Critique of Standard Plan A

A Critique of Standard Plan A


The author of this article was a member of the committee that put Standard Plan A together over 12 years ago.  It was written in attempt to stop the flood of ineffective retrofits that have overwhelmed building departments.  Even though this standard will make your house stronger than it is now, recent developments in hardware manufacturing and retrofit procedures have shown that better retrofits at lower cost are possible.

When Bay Area Retrofit is required to use this standard rather than a designed retrofit, we improve on this standard by applying these new developments.  All of our retrofits adhere to the state of the art engineering principles behind Standard Plan A as it is the most accurate engineering available..

Below is a chart found in Standard Plan A that gives the earthquake-resisting capacities of various types of hardware.  This hardware is rated in terms of pounds of resistance.  For example, according to this chart, an L70 can resist 458 pounds of earthquake force pushing against it before it fails.

                                                                                           Table 1



The L70

In Table 1, an L70 can resist 458 lbs. of force.  Table 2 shows the actual value found in the manufacturer’s catalog.  The actual value of an L70 is 740 lbs, a difference of 282 lbs.   If the current catalog value is multiplied by 0.88 because it is close grain redwood, it is rated at 651 pounds.

The L90

In Table 1, an L90 can resist 600 lbs. of force, in Table 2 it can resist 925 lbs of force, a difference of 325 lbs.  If the current catalog value is multiplied by 0.88 because it is close grain redwood, it has a capacity of 814 pounds.

It is understandable that for simplicity’s sake one would want to assign these components the lower of two values.  In this case the lower values would make sense.  Inaccurate values however do not make sense.


                                                                                                  Table 2

L90 Table



In Table 1 an H10 (this hardware still exists but its name has been changed to H10A) can resist 505 pounds of force, though its value is uncertain because the wrong nails are specified.  The actual value in the equivalent H10A hardware using the correct nails found in Table 3 (see below) is 590 pounds of force.  Notice that the nails in Table 1 specified for the H10 are 8d nails 0.131 in diameter and 1 1/2 inches long.  Again, this is the wrong nail.  As shown in Table 3, the correct nails are 10d,  o.141 in diameter and 1 1/2 inches long.

In short,  as shown in Table 3, the earthquake resistance of H10A hardware designed for newer lumber and the H10AR for older lumber are not the same as claimed in Table 1 under the “H10” heading.   According to the latest manufacturer’s catalog, 505 pounds is incorrect for either hardware.   Using this low value increases cost.

                                                                                                                                                                      Table 3

H10 Values




Table 1, found in Standard Plan A, rates the capacity of a half-inch bolt as being 820 lbs.  In fact it is 1037 pounds according to recent tests.  Table 1 also rates the 5/8 bolt as having a value of 1070 lbs. while the actual value is 1485 lbs.  These actual higher values mean Standard Plan A requires more bolts than necessary, which increases cost.  The bolting would be especially effective if splitting of the mudsill and increase in bolt strength were increased with Mudsill Plates. The values below were found using the connector calculator found on the American Wood Council website.



half bolt

five eight bolt













 Half inch in Redwood.JPG

As part of the research done by the author of this article, a questionnaire was sent out to a number of structural engineers with an average of 35 years experience.  Many of these engineers lived in Southern California and had seen Northridge Earthquake damage first hand.   The single capital letters are the first letters found in each engineer’s names.  Below are the highlights.

QUESTION: Is it true that all the earthquake force goes to the stiffest element and therefore only to the shear panels?

Yes. Ke, L, R, S, T,K   No. Indeterminate. B

QUESTION: If it is true that most of the force goes to the stiffest element, how much earthquake load is transferred to the part of the cripple wall that has no shear walling?


L: Only a negligible amount of earthquake force transfers to the un-braced sections of cripple wall.

S, T, K, agreed with this statement made by Ke- “No shear forces will travel to the sections of cripple wall that have no plywood shear-walling. It is therefore not necessary to install bolts along the mudsill at non-shear wall locations. Whatever is currently holding the mudsill in place at non-shear wall locations should be enough.”

B: The stucco has a lot of resistance but breaks apart.  Once that happens it no longer functions as a shear wall and will no longer protect the house.

R: Very little shear force will travel to the sections of cripple wall that have no plywood shear-walling, depending on the existing wall sheathing – stucco in good condition will carry a lot of shear, but once the nails have corroded or failed in an earthquake, it’s not worth much.

This is reflected in the requirement that all of the bolts or UFP anchors must be installed within the plywood braced panels.

Bolts only at panels




This accurate understanding of retrofit engineering is further reinforced with this requirement

Bolts on each end






Elsewhere in Standard Plan A there is a requirement that bolts be placed outside of braced panels.  If we assume that bolts attached to braced panels are the only bolts that do anything, these bolts are extraneous and increase cost.

Bolts outside of panels.JPG

Empirical evidence points to the fact that in actual earthquakes, mudsills do not slide on foundations.   Anyone who has removed a mudsill on an older home knows that the mudsill was well connected to the concrete.  During construction, contractors could not allow the mudsill to flop around while the house was being built.  So they drove very large nails, almost 1/4 inch in diameter, through the mudsill, and then embedded the nails into the wet concrete.  If any mudsill should have slid, it would have been the one in the house shown here.

