Standard Plan A was written by an ad hoc committee of volunteers consisting of 3 engineers, 3 building officials, the entire San Leandro building depart, 1 seismologist, a representative from the National Science Foundation, and 1 contractor who is the author of this article. The importance of the San Leandro building department cannot be overstated because this building department had been teaching courses crawl space seismic retrofitting to contractors and homeowners for many years. Standard Plan A consists of only 2 pages and was published after several years of work.

This building department therefore had years of practical experience teaching and inspecting crawl space retrofits.  This building department therefore had years of practical experience teaching and inspecting crawl space retrofits. Crawl space retrofits primarily consist of cripple wall retrofits.  If you do not know what a crawl space retrofit looks like I suggest you review this webpage. 

I remember the chair of the committee Jim Russell, telling me this committee was much different than the previous committee he chaired (currently Appendix Chapter A3 of the IEBC) because it included people who had actually spent a great deal of time in crawl spaces doing, inspecting, and studying the work. To my recollection the Standard Plan A committee only used the 2001 California Building Code as its information source. And of course the seismologist was important because he let us know how earthquake force Standard Plan A retrofit designs needed to resist. Below is a list of approving agencies for Standard Plan A. 

 The webpage describes its current deficiencies and what you can do to correct them so that you get a high quality retrofit at a reasonable cost. 

 

This 38 minute video will tell you everything there is to know about Standard Plan A.

This takes a lot of time.  The most  important things to know can be found in the video Standard Plan A Top Plate Nailing and the text and tables below.

This video describes a problem with standard plan which causes the retrofits to be half as strong as they should be.

 

 

 

 

Standard Plan A: The Good And The Bad.

Of all the seismic retrofit guidelines, Standard Plan A is the easiest to understand and install.  If you apply the information on this website to your Standard Plan A retrofit you should get a better retrofit at lower cost.

The Connector Table

This table found in Standard Plan A gives the earthquake-resisting capacities of various types of hardware made by Simpson Strong-Tie, the only brand of hardware used in Standard Plan A, measured in pounds of resistance to lateral earthquake forces.

Just imagine a man weighing 300 lbs. standing on a scale.  300 lbs. is the amount of vertical force he is putting on the scale.  When these 300 pounds of force is applied sideways, you have a lateral force.

Imagine a piece of hardware nailed to a to a 2 x 4.  If the nails separate from the hardware when 300 lbs. of lateral force is applied, we say this hardware has a capacity of 300 lbs.  For example, in the first row of Standard Plan A’s CONNECTOR CAPACITY Table we see a Simpson Strong-Tie L70 has a capacity or lateral resistance of 458 pounds.

TABLE 1

Table

                                   CONNECTIOR CAPACITY TABLE IN STANDARD PLAN A.

 

The Actual Resistance of the Simpson Strong-Tie L70

Table 2 below shows the actual capacity of the L70 in Simpson Strong-Tie’s catalog.  It is 740 lbs., not the 458 lbs. as shown in the CONNECTOR CAPACITY table. This is nearly a 40% difference.  How this impacts retrofit costs will be looked at later.

 

                                                        TABLE 2

L90 Table

The Simpson Strong-Tie L90

In the CONNECTOR CAPACITY Table a Simpson Strong-Tie L90 can resist 600 lbs. of lateral force. In TABLE 2 it can resist 925 lbs. of force based on the Simpson Strong-Tie’s catalog.  A 35% difference.

Cost-effective system such as Standard Plan A should be based on an accurate understanding of hardware capacities.  The capacities in the CONNECTOR TABLE are inaccurate.  Using accurate values will reduce the cost and provide for a just as effective retrofit.

L70, L90, and H10 hardware are also known as shear transfer ties. 

 

Simpson StrongTie L90

                                                       L70 OR L90 SHEAR TRANSFER TIE.

Standard Plan A Table

                                                                 505 LBS CAPACITY OF THE H10 HARDWARE.

 The H10 Anchor (Shear Transfer Tie)

                                              PHOTOGRAPH OF THE L90 AND H10R (now called the H10AR)

The CONNECTOR CAPACITY Table contains a value for an H10 shear transfer tie.  H10 shear transfer ties are no longer made and have been replaced by the Simpson Strong-Tie H10A hardware for new construction and the H10AR hardware for older homes.  Seismic retrofits almost always use the H10AR.  The “R” stands for rough sawn lumber which is 2 inches thick and is found in almost all older homes.  The 505 lb. capacity in the CONNECTOR CAPACITY Table does not apply to the current H10A and H10AR hardware.  The H10AR has a capacity of 490 lbs. as shown by the green arrow and the H10A 590 lbs.  Neither of them has a capacity of 505 lbs.

                                                 TABLE 3:  Actual earthquake resistance of H10A and H10AR hardware shown by the green and blue arrows.

