Blocking Method, the Reverse Blocking Method, and the Flush Cut Method. This letter summarizes their findings. A much more technical version for engineers is available in the The Retrofit Mudsill Connection. You can view a video that addresses the same issues below.
There are four ways to modify the framing so that the plywood is attached to the bolts.
In this method the 6-inch-wide mudsill is cut flush with the 2 by 4s. The plywood is then nailed directly to the mudsill.
Below is the same illustration showing the nailed plywood.
Shear walls slide on the foundation but they also try and overturn, no matter how tall they might be. When they do this they must push up on the wall as shown by the red arrow pushing up. The wall has a tremendous amount of weight on it (the weight of the wall above the cripple wall, ceiling, and roof) which protects the shear wall from damage by resisting overturning (you will find out what that is shortly). Other retrofit mudsill connections do not have this ability.
One of the reasons this method is used is because the plywood can now be nailed into old growth redwood that is many times less prone to splitting than wood grown on tree farms.
- A 2 by 4 is placed along the the long edge of a piece of plywood.
- The plywood has been nailed to the 2 by 4 from the back where you can’t see the nails.
- The 2 by 4 of this assembly is nailed to the top of the mudsill and along all edges.
The Nailed Blocking Method
Here is a construction detail from a set of engineered plans that tells a contractor how to do this. As shown by the red arrow 8 nails are specified through the mudsill blocking which has a very high potential of splitting the block. Unfortunately, this is the method recommended by all extant retrofit building codes These guidelines were written by building officials and engineers who never checked to see if it worked in a practical way.
Why are Blocks a Problem?
The problem with this method is that the blocks split. The blocks, usually 14 inches long and often dry, are installed between the studs on the cripple wall as shown in the photo above. If the blocks split, then the shear wall will fail.
When the blocks split, it is quite tempting for the installer to leave those split blocks in place because of the time and labor involved in removing them. The plywood covers the blocks when nailed to the wall framing. Private home inspectors cannot tell if the blocks split when installed. This can leave lingering doubts in the mind of the home buyer. Seismic retrofit contractors often believe “the more and bigger the nails, the stronger the shear wall” which further exacerbates the problem.
How to Stop the Shear Walls from Overturning
Shear walls must resist lateral forces that would make the house slide off its foundation. At the same time the earthquake will try to flip the shear wall over. This is known as overturning. Overturning is resisted with special hardware called Hold Downs. This video and text explain the basics behind overturning of shear walls and how hold downs prevent this from happening. Watching the video first will make the text easier to understand.
Overturning forces primarily act upon tall shear walls, much as a tall chest of drawers flips over if pushed across the floor from the top. A short squat dresser will not have this tendency, which is why overturning of shear walls is not a major concern in most cripple wall retrofits. Generally it is a waste of money to install overturning hardware, known as hold downs, on short shear walls.
Hold Downs Are Often Unnecessary
Many engineers and contractors are not aware of this and unfortunately recommend this expensive hardware when it is not necessary. The 2013 California Building Code does not even require their use so long as an 8 foot tall is at least 4 feet wide. You will see this on either side of too many garage doors, even when they support living areas above. This hardware is made by Simpson StrongTie and is abbreviated as HDU2, 4,5,8, etc.
Tall narrow shear walls can twist and deform and if extremely narrow, they will function like a post and simply tip over.
Why Short and Wide Shear Walls/Cripple Walls Don’t Need Hold Downs
The will scoot along the top of the foundation like a short chest of drawers.
Here is an example of a shear wall overturning. This drawing is exaggerated in order to illustrate what happens. Most of the damage occurs where the plywood lifts up and away from the mudsill.
The pink 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.
Hold Downs Resist Overturning
The hold-down hardware shown at the ends of the shear wall in the figure above are designed to resist overturning forces. One hold down is connected to the vertical framing at each end.
As the shear wall tries to overturn, the left end of the shear wall pulls up on the hold down, which in turn pulls up on the hold down bolt, which in turn pulls up on the foundation. When the earthquake changes directions, the exact same thing happens but in the other direction.
Sometimes the overturning forces are so great that an un-reinforced concrete foundation breaks. However, the segment of foundation that the shear wall is built upon should remain in one piece and maintain the integrity of the plywood nailing.
Getting New Concrete Under the Hold Downs
The best solution to prevent overturning in shear walls with un-reinforced concrete foundations is to provide additional weight to anchor the hold-downs. The existing un-reinforced concrete foundation shown here is only 8 inches deep and 12 inches wide. This is clearly insufficient to resist the potential overturning forces.
Here is the same view after the concrete has been poured and the plywood installed.
Designing a Retrofit
Calculating the Amount of Materials Needed and where they go
Perhaps the most critical decision is knowing how many linear feet of plywood, how many bolts, and how many shear transfer ties a house needs and where to put them. This is fully explained in this Video.
Installing more than required can strain a budget; not doing enough can cause the seismic retrofit to fail. This is determined by using a simple formula called the base shear formula.
Earthquake forces to be resisted at the base of the house (Foundation Level) will equal its weight times 0.2 (G force/ground acceleration).
Example: We have a two-story house that measures 25 feet by 40 feet, or 1,000 square feet (25 x 40 = 1,000).
The first thing we do is figure out how much earthquake force is going to strike the house. Engineers tell us a two story house weighs 80 pounds per square foot and to figure out how much this house weighs we simply multiply the square footage (1000 square feet) by the weight (80 pounds per square foot) or 80,000 pounds.
Finally, we take the ground acceleration of 0.2 Gs and multiply it by 80,000 pounds with the result that we now know the house will be attacked by 16,000 pounds of force. Our retrofit must resist this amount of force.
An Example of Seismic Retrofit Design
This house must have enough bolts to resist a minimum of 16,000 lbs of force AND enough plywood on the cripple walls to resist a minimum of 16,000 lbs of force AND enough shear transfer ties to resist the same 16,000 lbs.
Government recognized tests rate bolts, shear transfer ties, and types of plywood in their ability to resist earthquakes. These tests measure the retrofit component’s ability to resist earthquakes. For example, if tests show a bolt can resist 1500# of force, the bolt will fail when 1600 pounds of force push on the side of it.
In this example we need the bolts, plywood, and shear transfer ties to resist 16,000 lbs of force.
Foundation Bolt Quantity
8,000 pounds of earthquake force is striking this house on each side. One bolt can resist 1,200 lbs. Divide 8,000 lbs by 1, 200 lbs and the answer is 6.7 bolts. We round this up to 7 bolts needed for each side.
Linear Footage of Plywood Required
Let’s figure out how many linear feet of plywood we need. First thing we need to be aware of is that good quality plywood can resist 600 pounds of force. We that that, divide it into 8000 lbs (the amount of force that will be attacking our house), and voila! we discover we need 13.3 linear feet of plywood. Just to make things easier to work with we round that up to 14 linear feet. That’s it! We can now mover onto shear transfer ties.
Shear Transfer Ties Quantity
The required number of shear transfer ties use the same method. Good shear transfer ties can resist approximately 600 pounds of earthquake force. 8,000 divided by 600 equals 13.3. We round this up to 14 and need this many shear transfer ties along each side of the house. The purpose of shear transfer ties is to prevent movement of the floor framing on the cripple wall top plate as illustrated below.
A complete cripple wall with bolts, plywood and shear transfer ties looks like the image below. Be sure and notice the legend on the lower right.