Blue House With Arrows


Or this house.

Mudsill did not move


Existing Bolts





This is absolutely correct if the existing bolts have over-sized holes in the mudsill and these bolts are to be considered part of the shear wall.   At non-shear wall locations they will be fine even though tests show they are only half the strength of a properly-sized hole because they will be absorbing so little force.

Oversized Hole No Washer


Oversized Hole Photo

Damage Caused by Over-Sized Bolt Holes

Floor to Cripple Wall Connections

Based on the principle that  the shear wall is going to take all of the force, it is a natural corollary that all floor-to-cripple-wall connections be placed as close to the braced panel as possible.


Table S1


This is further reinforced in the following construction detail:
STT Sixteen o c



Schedule 4/S1 does not specify intervals, it only specifies quantity. 

Plywood Installation

In the detail, the red arrow points at a recommendation that the top of the plywood be nailed 4 inches apart in the upper top plate and 4 inches apart in the lower top plate.  The 4 inch nailing in the upper top plate is called edge nailing, which determines the strength of a shear wall.  Nailing into the lower top plate is not edge nailing and contributes nothing to the strength of the shear wall.  Nailing the shear wall in this manner creates a shear wall that is only half as strong as it should be.  The green arrow points at some wording that the author has yet to decipher.  Something is wrong with the syntax.




















Fasteners in Pressure Treated Wood

Standard Plan A recommends galvanized nails be used in pressure-treated lumber, also known as ACQ.  Tests done in New Zealand have shown even hot dipped galvanize nails deteriorate in ACQ lumber and should never be used.


Presssure treated


Galv Nalls Corroded


The Retrofit Mudsill Connection

Blocking PSA


Blocks and plywood












This is a construction detail from Standard Plan A.                                                    This is a similar perspective of how blocking is used.             The 2 by 4 Blocking is behind the plywood and
the plywood is nailed to it.


Cripple wall to shear wall conversions are complex in nature and retrofit shear walls are even more complex.  When building a retrofit shear wall, the studs are  2 by 4 Douglas Fir and the mudsills (piece of wood on the concrete) are full sized 2 by 6 redwood.

The size differential between the 2 by 4 studs and the 2 by 6 redwood mudsill presents a problem that requires a modifications to the mudsill that does not produce split blocks.  The American Plywood Association has conducted thousands of tests on various ways of constructing shear walls and their tests always used 2 by 4s on the top, sides, and bottom.  The challenge is to make our retrofit shear wall  the same configuration as those tested by the American Plywood Association.

Screenshot at Sep 07 18-40-55


The four ways of making this connection are also explored more thoroughly in this paper, though the material below and watching the video will tell you most of what you need to know.


The Nailed Blocking Method found in Standard Plan A


2 by 4 blocks put between 2 by 4 studs and plywood 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.

Shear Wall Blocks being Installed

Why are Blocks a Problem?

The problem with this method is that the blocks split. The blocks, usually 14 inches long, are installed between the studs on the cripple walls as shown in the photo above.   If the blocks split, then the shear wall will fail.

Photo: Block Split in an earthquake retrofit

                                         Split block found in an earthquake retrofit











Retrofit Shear Wall built with a Split Block

                                      Obviously, this Split Block will not work.


Another Split Block on a Cripple Wall Retrofit

Standard Plan A requires the blocks be nailed into the blocks with nails that are quite large.   They need to be large  in order to resist an earthquake.  These large nails are what caused the split here.  Blocks hidden behind plywood are also a problem for private home inspectors, who must give informed opinions based on their evaluations.  If there are nailed blocks behind the plywood they can have no idea if they are split or not.   They can only tell the buyer he hopes they are OK.

The Best ways to Avoid Split Blocks

The most common solution is to use smaller nails.  That, however, seriously compromises the effectiveness of the retrofit.  It is not possible to tell the size of the nail unless it has been color-coded, which is the exception rather than the rule.   8d nails and 10d nails have the same head size so color coding is the only way to tell the difference.



Color Coded Chart

Color Coded Nails

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

In this method, the 6-inch-wide mudsill is cut flush with  the 2 by 4s.  The plywood is then nailed directly to the mudsill.

Screenshot at Sep 07 19-07-15


The Flush Cut Method is the only way to build retrofit shear walls that is very close to those tested by the American Plywood Association, a National Laboratory for Testing Shear Walls.  These tests are the basis of the building code provisions for new shear wall construction for the entire country.     The following photographs show the step-by-step process used in this method of retrofit shear wall construction.

Carpenter Cutting Mudsill Flush for an earthquake retrofit

Removing mudsil from foundation bolts

One of the reasons this method is used is because the plywood can now be nailed into old growth redwood that is many times less prone to splitting than wood grown on tree farms.


Pnoto: Flush cut sill is best for earthquake retrofits

The 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.


