Cripple Wall Seismic Retrofit Methods – VIDEO

 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. We suggest that you watch the video and read the material to help you understand your retrofit. If you are ready to calculate materials for your job, you can jump to that section.

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

HOME WITH CRIPPLE WALL NEEDING A RETROFIT

THIS IS A GOOD EXAMPLE OF A CRIPPLE WALL. IF YOU HAVE MORE THAT 3 STEPS LEADING INTO YOUR FRONT DOOR YOU PROBABLY HAVE A CRIPPLE WALL AND NEED A SEISMIC RETROFIT

THIS IS A PHOTO OF A TYPICAL CRIPPLE WALL THAT WAS PARTIALLY RETROFITTED. YOU CAN SEE BLOCKS WITH RECTANGULAR CUT OUTS WHICH IS WHERE THE BOLTS GO.

A TYPICAL CRIPPLE WALL

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.

THIS ILLUSTRATION SHOWS ALL THERE AREAS WHERE A SEISMIC RETROFIT MIGHT FAIL. IF ANY AREAS ARE OVERLOOKED THE SEISMIC RETROFIT WILL FAIL

THIS ILLUSTRATION SHOWS ALL THE AREAS WHERE A SEISMIC RETROFIT MIGHT FAIL. 

                                                                   CRIPPLE WALL FAILURE.

 

Step One: Foundation Bolting

From left to right, Epoxy Bolt, Wedge Achor, and Titen Bolts

FROM LET TO RIGHT, EPOXY BOLT , WEDGE ANCHOR BOLT, AND TITEN BOLT

Bolting Cripple Wall Shear Walls When Standard Bolts Will Not Work.

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.
Short shear wall with bolt substitues

SHORT SHEAR WALL USING FOUNDATION ANCHOR BOLT SUBSTITUTES WHERE STANDARD BOLTS CAN’T BE INSTALLED BECAUSE DRILLING EQUIPMENT WON’T FIT

"

BOLTS PREVENT SLIDING OF THE CRIPPLE WALL OFF THE FOUNDATION

THE RED ARROW SHOWS HOW EARTHQUAKE FORCES PUSH AGAINST THE BOTTOM OF THE CRIPPLE WALL. RETROFIT FOUNDATION BOLTS COUNTERACT THIS FORCE. 

This is how bolts provided a counter force to an earthquake

Bolts Resisting Earthquake Forces Pushing on Bottom of Cripple Wall

BOLT RESISTING EARTHQUAKE FORCE BY CREATING A COUNTER FORCE OR RESISTANCE

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.

Cross grain bending tearing mudsill apart

CROSS GRAIN BENDING IS SOMETHING WOOD CANNOT RESIST WITHOUT SERIOUS DAMAGE

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.

Plate Washer on Foundation Bolt

PLATE WASHERS RESIST CROSS GRAIN BENDING. 

 

DAMAGE CAUSED IN THE BOLT TO WOOD MUDSILL CONNECTION CAUSED THE DEVELOPMENT OF THIS HARDWARE 

 

THIS HARDWARE CAN INCREASE FOUNDATION BOLT STRENGTH BY 143%

THIS HARDWARE CAN INCREASE FOUNDATION BOLT STRENGTH BY 143%

Step Two: Putting up the 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.

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TECHNICIAN NAILING PLYWOOD WHICH IS THE FINAL STAGE OF BUILDING A SHEAR WALL

MARTIN NAILING PLYWOOD WHICH IS THE FINAL STAGE OF BUILDING A SHEAR WALL

 

THIS HOME FELL OFF A CRIPPLE WALL THAT WAS AROUND 6 FEET TALL

HERE IS A TYPICAL EXAMPLE OF A CRIPPLE WALL COLLAPSE

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.  

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.

THIS IS WHY EVERY SEISMIC RETROFIT MUST ATTACH THE FLOOR TO THE TOP OF THE PLYWOOD BRACED CRIPPLE WALL. THIS IS BY FAR THE MOST COMMON OMISSION WE SEE IN THE EXISTING RETROFITS WE EVALUATE

THIS HOUSE SLID OFF  THE BOLTED AND PLYWOOD BRACED CRIPPLE WALLS.  AN EFFECTIVE RETROFIT CONNECTS THE FLOOR AND THE BRACED CRIPPLE WALL TOGETHER. 

  • 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 that don’t do anything at all. 
SHEAR TRANSFER TIES CREATE A COUNTER FORCE TO THE EARTHQUAKE FORCE STOPPING MOVEMENT OF THE FLOOR

THESE SHEAR TRANSFER TIES RESIST EARTHQUAKE MOVEMENT.  THESE ARE VITAL TO ANY EFFECTIVE WOOD FRAME HOME RETROFIT

 

Earthquake simulation testing on shear transfer ties tell us ow 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. 

