What is a cripple wall retrofit?
Earthquake retrofitting is not the same as foundation bolting. Previous earthquakes have shown houses retrofitted with bolts still suffer earthquake damage. Some say cripple wall bracing is a seismic upgrade. If that is true, why can these houses also suffer significant damage? Some say an earthquake retrofit is achieved by following the building code. If that is true why do houses “built to code” often suffer extensive damage? Some say a house can only be seismically retrofitted if a structural engineer designs the retrofit while others point out most structural engineers do not understand cost and make retrofits unaffordable.
The Ultimate Purpose of a Seismic Retrofit
Seismic retrofitting refers to attaching the floor of the house to the concrete foundation with bolts, plywood, and different kinds of metal using different retrofit methods. This is done to prevent the house from sliding off its foundation when the earthquake shakes it back and forth. In most houses built before 1940 this involves building shear walls. A shear wall is a structural plywood element that transfers the side-to-side force of an earthquake out of the house itself into the foundation, and finally into the ground.
The 1989 Bay Area Loma Prieta earthquake proved beyond the shadow of a doubt that retrofitting works. After the 1994 Northridge Earthquake, President Clinton took a photo opportunity in front of a retrofitted house to make a point that retrofitting works and directed the Federal Emergency Management Agency (FEMA) to give grants to homeowners for earthquake retrofits.
Earthquake Retrofitting a House is Simple
Seismic Retrofitting in Four Simple Steps
If you have more than 3 steps you need a cripple wall Seismic Retrofit
The Whole House Does not Need a Retrofit.
This house was thrown off its foundation in the 1964 9.2 Alaska earthquake. The house is more or less intact and would have been fully habitable had it remained attached to its foundation. The whole purpose of a retrofit is to keep the house on its foundation with foundation bolting and cripple wall to shear wall conversions.
Above the crawl space where people live, houses are protected by the honeycomb of cross-walls in hallways, bedrooms, bathrooms, kitchens etc. These walls are usually made of plaster or sheet rock and together they have a great deal of earthquake resistance.
The house below was inspected by the author while working for FEMA and held together so well that the windows did not even crack.
Below is the Same House from the Inside
On the other hand, damage to interior walls, plumbing, and electrical systems can be catastrophic.
The author was the FEMA inspector who evaluated this beautiful house after the 1989 San Francisco Earthquake. Two weeks after the evaluation I drove by and saw an empty lot.
How does Ground Movement Cause Damage?
When the ground movement accelerates, the house and foundation go with it. When the ground stops suddenly and then reverses direction, the house wants to keep on going and slide off the foundation. It is similar to riding in the back of a pick-up truck which accelerates very quickly and stops suddenly. When it stops, your body is pushed forward at the same speed as the original acceleration and slides forward. Earthquakes attack a house in the same way.
The purpose of a seismic retrofit is to prevent this sliding.
Converting Cripple Walls to Shear Walls
Preventing this sliding is achieved by converting a home’s cripple walls (see photo below) into shear walls. Once the cripple walls are converted into shear walls, the house can no longer slide off its foundation.
Cripple Wall Collapse
The illustration above shows what can happen to a cripple wall if plywood is not put on it.
In the photo to the left, look at the top of the concrete steps, then see the porch. The steps are a few feet higher than the porch because the house fell a few feet to the left due to the cripple wall collapse.
1. Bracing the Cripple Wall with Plywood
This man is preparing for the aftershock. His cripple wall is about to collapse.
If the initial shock does not collapse the cripple wall, an after shock might collapse it. In the photo above, the man is trying to prevent his already leaning cripple wall from fully collapsing in an after shock.
This is what happens when Cripple Walls are not Converted into a Shear Walls
How do Bolts work?
The hatched area represents a cripple wall partially braced with plywood. If it is not attached to the foundation it can slide on top of it. The purpose of bolts is to prevent this from happening.
