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What is an Earthquake Retrofit?

What is Residential Earthquake Retrofitting?

THIS DAMAGE WAS CAUSED IN THE 1994 NORTHRIDGE EARTHQUAKE DUE TO A CRIPPLE WALL COLLAPSE

THIS DAMAGE WAS CAUSED IN THE 1994 NORTHRIDGE EARTHQUAKE DUE TO A CRIPPLE WALL COLLAPSE

This article discusses the retrofit of homes frequently found in the older parts of the Bay Area such as Oakland, Berkeley, and El Cerrito.  This type of retrofit is rarely found in cities such as San Jose or Fremont because the housing stock is much newer.

Retrofitting Residential seismic retrofitting refers to attaching the floor of a house to the 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.

This amazing story from the 1989 Bay Area Loma Prieta is proof that retrofitting works.

 

Seismic Retrofitting in Four 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

Here you can see all the components of a cripple wall.  Each component is connected to another component.  If any of the connections between these components fail the the cripple wall will collapse.  One must make sure all these connections are tight because you are only as strong as your weakest link. 

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. IF ANY AREAS ARE OVERLOOKED THE SEISMIC RETROFIT WILL BE WEAK

Cripple wall collapse is the number one house killer in any earthquake.  The proper application of seismic retrofit engineering principles prevents this.

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.

IF A HOUSE HAS A CRIPPLE WALL THIS IS HOW PLYWOOD KEEPS IT FROM COLLAPSING.  THIS IS THE MOST IMPORTANT PART OF ANY RESIDENTIAL SEISMIC RETROFIT

IF A HOUSE HAS A CRIPPLE WALL THIS IS HOW PLYWOOD KEEPS IT FROM COLLAPSING.  THIS IS THE MOST IMPORTANT PART OF ANY RESIDENTIAL SEISMIC RETROFIT

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.  

Foundation Bolting

 THE HATCHED AREA REPRESENTS A CRIPPLE WALL THAT HAS BEEN BRACED WITH PLYWOOD ON EACH END. IF THE CRIPPLE WALLS ARE NOT ATTACHED TO THE FOUNDATION WITH BOLTS THE BRACED CRIPPLE WALLS CAN FALL OFF

THE HATCHED AREA REPRESENTS A CRIPPLE WALL THAT HAS BEEN BRACED WITH PLYWOOD ON EACH END. IF THE CRIPPLE WALLS ARE NOT ATTACHED TO THE FOUNDATION WITH BOLTS THE PLYWOOD BRACED CRIPPLE WALLS CAN SLIDE OFF THE FOUNDATION

 

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. THE RETROFIT FOUNDATION BOLTS ACT AS A COUNTER FORCE TO PREVENT THIS AND ARE PART OF EVERY SEISMIC RETROFIT

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. 

 

THIS HARDWARE WAS DEVELOPED AFTER THE 1994 NORTHRIDGE AS A RESPONSE TO PERVASIVE DAMAGE TO FOUNDATION BOLTS.

 

THIS HARDWARE CAN INCREASE FOUNDATION BOLT STRENGTH BY 143%

THIS HARDWARE CAN INCREASE FOUNDATION BOLT STRENGTH BY 143%

Attaching the Floor to the Cripple Walls

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 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 IS WHAT CAN HAPPEN IF THE FLOOR IS NOT ATTACHED TO THE OTHERWISE RETROFITTED CRIPPLE WALL.

  • All seismic retrofit guidelines require the installation of shear transfer ties and the types of shear transfer ties these guidelines specify should always be used.  There are a number of 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

THE WAY THE FLOOR IS ATTACHED TO THE TOP OF THE CRIPPLE WALL IS BY CREATING A COUNTER FORCE WITH METAL FRAMING ANCHORS ALSO KNOWN AS SHEAR TRANSFER TIES.  THESE ARE VITAL TO ANY EFFECTIVE WOOD FRAME HOME RETROFIT

 

Each type of shear transfer tie as is the case with all other seismic retrofit hardware has been evaluated in terms of its ability to resist earthquakes.   Always check to see that you are using the highest strength shear transfer ties available

Movement of floor being resisted by shear transfer ties

THE PIECES OF STEEL HERE ARE EXAMPLES OF TWO COMMONLY USED SHEAR TRANSFER TIES THOUGH THERE ARE MANY OTHERS. EACH ONE HAS BEEN EVALUATED TO SEE HOW MUCH EARTHQUAKE FORCE IT CAN RESIST

 

Below is another kind of Shear Transfer Ties, there are many others. 

