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Using Plywood to Convert Cripple Walls into Shear Walls

 

Things you need to know about plywood

Screenshot at Aug 09 16-49-20

The trickiest and most important part is the plywood.

The most important part factors determine plywood’s ability and a shear walls performance to resist earthquakes and each of these factors must be considered.  The type of nails or staples used, their size and length, plywood thickness, the species of wood, the manufacturing of the plywood, and the wall framing all play a part in the plywood installation.

The table below is from APA Research Report 154.  Below are some instructions on how to read it.

The black and red arrows point at numbers that represent the pounds of earthquake force each linear foot of plywood can resist if nailed in a manner consistent with the table.  For example, if we go the bottom row of the table that starts with 15/32 and follow it left to right we see that:

154 3 Arrows(1) The Panel Grade is Structural 1 plywood.

(2) The plywood’s thickness is 15/32″

(3) The penetration into the framing is 1 1/2 inches.

(4) The nail size is 8d  (8d or 10d is simply a way of describing nails a certain length and diameter)

(5) The plywood is nailed on the edges 6″, 4″,3″ or 2″ apart.

Plywood Strength Makes All the Difference

In this case we get different strengths, for example a shear wall will have 430 pounds of resistance per linear foot if the “Nail Spacing at Edges” is 4″ apart with 8d nails and 870 pounds of resistance when nailed with 10d nails 2″ apart.  What kind of nailing would you want for your house?

These higher capacity shear walls are  not always feasible, and only the person who determines the shape, size, and condition of your existing house can make that determination.

In this case we get different strengths, for example a shear wall will have 430 pounds of resistance per linear foot if the “Nail Spacing at Edges” is 4″ apart with 8d nails and 870 pounds of resistance when nailed with 10d nails 2″ apart.  What kind of nailing would you want for your house?

The Science of Plywood Nailing

The best plywood in a seismic retrofits is called Structual 1.  It has been specially formulated to brace walls against earthquakes, including cripple walls.  The closer the nails are on the edges of the plywood when nailed to the 2 by 4 wall framing, the stronger the plywood will be.   All retrofit guidelines  require nailing that is 4″ apart, even though plywood nailed at 2″ apart makes the earthquake resistance of the plywood 58% stronger.  Depending on how your retrofit was designed, your proposal may mention that the plywood will be nailed 2″ apart.

It is very clear that these guidelines only allow this low strength plywood nailing.  These guidelines recommend the use of the Nailed Blocking Method.  Because these blocks split so readily it was decided that no more than 4 nails would be allowed for each block.  Each nail can resist 117# of force.  Multiply that by 4 and you get 471# of resistance.  There is no point in nailing the plywood such that it exceeds the strength of these blocks.  It was therefore decided that 8d nails would be spaced 4″ apart which provides 484# of resistance per linear foot.  An almost exact match.

If the Nailed Blocking method is not used closer nailing is not a problem as born out by the hundreds of tests done by the American Plywood Association.  The APA’s allowance of this spacing is quite conservative meaning it has no problem in condoning this nail spacing in the nationally adopted International Building Code.  Retrofit has not seen any problems with nail spacing even as close as 1″ apart, let alone 2″ apart as approved

2 by 4 block with plywood

 

These higher capacity shear walls are  not always feasible, and only the person who determines the shape, size, and condition of your existing house can make that determination.

Non-Conventional Shear Walls are Vital to a Good Seismic Retrofit

 

Double Sided

Double sided shear walls are useful when space for a shear wall is limited and a new shear wall is required.   By using a double sided shear wall one can have 6 linear feet of foundation, install a 6 foot long shear wall on each side, achieve the strength of 12 linear feet of shear wall that normally would have take 12 linear feet of foundation.

Further on in the report it states:

“Typical failure of these walls was in compression and crushing of the lumber framing where the end studs bore against the bottom and top plates..The designer should carefully consider column buckling (snapping like a pencil) of the end framing members and bearing on the bottom plate in order to transmit these forces in compression to the foundation and in tension to hold-downs. In some cases it may be desirable to stop the plate short of these end studs and allow the end studs to bear directly upon the foundation.  In light of this the designer should carefully consider column buckling of the end framing members (they can buckle and snap).  This is done by carefully sizing the end framing members, reinforcing them with steel, or having the end studs bear directly on the foundation.”