Types of Retrofit Foundation Bolts
There are a few types of retrofit foundation bolts: epoxy bolts, wedge anchors, Titens, and Simpson Strong-Tie UFP IOs. These all are used to attach the base of the shear wall (the mudsill) to the foundation.
It is critical that the bolts have been tested in independent research libraries to make sure they will be effective in your retrofit. We use industrial bolts Manufactured by Hilti, a Liechtenstein-based company known for producing the highest quality bolts available Tested bolt shear values are on page 8 of the International Code Council’s Evaluation Service Report 302. All of our Hilti bolts have a minimum of 4 inches of penetration to maximize capacity.
Bolt that is epoxied into Concrete Wedge Anchor Bolt Part of Wedge Anchor that opens and grabs the Concrete.
Plywood and Bolts work Together
The hatches on either side of the cripple wall in the figure below represent plywood that prevents collapse. In other words, the cripple wall can no longer collapse. However, the cripple wall can still slide on top of the foundation if not bolted. The purpose of bolting is to keep the plywood braced cripple wall from sliding on the foundation.
Shear Transfer Ties
As shown in Figure 3 below, once the plywood is on the cripple walls and they are bolted to the foundation, we still need to prevent the house from sliding on top of the cripple walls. This is done with Shear Transfer Ties. Shear Transfer Ties are usually made of steel and attach the floor to the cripple wall. This is the final stage of a cripple wall to shear wall conversion.
The pieces of steel shown here are Shear Transfer Ties as shown from inside the crawl space.
Below is a pictorial summary of the three components of every cripple wall retrofit.
Retrofit Shear Walls can be Complex
Converting a cripple wall into a shear wall may appear simple but shear walls are actually quite complex as evidenced by a 180 page book on shear wall construction. Still, this website can teach you enough to make sure the retrofit you pay for is a retrofit worth buying.
The Load Path
The load path is the process by which side to side movement from earthquakes is transferred downward though the shear walls into the foundation. Make sure your earthquake retrofit contractor can explain how the load path will work on your retrofit before you hire him. You should also check it once the job is done.
This chain of movement from the floor to the foundation through various structural components is the load path. If any component within the chain is missing or weak, the retrofit can fail.
Connecting the Plywood to the Mudsill
The trickiest part of building a retrofit shear wall is the attachment of the plywood to the mudsill. This is the piece of wood that lies on top of the foundation and where the bolts go. It is usually is wider than the rest of the cripple wall framing.
One way to address this is with The Nailed Blocking Method
On the left, blocks have been nailed onto the mudsill. On the right, the plywood has been nailed to the blocks.
Below is a technician installing mudsill blocks
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.
Unfortunately, most contractors use this method and it is found in all government retrofit guidelines. If you are going to hire someone, make sure they are NOT using this method or if they do, make sure to check all the blocks before they nail up the plywood.
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. Furthermore, once the plywood is nailed, there is no way for home inspectors to tell if the side of the blocks were split when penetrated by the plywood nails. Seismic retrofit contractors often believe “the more and bigger the nails, the stronger the shear wall” which further exacerbates the problem.
Stapled Blocking Method
Staples are an excellent way to prevent splitting of the blocks and also have a very high shear capacity. The staples in this block have the strength of 35 nails.
The Reverse Blocking Method of Building a Shear wall
The image on the left shows a 2 by 4 being attached to the plywood. The center image shows how it is then placed on top of the mudsill. The 2 by 4 is then nailed to the top of the mudsill. Because the 2 by 4 is so long it will not split.
The Carpenter is nailing directly into the reverse block with his nail gun
The Flush Cut Method of Attaching the Plywood to the Mudsill.
The Flush Cut Method is the only way to build retrofit shear walls that are identical to those tested by the American Plywood Association, a National Laboratory for Testing Shear Walls. These tests are the basis of the International Building Code, the national building code that governs acceptable shear wall construction standards.