Earthquake Force Being Resisted by Shear Transfer Ties

EARTHQUAKE FORCE BEING RESISTED BY SHEAR TRANSFER TIES

Below is a pictorial summary of the three components of every 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

HERE IS A FULLY RETROFITTED CRIPPLE WALL. IT HAS BEEN BOLTED, BRACED WITH PLYWOOD, AND CONNECTED TO THE FLOOR.

A Well Built Shear Wall Must Have a 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.

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 called a Load Path.  If any Component within the Load Path Chain is missing or weak the Retrofit will Fail.

The Whole House Does Not Need a Retrofit.

This house was thrown off its foundation in an 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, plywood, and shear transfer ties to convert the cripple wall into a shear wall.

HIS HOUSE FELL FROM ITS FOUNDATION IN THE 1989 SAN FRANCISCO EARTHQUAKE WITHE CATASTROPHIC CATASTROPHIC CONSEQUENCES

THIS HOUSE WOULD HAVE REMAINED FULLY HABITABLE IF A RETROFIT  HAD ATTACHED IT TO ITS FOUNDATION.

Why so little damage?

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. Unfortunately, the house was not attached to its foundation, so in spite of holding together well above the floor level, it still suffered severe damage. 

THIS HOUSE SLID OFF ITS FOUNDATION IN THE 1992 FERNDALE EARTHQUAKE IN CALIFORNIA BECAUSE OF ITS TALL CRIPPLE WALLS.

THIS HOUSE FELL FROM ITS FOUNDATION IN THE 1992 FERNDALE EARTHQUAKE IN CALIFORNIA. THE AUTHOR OF THIS ARTICLE WAS THE FIRST FEMA INSPECTOR TO EVALUATE IT. 

 Below is the Same House from 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.

THIS IS THE SAME HOUSE AS I SAW IT WORKING FOR FEMA WHEN I WENT INSIDE. WHEN HOUSES FALL PLASTER CAN FALL FROM THE ROOF AND CEILING

THIS IS THE SAME HOUSE AS I SAW IT WORKING FOR FEMA WHEN I WENT INSIDE. WHEN HOUSES FALL PLASTER CAN FALL FROM THE ROOF AND CEILING.  PLUMBING AND ELECTRICAL SYSTEM WERE ALSO RUINED. 

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.

THE CRIPPLE WALL COLLAPSED UNDER THIS BEAUTIFUL HOUSE THAT THE AUTHOR SAW IN WATSONVILLE AFTER THE LOMA PRIETA EARTHQUAKE. THE LAST TIME HE DROVE BY IT WAS A VACANT LOT

THE CRIPPLE WALL COLLAPSED UNDER THIS BEAUTIFUL HOUSE THAT THE AUTHOR SAW IN WATSONVILLE AFTER THE LOMA PRIETA EARTHQUAKE. THE LAST TIME HE DROVE BY IT WAS A VACANT LOT

How does Ground Movement Cause Damage?

MOVEMENT OF EARTH SHOWING HOW EARTHQUAKES DAMAGES HOUSES

MOVEMENT OF EARTH SHOWING HOW EARTHQUAKES DAMAGE HOUSES

When the ground 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 slides 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 moves as it is pushed forward.  A retrofit prevents this movement.

Cripple Wall Collapse Photos

Below are some Examples of Homes that fell off their Foundation either because the cripple walls were not braced with plywood, were not bolted, of the floors were not attached to the cripple walls.

CRIPPLE WALL COLLAPSE WATSONVILLE LOMA PRIETA 1989

THIS IS A HOUSE THE AUTHOR INSPECTED WHILE WORKING FOR FEMA. THIS HAPPENED TO AT LEAST HALF THE HOUSES ON THE BLOCK

In the photo above, look at the top of the concrete steps, then see the porch which is a few feet below it.  This is the first sign that the cripple wall has collapsed.

