What is an Earthquake Retrofit?

By Howard Cook, Founder and General Manager of Bay Area Retrofit

What is 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 hardware.  This is done to prevent the house from sliding off its foundation when the earthquake shakes it back and forth.  In most houses this involves building shear walls.  A shear wall is a structural element that transfers the side-to-side forces of an earthquake out of the house and 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

This is a good example of a cripple wall

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.

This Oakland house fell in the Loma Prieta Eartquake

Don’t Retrofit the Whole House

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.

Tall cripple wall failure

This house fell 6 feet

 Below is the Same House from the Inside

On the other hand, damage to interior walls, plumbing, and electrical systems can be catastrophic.

Earthquake damage in house from cripple wall failure.

Plaster falls off walls in earthquakes

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.

The house had cripple walls that collapsed in an earthquake

This house had a cripple wall collapse and was torn down

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.

Effect of ground movement on a house in large earthquakes

Earthquake Attacking House

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.

This is a photo of a cripple wall that was partially retrofitted with bolts and plywood

The Foundation, cripple wall, and floor must all be connected together.

 Cripple Wall Collapse

The illustration above shows what can happen to a cripple wall if plywood is not put on it.

These houses suffered damage because of no bolts or retrofit hardware

Cripple wall Collapse in the Northrdge Earthquake

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.

This house is about to collapse because it was never seismically retrofitted

Cripple wall ready to fall in the aftershock.

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.

The three main components of an effective earthquake retrofit

All the components of a Typical Cripple Wall that need Retrofitting

This is what happens when Cripple Walls are not Converted into a Shear Walls

This is the kind of damage plywood prevents

This is how Cripple Walls Collapse in Earthquakes

This is how plywood prevents earthquake damage

Structural plywood, made specifically to prevent cripple wall collapse has an almost miraculous ability to prevent damage like this.

 Foundation Bolting

 How do Bolts work?

Foundation bolts prevent this kind of earthquake damage

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.

Earthquake attacking house and saved with retrofit foundation bolts

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.

epoxy bolt used in foundation bolting A wedge anchor foundation bolt also used in many retrofits bolt-wedge-mechanism

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.

This is why it is important to attach the floor to the seismically braced cripple wall

this hardware keeps the floor attached to the cripple wall

A photo of the most common hardware used in cripple wall to floor connectoins

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.

The three components of an effective seismic retrofit

A cripple wall converted to shear wall with bolts,plywood and floor connectors.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 is how a seismic retrofit transfers forces to the ground

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.

The plywood connection in the shear wall is problematic

Plywood rests on Mudsill and cannot be attached to the bolts.

One way to address this is with The Nailed Blocking Method

Bolts and plywood are connected to the foundation with this blocking

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

Seismic retrofit contractor installing blocking for bolts and plywood

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.

This photo shows how nails can split the sill blockingUnfortunately, 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.

This block has been split by the nails and should not be used

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.

Sill blocking should use staples

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.

This method has been tested by the American Plywood Association

Contractor installing this tested shear wall system

The Carpenter is nailing directly into the reverse block with his nail gun

The Flush Cut Method of Attaching the Plywood to the Mudsill.

Another fully tested system for building seismic retrofit shear wallsThe 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.

best retrofit method seismic retrofit shear wall

Removing mudsil from foundation bolts

seismic retrofits work best with old growth lumber

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.

This is necessary to transfer earthquake forces to shear walls

The Problem with Top Plate Breaks in Shear Walls

  • 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.

This steel strap used to connect floor to shear wall

Another way to do this is with nails

This is another method of attaching the shear wall to floor

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.

 overturning forces acting on shear wall caused by earthquakes

Below is a close up of this kind of damage and shows how the plywood is no longer attached to the bolts

Overturning forces cause nails in shear wall to tear out of plywood

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.

The cripple wall has been retrofitted with StrongTie hold downs

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.

StrongTie hold downs resisting overturning of retrofitted shear wall

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.

StrongTie hardware using concrete to prevent overturning

It is extremely rare for the additional concrete to cause settling

A photograph of hole dug under foundation for hold down

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

earthquake forces attacking house

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.

how bolts in a seismic retrofit resist earthquakes and protect a house


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.

Amount of plywood needed in an effective earthquake retrofit

This is how plywood prevents a cripple wall collapse

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.

Simpson StrongTie hardware resists earthquakes

This is how Simpson Strongtie hardware resists earthquakes

Earthquake Movement Prevented by Shear Transfer Ties

The Completed Seismic Retrofit

photograph Simpson StrongTie seismic retrofit floor connector hardware

A complete retrofit with bolts, plywood, and floor connectors



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Bay Area Retrofit
427 San Pablo Ave
Albany, CA 94706
With field offices in San Jose and Hayward.

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