In this method the mudsill is cut flush with the 2 by 4’s. The plywood is then nailed directly to the mudsill without any type of blocking. This is the only earthquake retrofit shear wall construction method that exactly mimics the tested shear wall configurations.
The following photographs show the step-by-step process.
While on a committee developing Standard Plan A, a regional retrofit guideline, the author had these methods evaluated by the largest national shear wall research laboratory in the world. Their evaluation of these 4 methods is a definitive opinion based on scientific research.
A much more technical version for engineers can be read in the The Retrofit Mudsill Connection. A video that addresses the same issues can be viewed below.
Breaks in Top Plates
When we say “Connect Breaks in Top Plates,” we are stressing the importance of connecting the entire cripple wall to section of shear wall that has been converted into a shear wall. This is necessary because cripple walls are actually segments of wall that can move independently of each other. Top Plates in actual houses are not one continuous horizontal pieces of lumber. Instead, they are made of boards 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. A continuity tie is used to bridge these breaks and restore continuity.
- Earthquake forces push the top plate to the left.
- 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 was 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 entire cripple wall is attached to the shear wall.
One way to do this is with a Steel Strap or a 2 by 4.
If the two top plates are nailed together to form one single member, and the top plate break (either single or doubled) is bridged with a 2 by 4, the value of an MST48 can be achieved by installing 24 12d nails through the 2 by 4 on each side of the break.
Another way to do this is with nails
This allows a “load path” for the force to bypass the break.
Overturning of Shear Walls
When shear walls are long relative to their width, they want to tip over. This is especially common in houses found in San Francisco where much of the parking is underneath the first floor and there are narrow walls on either side of a garage door opening. This force must be resisted or the shear wall can be severely damaged.
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.
Below is a close up of this kind of damage and shows how the plywood is no longer attached to the bolts
Once the nails at the bottom of the shear wall are uprooted from the mudsill, the shear wall will no longer be attached to the bolts and will be unable to resist lateral forces caused by earthquakes. This kind of damage is called overturning damage.
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.
Retrofit Engineering Principles
Perhaps the most critical decision regarding a seismic retrofit is knowing how many linear feet of plywood, how many bolts, and how many shear transfer ties a house will need. This is explained in this Video.
Doing more retrofitting than is required can strain a budget and is not necessary for a seismic retrofit; not doing enough can cause the seismic retrofit to fail. This is determined by using a simple formula called the base shear formula.
Shear Forces at the the base of the house (Foundation Level) will equal its weight times 0.2.
Example: We have a two-story house that measures 25 feet by 40 feet, or 1,000 square feet (25 x 40 = 1,000). We multiply this by 80 pounds because two story houses weigh 80 pounds per square foot based on the footprint of the house. 1000 square feet times 80 pounds per square foot means the house weighs 80,000 pounds. The earthquake force that is anticipated to strike this home at its base during a major earthquake will be 0.2 times 80,000 or 16,000 pounds of force. If you want to use more precise weights you can use the weights used for Standard Plan
Installing enough Retrofit Components
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. If each side can resist 8,000 lbs (2 x 8,000 = 16,000) then the house will resist 16,000 lbs of force no matter which direction the earthquake force comes from.
This information establishes the quantity of bolts, plywood, and shear transfer ties that are needed to strengthen these potential failure points on each side of the house. All bolts, shear transfer ties, and types of plywood are rated in terms of their earthquake resistance.
Foundation Bolt Quantity
The house in our example could be attacked by 8,000 pounds of earthquake force on each side. in any direction. 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
Each linear foot of good quality plywood can resist 600 pounds of force. If we divide 8,000 lbs (the amount of force each wall must resist) by 600, we get 13.3 linear feet of plywood. We round this up to 14 linear feet. That is all the plywood we need on each side of the house.
Shear Transfer Ties Quantity
The same method is used to determine the required number of shear transfer ties. 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.
Earthquake Movement Prevented by Shear Transfer Ties
The Completed Seismic Retrofit
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