 

 

SEISMIC RETROFIT FOR OLD HOUSE WITH CRIPPLE WALL

HERE IS ANOTHER CASE OF A CRIPPLE WALL COLLAPSE. SEISMIC RETROFITS PREVENT THIS KIND OF DAMAGE THE RED NOTICE ON THE WALL MEANS THE AUTHORITIES DEEMED THIS HOUSE DANGEROUS TO ENTER. 

 

  

NEWER HOME ON A HILLSIDE WITH EARTHQUAKE DAMAGE TO THE CRIPPLE WALL

NEWER HOME ON A HILLSIDE WITH EARTHQUAKE DAMAGE TO THE CRIPPLE WALL

 

 

CRIPPLE WALL COLLAPSE COALINGA EARTHQUAKE

ONE OF MANY HOUSES WITH A CRIPPLE WALL COLLAPSE IN THE PETROLIA  EARTHQUAKE IN NORTHERN CALIFORNIA

 

Prepare for aftershocks.  The earthquake you just felt is probably only an appetizer

THIS CRIPPLE WALL HAS ALMOST COLLAPSED AND THE HOME OWNER IS USING 2 BY 4 BRACES TO TRY AND PREVENT COMPLETE COLLAPSE IN AN AFTER SHOCK

THIS CRIPPLE WALL HAS ALMOST COLLAPSED AND THE HOME OWNER IS USING 2 BY 4 BRACES TO TRY AND PREVENT COMPLETE COLLAPSE IN AN AFTER SHOCK

 

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.

MOVEMENT OF PLYWOOD BEING RESISTED BY FOUNDATION BOLTS IN THE MUDSILL

MOVEMENT OF PLYWOOD BEING RESISTED BY FOUNDATION BOLTS IN THE MUDSILL

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.

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

 

Using Plywood Blocks for Shear Transfer Ties

Sometimes construction blocking was not placed between the floor joists when a house was built. This means the joists can roll over like a deck of cards. To prevent this, lock blocks are used.

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

 

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 PLYWOOD NEEDS TO BE CONNECTED TO THE BOLTS IN ORDER TO MAKE A SHEAR WALL. MODIFICATIONS TO THE WALL FRAMING ARE NECESSARY.

One way to address this is with the Nailed Blocking Method

THIS SHOWS HOW THE PLYWOOD CAN BE CONNECTED TO THE BOLTS BY NAILING 2 BY 4 BLOCKS BETWEEN THE 2 BY 4 STUDS. THE PLYWOOD IS THEN NAILED TO THE BLOCKS.

On the left, blocks have been nailed onto the mudsill.  On the right, the plywood has been nailed 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. The blocks, usually 14 inches long, 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 IS A PHOTO OF A SPLIT BLOCK. ONE WAY CONTRACTORS AVOID SPLITTING BLOCKS IS BY USING SMALLER NAILS; USUALLY SO SMALL THE RETROFIT WILL BE COMPROMISED

THIS IS A PHOTO OF A SPLIT BLOCK. ONE WAY CONTRACTORS AVOID SPLITTING BLOCKS IS BY USING SMALLER NAILS; USUALLY SO SMALL THE RETROFIT WILL BE COMPROMISED

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.

THIS 2 BY 4 BLOCK HAS BEEN SPLIT BY THE NAILS IN TWO LOCATIONS. SPLIT BLOCKS ARE EXTREMELY DIFFICULT TO REMOVE SO CONTRACTORS ARE TEMPTED TO USE SMALLER NAILS

THIS 2 BY 4 BLOCK HAS BEEN SPLIT BY THE NAILS IN TWO LOCATIONS. SPLIT BLOCKS ARE EXTREMELY DIFFICULT TO REMOVE SO CONTRACTORS ARE TEMPTED TO USE SMALLER NAILS

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 added it covers the blocks.  There is no way for home inspectors to tell if the side of the blocks were split which can be a problem when you sell the house.  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.

ONE METHOD OF ATTACHING 2 BY 4 BLOCKS TO THE MUDSILL IS BY USING STAPLES. IN THIS CASE OVER 100 STAPLES WITH THE CAPACITY OF 50 NAILS AND NO SIGN OF THE BLOCK SPLITTING

ONE METHOD OF ATTACHING 2 BY 4 BLOCKS TO THE MUDSILL IS BY USING STAPLES. IN THIS CASE OVER 100 STAPLES WITH THE CAPACITY OF 35 NAILS AND NO SIGN OF THE BLOCK SPLITTING

The Reverse Blocking Method of Building a Shear wall

The image on the left shows a 2 by 4 ready to nail to the plywood.   The 2 by 4 with the attached plywood is then nailed to the top of the mudsill. Because the 2 by 4 is so long it will not split.