The Most Technical Parts of Building Retrofit Shear Walls

1/8” crushing of the end studs into the mudsill at the bottom plate can cause over 1″ of at the top on a narrow shear wall.  The magnification factor is the height of the shear wall / width of the shear wall x the amount of mudsill compression.  If the walls are too flexible they will not resist much earthquake force when the whole house deforms (twists) because of the earthquake.

For example, if the end framing member studs on an 8 foot tall by 4 foot wide shear wall  crush the mudsill 1 inch, the deflection at the top will be 2 inches, which is significant.  (8/4 = 2,  x 1 inch compression = 2 inch movement.

Note that a normal shear wall has deflections at the top of the wall in the range of about 0.2 inches at their design loads, so you can see that a little crushing can cause big problems when a narrow shear wall is used in line with other conventional shear walls.

This is more of a deflection issue (lateral movement of the top of the shear wall) rather than a strength issue.   1/8” crushing at the bottom plate can be translated to over an inch of deflection at the top for a narrow shear wall.  The magnification factor is the height of the shear wall / width of the shear wall.  If the walls are too flexible they will not resist much earthquake force when the whole house deforms (twists) because of the earthquake.

High Capacity Shear Walls

In a series of experiments done by the American Plywood Association to test the limits of shear walls strength.  Their finding can be found in APA Research Report 138

138 Word

Looking at this table we are using:

(1) Structural 1 Plywood

(2) Plywood that is 23/32 ” thick.

(3) The stud framing that is 4″ wide.

(4) The rows of nails.  For example, 3 lines of fasteners means there are 3 rows of nails going up and down on the studs.

APA Research Report 138 :  describes tests prove show very high strength shear walls can be produced by using multiple rows of nails or staples in wood framing that is wider than the normal 1-1/2 inch wide framing used in new construction.  These high capacity shear walls also need high capacity foundations.  As you can see a shear wall built in this way can resist 1900 pounds of earthquake force and represents the strongest shear wall ever tested.  Even though it has never been tested, a two sided shear wall of this type could have an enormous ability to resist earthquakes.

Overturning Forces in High Capacity Shear Walls

A typical application would be typical building in San Francisco where much of the front lower story is taken up by a garage and the rest is taken up by a stair wall.  In these circumstances the front of the building is not connected to any foundation.  This is also a preferred method in terms of effectiveness and often cost compared to moment columns. 

The next consideration is uplift or overturning forces.  High capacity shear walls must resist a tremendous amount of overturning.  For example, if the shear wall can is resist 1900 pounds per linear foot of lateral earthquake force, it will also need to resist #15,200 pounds of overturning force.    For this reason proper sizing of hold downs is critical.  Under Model No. is the name of the hold down.

Hold Downs

Stapled Shear Walls

 

Stapled SW

Stapled shear walls are a consideration when one is concerned about the framing behind the plywood splitting.

The thickness of plywood has no bearing on shear wall strength except in the case of high capacity shear walls.  For these shear walls, plywood up to 19/32″ (3/4) thick was tested and it was discovered shear wall built in this manner were on average three times stronger than the shear walls found in  the California Building Code.

This type of shear wall is extremely useful when one needs the strongest shear wall possible and a foundation strong enough to withstand this force.  A typical application would be shear at the front or back of a long apartment house, such as those found in San Francisco, built on a new foundation with extensive steel reinforcing.

Stapled shear walls are a consideration when one is concerned about the framing behind the plywood splitting.  According to the American Plywood Association one should be able to double the number of nails and double the strength of the shear wall even though this was not tested.  In other words, if you double the staple spacing, you should be able to double Target Shear.  

Target shear is another name for “allowable load” which is the value the building code says you can use when designing a shear wall.  “Ultimate Load” is the point at which the test specimen actually failed.  “Load Factor” is the safety factor.  Meaning if you have a shear wall that fails on the testing table (ultimate load) and divide this by the safety factor (load factor) the result is the “Target Design Shear” or allowable load.  Scientist have a way of making everything more complicated than they need be.