H10 Values

                                                                                 TABLE FROM SIMPSON STRONGTIE CATALOG.

Using The Wrong Nails

The CONNECTOR CAPACITY Table tells us the H10 hardware needs (8) – 0.131 (diameter) x 1 ½ (long) nails.  Neither the H10A nor H10AR hardware uses this size nail.  Instead, they use (9) 0.148 x 1 1/2″ nails as shown in the Simpson Strong-Tie Catalog shown below.

                                         TABLE FROM SIMPSON STRONGTIE CATALOG.

Contractors will automatically choose the hardware listed in the CONNECTOR CAPACITY Table because they assume the table is correct.  Using the wrong nail reduces the capacity of this hardware and violates Simpson Strong-Tie’s installation requirements.  Not only this, but it creates a weak point in the Standard Plan A system.

Standard Plan A Bolting Hardware

The CONNECTOR CAPACITY table rates the earthquake resisting capacity of a half-inch bolt at 820 lbs.  According to the American Wood Council, whose tests are the basis of the current building code, the actual capacity is 1119 lbs. in 2″ old growth redwood.

Similarly, the CONNECTOR CAPACITY Table rates the capacity of a 5/8 bolt at 1170 lbs., while the actual capacity is 1555 lbs.  Just like with the L70 and L90, these higher capacities mean you need less bolts to resist the amount force Standard Plan A is designed to resist.

For example, let’s say Standard Plan A’s calculations tell us we need to resist 15,550 pounds of force.  If we use the old inaccurate capacity of 1170 lb. capacity, we will need 14 bolts (15,550/1119). If we use the actual capacity of 1,555 we only need 10 bolts.  This means if accurate bolt capacities are used, fewer bolts are required and cost will be reduced.

The accurate capacities below were found using the connector calculator found on the American Wood Council website.  This webpage shows you how to use it.

This is the capacity for 1/2-inch bolts. 

 

                      CAPACITY OF A 1/2′ BOLT IN 2″ THICK OLD                                                           GROWTH REDWOOD.

NOT the 820 lbs. capacity found in the CONNECTOR CAPACITY table

This is the capacity of 5/8 inch bolts.

NOT the 1170 lbs. capacity found in the CONNECTOR CAPACITY table


This video summarizes the information provided so far.

Standard Plan A’s Hardware and Plywood Requirements Create EXPENSIVE retrofits.

A basic engineering fact is that only the bolts at plywood locations do anything and the strength of the bolts should match the strength of the plywood.  Please see questions 1-3 in this list of questions I put to numerous structural engineers while on the Standard Plan A Committee.  Cost-effective retrofit engineering stipulates that if you have linear footage of plywood that can for example resist 2000# of force, then you need bolts connected to at plywood locations that can also resist 2000# of force.  Any bolts more than that are a waste of money.

Bolts on each end

 

 

 

 

Standard Plan A requires bolts 32″ apart where the plywood is AND extra bolting 6 feet apart PLUS bolts on either end of each piece of mudsill.  All these extra bolts adds tremendously to the cost of the retrofits.

Bolt Placement

Bolts outside of panels.JPG

 

 

As part of the research done by the author of this article while on the committee, he sent a questionnaire 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 firsthand.  The single capital letters are the first letter 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. 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. This depends 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 means only the bolts located where the plywood is located actually do anything.  All these extra bolts do nothing more than make Standard Plan A more expensive than it needs to be.

The same goes for the plywood.  Standard Plan A uses twice as much plywood as is needed because the plywood is low quality and Standard Plan A demands that it is improperly installed.

The Plywood Connection

 

 

Here is a comparison between two retrofit approaches on the same house.   The one on the right uses correct L90 and bolt capacities.  In addition, the plywood capacity is based on properly installed plywood (as you will see Standard Plan A requires improper installations).   The one on the right requires almost twice as much hardware and over twice as much plywood as Standard Plan A in its current form.  I  will leave it to your imagination to figure out the cost differential.

 

What’s Wrong With The Plywood?

Here are some notes found in Standard Plan A concerning the plywood.  It is important to understand what the parts  underlined in red mean.

 

                               FROM THE FIRST PAGE OF STANDARD PLAN A.

 

Nailed at 4″ on center” is engineer lingo for 4″ apart

Paragraph 6 also tells us to use these on all the edges of the plywood.  The edges are always on the top plate, the mudsill, and the vertical 2 x 4s on each end of the plywood.

Nails shall be 8d common means use nails 2 1/2″ long and 0.131 inches in diameter.

“Plywood braced panel” is very confusing.  Just get rid of the “braced panel” so it says “Plywood shall be 5-ply, 15/32″ exterior grade.” and it will make a lot more sense.