Wood Blocks in Cripple Wall Retrofits should be Stapled.

This Block has over One Hundred Staples and never Split.

 Stapled Block



The Reverse Blocking Method

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.





Which Method is Best?

None of them is “best”.  They all have pros and cons.  The Nailed Blocking Method is a good choice when tools are limited.  This method is a good one for non-professionals who want to do the work themselves and who can be relied upon not to split the blocks.   The Stapled Blocking Method is good in that you don’t have to worry about splitting the blocks, but the staple gun needed is very expensive.  The reverse blocking method is excellent in that you don’t need to cut lots of little blocks, and long 2 by 4s are much more difficult to split than blocks.  The Flush Cut Method is probably the best of all because it saves on blocking material and labor on cutting the blocks.  It is also the method that most closely resembles tested shear walls.  The big downside is that the flush cut saw is extremely dangerous.

Below is a letter written by the chief engineer in charge of research at the American Plywood Association, the organization that tests shear walls and informs the building code about how they should be built.  This letter was in response to a query made by one of the Standard Plan A committee members.  You can review the complete letter here.  





The Matter of Plywood Nailing

Nail Spacing




This part of Standard Plan A arose out of a concern the committee had that the blocks would split.  However empirical evidence does not support this claim, and has the effect that homeowners and contractors are building shear walls considerably weaker than they could if nails were allowed to be 3″ or even 2″ apart.  Both empirical evidence and testing by the American Plywood Association support this.   The 14-inch block below was nailed with 8d nails 1 inch apart.

2 by 4 block with plywood


The table below is a summary of test results published by the American Plywood Association, a national laboratory that informs the building code and the public about the best ways to nail plywood in shear walls.

The black and red arrows point at numbers that represent the pounds of earthquake force that each linear foot of plywood can resist if nailed in a manner consistent with the table.  For example, if we go to 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 of a certain length and diameter)

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

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.

From a contractor’s point of view, the less linear footage of plywood required translates into less materials and labor required.  If the same shear wall strength can be gained by changing the nail spacing, the homeowner and the public stand to gain because the cost of the retrofit will be reduced.


Top Plates and Cripple Wall Retrofits


Cripple wall top plates are actually segments of wall that can move independently of each other–boards that are 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, meaning the pull and push of the earthquake force is not continuous along the cripple wall top plate, and therefore the earthquake force does not have a continuous connection to the shear walls.  Steel straps, nails, and 2 by 4s are the three methods by which these breaks are joined together and continuity is restored.

For example, if you have a shear wall at one end of a cripple wall and it is not connected to the floor of the rest of the house, when this floor  moves there will be no way for the shear wall to restrain it.

Cripple Wall Retrofits and Top Plate Continuity

One way to do this is with a Steel Strap

Cripple Wall Retrofits and Steel Strap


Another way to do this is with nails

Cripple Wall Retrofits improved with Nails

Cripple Wall Retrofits improved with Nails

This allows a “load path” for the force to bypass the break.

Screenshot at Aug 24 08-57-49


Screenshot at Aug 24 08-58-04

 Another way to do this is with a 2 by 4

High values can also be achieved by using straps made of 2 by 4s.  A 2 by 4 can resist 7500#, which is more that all but one streel strap.  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.

The value of a higher strength MST72 strap can be achieve with 36 nails on each side of the break.

If there is a break created by a vent stack, a 2 by 4 must be used because it will resist both tension and compression forces.


Top Plate Break

More on Steel Straps

It is fairly common to find breaks in a cripple wall retrofit that need to be addressed so the shear walls all work together in protecting the house.  A common way of bridging breaks in a top plate is through the use of MST straps.  Below is a table representing their capacity.

Simpson StrongTie Continuity Ties for Top Plate Breaks

Simpson StrongTie Continuity Ties for Top Plate Breaks

MST straps require 16d nails, though substitutions are allowed.  For example, as shown in the adjustment factor table below, if 10d by 1-1/2 nails are used instead of 16d commons, the strength of the strap must be reduced by 16%.  This is accomplished by multiplying the value in the table above by 0.84.  Thus, an MST48 with a tension value of 5,310 pounds is actually worth 4,460 if 10d ticos are used (0.84 x 5,310).

Load Adjustment

High values can also be achieved by using straps made of 2 by 4s.  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.  5310/188=23.70.

The value of a higher strength MST72 strap can be achieve with 36 nails on each side of the break- 6730/188 = 35.8 nails.

If staples are used, which have a shear value of 80 pounds each and have no potential for splitting, the capacity of this connections is almost limitless.

Narrow straps such as CS16 used in single top plates have a much lower value

Simpson StrongTie Continuity Tie Steel Straps

Simpson StrongTie Continuity Tie Steel Straps

There is no reduction from using 10d nails 1-1/2 inch long rather than the 10ds in the catalog.

This can be greatly increased with a 2 by 4 strap.  If 20 nails are nailed through the 4 foot section of 2×4 on each side of the break, the tension connection will now be worth 3,760 pounds.  If 2 1/2″ long staples are used, the capacity of this connection is almost limitless.

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