Staples not create high capacity shear walls, but if spaced close enough together they are extremely stiff.  This can be useful when designing a shear wall that will be working in tandem with other shear walls made of a stiffer material such as plaster. 

Plywood to Plywood Connections

PLy to Ply

In this test plywood was stapled to plywood to see if how strong this connect would be.  This is useful when it is necessary to attach to pieces of plywood together which is often the case when the contractor did not attach the first layer of plywood to the mudsill. 

Sill Nailing

 The next consideration is uplift or overturning forces.  High capacity shear walls must resist a tremendous about of overturning.  For example, a high capacity shear wall can create 14,400 pounds of force trying to lift the ends of the shear wall up off of the foundation.  For this reason proper sizing of hold downs is critical.  

 

Quality of Cripple Wall Framing

Older homes were built with old growth Douglas Fir and redwood that was centuries old.  This wood has very different properties compared to wood grown on modern tree farms.  The old wood is much denser and is very difficult to split compared to  tree farm lumber.  For this reason the retrofit guidelines found in the International Existing Building Code, the Bay Area.s Standard Plan A, Seattle’s Project Impact,and  the Los Angeles Retrofit Building Code only allow 8d nails spaced no closer than 4 inches apart.  Old growth lumber should always be used whenever possible because it can easily be nailed with larger 10d nails 2 inches apart on the edges without splitting.Pnoto: Flush cut sill is best for earthquake retrofits

Seismically Bracing a Failing Cripple Wall Photo

If the initial shock does not collapse the cripple wall, an after shock might. In the photo below, a  man trying to prevent his already leaning cripple wall from fully collapsing in an after shock.

 You need the right type of Plywood

The two types of plywood available are Rated and Structural One, but for shear wall use the plywood must have 5 plies. Rated Plywood can be made of any species of wood while 10% stronger Structural 1 must be made of denser Southern Pine or Douglas Fir.  Overall it is not that big a deal if you use Rated instead of Structural one.

Avoid 3-PLY PLYWOOD

3 ply plywood tore in Northridge Earthquake

Plywood made with only 3 plies tore in the 1994 Northridge Earthquake and should not be used

Shear walls made of 3 ply plywood tore in the Northridge Earthquake so the City of Los Angeles downgraded  the acceptable limits for 3-ply plywood to a maximum of #200 plf.  On page 10 of the Wood-Frame Subcommittee Findings Report published immediately after the Northridge Earthquake it says: “The performance of 3-ply construction has raised questions of its ultimate capacity.  Horizontal tearing has occurred on some outer face plies above the inner ply seam.  Values for all 3-ply panel construction were therefore reduced to 200lbs/ft maximum.”

 

Aspect Ratio-This is very technical

An aspect ration is the ratio between the height and the width.   For example, a shear wall that is 8 feet long and 4 feet wide has an aspect ration of 2h/1w (the height is twice as long as the width).

To use the values listed in the shear wall tables above, a shear wall must have a 2/1 aspect ratio or less.  If the aspect ratio is greater 2/1 but less than 3.5/1, the per linear foot of earthquake resistance of the shear wall must be reduced by what is called a reduction factor.

This maximum 3.5/1 aspect ratio translates into an 8 ft. shear wall 27.5″ wide.  Any narrower than this and you have a post.

The way you figure out the aspect ratio of a shear wall is to divide both the height and the width by the width.  For example: if a shear wall is 64″ high/18″ wide, we divide both the height and the width by 18 to get a ration of 3.5/1.  If the wall is 96″ tall then 96″/18″ = 5.3/1.  This is a post and not a shear wall.

Once we determine the aspect ratio, assuming it is less than 3.5/1, we use a reduction factor of twice the width 2w/h (height).  So if we have a shear wall that is 96″ tall and 32″ wide the reduction factor is twice the w/h = 2 x 32/96 = 0.666.  The allowable shear in the table is multiplied by this factor to get the reduced shear capacity of the narrow wall.

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