Plywood is only one element in a braced cripple wall.  A wall that is made earthquake resistant with plywood and other hardware is called a shear wall.  Notice the how the nails on the edges of the plywood are closer together than in the middle of the plywood.  This edge nailing determines the strength of the plywood.

Types of Plywood

There are two types of plywood which is referred to as “Panel Grade” in the table below: RATED SHEATHING AND STRUCTURAL 1.  This table can be found in he building code.  Sheathing is fancy engineer lingo for wood you put on the outside of a wall in new construction.  In this case it refers to plywood,

Below in bold is a note from Standard Plan A. that tells us rated sheathing must be used and how it is supposed to be nailed.

APA Plywood

Let’s look at this table from the building code and see what it means.  In the Panel Grade column we see  RATED SHEATHING, in the Minimum Nominal Panel Thickness column we see 15/32″ (contractors refer to this as 1/2″). The “Nail Penetration” and “Nail Size”  in the next two columns are self-explanatory.  The crucial column is the “Nail Spacing at Panel Edges” because this tells us the earthquake resisting capacity of the plywood.  Just as hardware has capacities, plywood also has capacities listed in this table.  The number represents the number of pounds of earthquake resisting capacity the plywood has if nailed in accordance with the table.

For example, the blue arrow tells us rated sheathing sheathing has a capacity of 380 lbs per linear foot if nailed with 8d common nails 4″ part on the the edges of the plywood.

Look how this same pattern in structural 1 plywood gives us a capacity of 430 pounds per linear foot (see the green arrow).  So why did the committee decide to use rated plywood when structural 1 has a higher capacity?

An important Note In The Building Code

The committee decided to use the weaker capacity because of a very limiting section in the building code at the time.  This section required that the shear wall framing that sits on the foundation (called the foundation sill plate  or mudsill) must be 2 1/2 inches thick (2 1/2″ thick is also called 3″ nominal) in order for the plywood to have a capacity over 350 pounds per linear foot.

Our hands were tied by this part of the 2001 California Building Code because it means each linear foot of plywood can have a capacity of 350 lbs. per linear foot no matter which grade of plywood, type of nail, or edge nailing is used.   Rated  sheathing  is  cheaper  than  Structural  1, and since either grade gives us plywood with 350 lbs. per linear foot,  so we  chose  rated  sheathing.

However,  350 lbs. per linear foot is very close to the 380 lbs. per linear foot found in the plywood capacity table so the Standard Plan A committee decided to violate the 350 lb requirement in the code and use the 380 lbs tested capacity.  See the green  arrow  in  the  table  above.   

 

This is how it plays out when using the CONNECTOR TABE.

Let’s say we looked at the calculations behind Standard Plan A and discovered a certain size and type of house must resist 4540 lbs of force on each wall line.   If the plywood can resist 380 lbs this translates into 12 feet of plywood.  4540/380 = 12 linear feet.

SNIPPET FROM STANDARD PLAN A CONNECTOR TABLE

The code no longer requires 2 1/2″ thick sill plates and now allows 1 1/2″ thick  sill plates.  For this reason the committee would now be allowed to use 870 pounds per linear foot plywood (that is 10d nails spaced 2″ apart on the edges).  See the red arrow in the table above.

In other words, if we need to resist 4,560 lbs. of lateral force we can either install 12 linear feet of plywood nailed with 8d nails 4″ apart on the edges on each wall, or 6 linear feet of 870 lbs per linear foot plywood on each wall.  4560/870 = 5.25 linear feet.  When we round up it equals 6 linear feet.  In other words, if we use 870 plf plywood we will get the same earthquake resistance we need with 1/2 the amount of plywood while the labor cost is practically the same.  This makes a retrofit A LOT cheaper.

Some designers would disagree with this approach because they would say these shear walls will need holdowns to resist large overturning forces.  Three is however evidence that overturning forces are not a major concern and can be ignored.  It appears overturning is a big factor in a laboratory and no so much in the real world.

If installing holdowns makes a retrofit unaffordable, I would forget about the overturning forces.  There is a good chance that it most circumstances they are not needed.

 

Standard Plan A actually achieves 190 Pounds of Resistance Per Linear Foot, NOT 380

 

As mentioned in the introductory video, the strength of a shear wall is dependent on the spacing of the nails on the edges of the plywood.  In a cripple wall retrofit the edge is the upper top plate, the bottom edge is the mudsill or sill plate, and the side edges are the 2 by 4s on each end.  This nailing on the edges is called edge nailing.  Edge nailing determines the strength of the plywood.

In the Standard Plan A image below, the blue arrow points at the lower top plate which is NOT an edge.  The green arrow points to the upper top plate which IS the edge and where the edge nailing should be.  It only makes sense.  The floor is going to be sliding on the upper top plate, the upper top plate will be absorbing all the force, which means only plywood nails attached to the upper top pate will do resisting anything.