Earthquake Force Being Resisted by Shear Transfer Ties

SHEAR TRANSFER TIES RESISTING THE EARTHQUAKE FORCE

Using Plywood Blocks for Shear Transfer Ties

In older homes, contractors on occasion did not install blocking between the joists.  This can cause the joists to roll over like dominoes.  Locks blocks prevent this.

Sometimes it is not possible to connect the floor to the floor joists with steel or plywood blocks in which case one must staple the plywood directly to the floor

STAPLING PLYWOOD DIRECTLY TO THE FLOOR IS SOMETIMES THE ONLY WAY TO CONNECT A SHEAR TO THE FLOOR IT IS TO RESTRAIN

Review of the Three Steps in a Cripple Wall Retrofit

 

The three components of an effective seismic retrofit

ALL THESE RETROFIT COMPONENTS WORK TOGETHER TO TRANSFER FORCES INTO THE FOUNDATION

The cripple wall has been converted into a Shear Wall that has Plywood, Bolts, and Shear Transfer Ties

HERE IS A FULLY RETROFITTED CRIPPLE WALL WHICH HAS BEEN BOLTED, BRACED WITH PLYWOOD, AND CONNECTED TO THE FLOOR. YOU SHOULD MAKE SURE YOUR CONTRACTOR UNDERSTANDS THIS

BOLTS, PLYWOOD, AND SHEAR TRANSFER TIES COMPLETE THE CRIPPLE WALL TO SHEAR WALL CONVERSION

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.

Earthquake Forces Travelling from Floor into Foundation through Shear Wall

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.

 

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.

 

The Problem

Below is an illustration showing why this is a problem.

Screenshot at Sep 07 18-40-55

One way to address this is with the Nailed Blocking Method

THIS SHOWS HOW THE PLYWOOD CONNECTS TO THE BOLTS BY NAILING 2 BY 4 BLOCKS BETWEEN THE 2 BY 4 STUDS.  THE CONTRACTOR WILL THEN NAIL THE PLYWOOD TO THE BLOCKS.


This seismic retrofit contractor is nailing 2 by 4 blocks to the mudsill.

THIS SEISMIC RETROFIT CONTRACTOR IS NAILING 2 BY 4 BLOCKS TO THE MUDSILL. THEN HE WILL BOLT THE BLOCKS TO THE FOUNDATION, AND NAIL PLYWOOD TO THE BLOCKS

The problem with this method is that the blocks  split. Contractors put blocks between the 2 by 4 wall framing and nail the blocks to the mudsill.  Splitting of the blocks is common.  Split blocks cause shear walls to fail. Almost all contractors use the nailed blocking method because it all government retrofit guidelines recommend it.  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 yourself before they nail up the plywood. There are actually much better ways.

The Importance of Connecting Top Plates Together

Top plates sit on top of the cripple wall and bear the entire weight of the house. If all of the top plates are not continuously connected, there will be problems.

Calculate the Amount of Materials Needed for Your Retrofit

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.  This is fully explained in this Video.

Retrofitting morerequired can strain a budget and is not necessary ; 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 (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.

EARTHQUAKE FORCES ATTACKING A HOUSE                                                            EARTHQUAKE FORCES ATTACKING A HOUSE

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.

DETERMINING THE NUMBER OF BOLTS REQUIRED

DETERMINING THE NUMBER OF BOLTS REQUIRED

                                                                                                                                                                           

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.

DETERMINING AMOUNT OF PLYWOOD NEEDED

DETERMINING AMOUNT OF PLYWOOD NEEDED

PLYWOOD PREVENTING A CRIPPLE WALL COLLAPSE

PLYWOOD PREVENTING A CRIPPLE WALL COLLAPSE

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.

DETERMINING THE NUMBER OF SHEAR TRANSFER TIES NEEDED

DETERMINING THE NUMBER OF SHEAR TRANSFER TIES NEEDED

HOW SHEAR TRANSFER TIES WORK

HOW SHEAR TRANSFER TIES WORK

TWO TYPES OF SHEAR TRANSFER TIES

HERE ARE TWO TYPES OF SHEAR TRANSFER TIES, THERE ARE MANY MORE

 

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.

 

A COMPLETE RETROFIT WITH BOLTS, PLYWOOD, AND SHEAR TRANSFER TIES

A COMPLETE RETROFIT WITH BOLTS, PLYWOOD, AND SHEAR TRANSFER TIES

This is a much more condensed and simplified way of describing the concepts found both in Appendix Chapter A3 of the California Building Code and the Bay Area’s regional guideline Standard Plan A. 

 

Areas Served

San Jose,Sunnyvale, Fremont,Oakland,Berkeley
And Surrounding Areas
408-664-6355
510-548-1111

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