THIS IS ANOTHER METHOD OF ATTACHING THE PLYWOOD TO THE BOLTS AND IS CALLED THE REVERSE BLOCKING METHOD. IT HAS BEEN EVALUATED BY THE LEADING TESTING LABORATORY IN THE COUNTRY AND IS CONSIDERED ONE OF THE BEST METHODS USED WHEN DOING RETROFIT WORK.

THIS IS ANOTHER METHOD OF ATTACHING THE PLYWOOD TO THE BOLTS AND IS CALLED THE REVERSE BLOCKING METHOD. IT HAS BEEN EVALUATED BY THE LEADING TESTING LABORATORY IN THE COUNTRY AND IS CONSIDERED ONE OF THE BEST METHODS USED WHEN DOING RETROFIT WORK.

THIS PHOTO SHOWS A CONTRACTOR NAILING THE REVERSE BLOCK INTO THE MUDSILL. THE PLYWOOD WAS NAILED TO THE REVERSE BLOCK BEFORE IT WAS PLACED IN THIS POSITION

THIS PHOTO SHOWS A CONTRACTOR NAILING THE REVERSE BLOCK INTO THE MUDSILL. THE PLYWOOD WAS NAILED TO THE REVERSE BLOCK BEFORE IT WAS PLACED IN THIS POSITION.  WITH THIS METHOD, ANY SPLITS IN THE 2 BY 4 ARE VISIBLE. 

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

THIS METHOD OF ATTACHING THE PLYWOOD TO THE FOUNDATION BOLTS IS CALLED THE FLUSH CUT METHOD AND IS THE ONLY ONE THAT HAS BEEN THOROUGHLY TESTED.

THIS METHOD OF ATTACHING THE PLYWOOD TO THE FOUNDATION BOLTS IS CALLED THE FLUSH CUT METHOD AND IS THE ONLY ONE THAT HAS BEEN THOROUGHLY TESTED.

The Flush Cut Method is the only way to build a retrofit shear wall 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 which is accepted by all building departments in the U.S.

In this method the mudsill is cut flush with the 2 by 4’s, using a special saw.  This allows the plywood to be 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.  Some people are concerned that removing part of the mudsill will weaken it.  According to research it does not.

The following photographs show the step-by-step process.

This shows how the mudsill is cut so the plywood can be attached to the bolts for the retrofit shear wall. This is the best method.

This shows how the mudsill is cut so the plywood can be attached to the bolts for the retrofit shear wall. This is the best method.

Removing mudsil from foundation bolts

THIS SAW IS QUITE DANGEROUS AND THIS SHOULD BE DONE BY A SKILLED CONTRACTOR. 

 

 

 

OLD GROWTH REDWOOD MAKES MUCH STRONGER SHEAR WALL AND IF THE FLUSH CUT METHOD IS USED THE PLYWOOD CAN BE NAILED DIRECTLY INTO IT. HERE IS A COMPARISON BETWEEN OLD GROWTH REDWOOD AND TREE FARM WOOD YOU BUY THESE DAYS

OLD GROWTH REDWOOD MAKES A MUCH STRONGER SHEAR WALL. HERE IS A COMPARISON BETWEEN OLD GROWTH REDWOOD AND TREE FARM WOOD AVAILABLE TODAY. WITH THE FLUSH CUT METHOD, PLYWOOD CAN BE NAILED DIRECTLY INTO THE OLD GROWTH MUDSILL & STUDS.  THE CLOSER THE GROWTH RINGS ARE SPACED, THE BETTER THE NAILS HOLD WHEN ROCKED BY AN EARTHQUAKE .

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.

Additions

FLOOR BEING PUSHED INTO SIDE AND BACK YARDS. THIS IS A VIEW FROM THE TOP AS IF YOU WERE IN A HELICOPTER

FLOOR BEING PUSHED INTO SIDE AND BACK YARDS. THIS IS A VIEW FROM THE TOP AS IF YOU WERE IN A HELICOPTER

 

 

CONTINUITY TIES AND FLOOR TO FLOOR CONNECTORS PREVENT A HOUSE FROM BEING PUSHED INTO THE SIDE AND BACK YARDS

CONTINUITY TIES AND FLOOR TO FLOOR CONNECTORS PREVENT A HOUSE FROM BEING PUSHED INTO THE SIDE AND BACK YARDS

Overturning of Shear Walls

When shear walls are tall 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

 

THE LATERAL FORCE OF AN EARTHQUAKE TRANSFERS ALONG ALL EDGES OF A SHEAR WALL

THE LATERAL FORCE OF AN EARTHQUAKE TRANSFERS ALONG ALL EDGES OF A 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 occurs where the plywood lifts up and away from the mudsill.

THIS DRAWING SHOWS HOW EARTHQUAKE FORCES TRY AND FLIP OVER THE SHEAR WALL. THIS IS CALLED OVERTURNING. IF THE OVERTURNING FORCE IS TOO GREAT THE PLYWOOD WILL BE DISCONNECTED FROM THE BOLTS

THIS DRAWING SHOWS HOW EARTHQUAKE FORCES TRY AND FLIP OVER THE SHEAR WALL. THIS IS CALLED OVERTURNING. IF THE OVERTURNING FORCE IS TOO GREAT THE PLYWOOD WILL BE DISCONNECTED FROM THE BOLTS

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

This shows how overturning forces cause the nails and plywood to be torn apart

THIS IS A CLOSE UP IMAGAE THAT SHOWS HOW OVERTURNING FORCES CAUSE THE NAILS AND PLYWOOD TO BE TORN APART

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.

Overturning forces are resisted by hold downs and place on each side of the shear wall

OVERTURNING FORCES ARE RESISTED BY HOLD DOWNS AND PLACED ON EACH SIDE OF A SHEAR WALL

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.

This image shows how overturning forces being resisted by hold downs pull up on the foundation

OVERTURNING FORCES BEING RESISTED BY HOLD DOWNS THAT PULL UP ON THE FOUNDATION

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.

Earthquake Forces pulling up Hold Down which is designed to resist Overturning Forces.

EARTHQUAKE FORCES PULLING UP ON HOLD DOWN WHICH ARE DESIGNED TO RESIST OVERTURNING FORCES.

This is a vertical Load Path.   As Earthquake Forces pull up on the 2 by 4 at the end of the Shear Wall it pulls up on the Hold Down which pulls up on the bolt which pulls up on the concrete which keeps the 2 by 4 in place and prevents overturning.

OVERTURNING FORCES RESISTED WITH CONCRETE MASS

IN SOME CASES THE OVERTURNING CAN BE SO GREAT THAT CONCRETE MUST BE PLACED UNDER THE HOLD DOWNS

 

 

A photograph of a hole dug under foundation for a hold down. On average 3000 pounds of concrete are put here

HOLE DUG UNDER THE HOLD DOWN.  IN THESE CASES 3000# OF CONCRETE ARE REQUIRED

Additions

 

 

 

 

 

 

 

 

 

 

 

 

Installing enough Retrofit Components

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 ; 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

 

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 lbs) then the house will resist 16,000 lbs of force no matter which direction the earthquake force comes from.

 All bolts, shear transfer ties, and types of plywood are rated in their ability to resist eartrhquakes rated in lbs.  We just need the bolts etc. to be of sufficient quantity to resist 16,000 lbs of force.

EARTHQUAKE FORCES ATTACKING A HOUSE                                                                                                                  EARTHQUAKE FORCES ATTACKING A HOUSE

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.

DETERMINING THE NUMBER OF BOLTS REQUIRED

DETERMINING THE NUMBER OF BOLTS REQUIRED

                                                                                                                                                                           

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.

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 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.

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

 

This is a complete seismic retrofit.   This has made it seem very simple but in the real world it is far more complicated than you see here.  A 180 page book was written that exclusively addresses the shear wall construction and design practices.

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

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

 

 

 

Areas Served

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

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Works' Comp RWCC64393236
Contractors Lic #558462
Bond #SC6334450
Liability Ins PCA 1045011
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