According to Standard Plan A the plywood should be nailed 4 inches apart staggered.  See the red arrow in the image to the right.  According to the drawing on the left this is done by putting one nail into the upper top plate, then move over 4 inches and put a nail into the lower top plate, then to 4 inches over to the upper top plate, etc.  Nailed this way the outer boundary nailing on the upper top plate is 8 inches apart, and not the necessary 4 inches apart required for effective plywood nailing.  This reduces the strength of the plywood from 380 pounds of earthquake resistance per linear foot to a completely inadequate 190 pounds per linear foot.

Shear wall boundary nailing mistake

CONSTRUCTION DETAIL FROM STANDARD PLAN A- INCORRECT NAILING.

StaggeredIn the illustrations to the left and right the red arrow shows the plywood nailed 8 inches apart in the lower and lower top plate.   The upper top plate is also the edge nailing. This 8″ plywood edge nailing is what happens when the nails are staggered as required by Standard Plan A

The Nailed Blocking Method Used In Standard Plan A

The aforementioned Nailed Blocking Method illustrates another weakness found in Standard Plan A.

 

 

Blocking

 

 

 

Untested Nailed Blocking Method of Connecting the Plywood to the Mudsill.

ON THE LEFT 2 BY 4 BLOCKS HAVE BEEN NAILED ONTO THE MUDSILL.  ON THE RIGHT THE PLYWOOD HAS BEEN NAILED INTO THE BLOCKS. 

Shear Wall Blocks being Installed

              WORKERS THIS BIG DO NOT NORMALLY DO SEISMIC RETROFIT WORK.

 

An Analysis of the 4 different ways of making the plywood to mudsill connection

RETROFITTING WITH THE FLUSH CUT METHOD

                                                                     THE METHOD THAT MOST RESEMBLES TESTED SYSTEMS.

THE REVERSE BLOCK METHOD OF SHEAR WALL CONSTRUCTION

                                                                    THE REVERSE BLOCK METHOD OF SHEAR WALL CONSTRUCTION.

 

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 is because blocks tend to split when nailed through the wide face.

Split Block due to Nailing into Wide Face of Block

                      SPLIT BLOCK DUE TO NAILING INTO THE WIDE FACE OF A 2 BY 4 BLOCK. 

 

This is from an evaluation report on the 4 different methods performed by the American Plywood Association, the largest shear wall testing laboratory in the world.

 

Committee members were concerned the nails would split the blocks when the plywood was nailed less than 4″ apart into the narrow face of the 2 by 4s.  As shown below, nails were space 1 inch apart with no sign of splitting.  This further confirms closer edge nailing is a very viable alternative.

2 by 4 block with plywood

 

So why does Standard Plan A use the most questionable untested system?

I was there when it was adopted, and it was a political decision.  The chair of the committee had been teaching people to use nails for decades and was not about to admit he was wrong.

Fortunately, there is an alternative

Plywood and the Building Code

The 2001 California Building Code did not recognize plywood earthquake resistance over 380 plf (pounds of resistance per linear foot) except in very unusual circumstances.  The current California Building code now allows plywood 870 pounds per linear foot, which is DOUBLE the that specified in Standard Plan A.

Using weaker plywood nailing patterns from a defunct building code makes using more plywood than needed necessary.  There is a big difference between 380 plf and 870 plf plywood. Using the stronger plywood decreases the amount of plywood needed and reduces cost.

 

Long Term Consequences

Standard Plan A calls for a plywood nailing pattern with 8d nails 4 inches apart on the edges and equals 380 plf.  Double the number of nails and use structural 1 plywood and the plywood connection will be 730 pounds per linear foot. If we use 10d nails in structural 1 plywood and keep the same nailing pattern, we can get an even stronger 870 pounds per linear foot.

The Standard Plan A chart specifies 20 linear feet of plywood bracing on a house with stucco siding and equals 860 pounds of earthquake resistance. The current California Building Code allows 870 pounds of resistance with only 10 feet of plywood.

 

Plywood Bracing

                                          Chart Showing Plywood Required by Standard Plan A.

 

Retrofits without cripple walls use bolt substitutes for standard bolts. Standard Plan A specifies the Simpson UFP10 for this bolt substitute.  The UFP10 is no longer made.  It resisted 1340 lbs. of force as shown in Table 1. The available URFP and FRFP can resist 1530 lbs. and 1810 lbs. of force respectively.  Modern and stronger bolting hardware requires less hardware overall.  Naturally, this will have a big impact on cost.

The Hardware below is used when there is no room to put in Standard Bolts.

The Simpson URFP

                                              THE SIMPSON URFP.

This is the strongest Foundation Anchor made.

                                                   THE SIMPSON FRFP.

 

Oversized Hole No WasherMSPOversized Hole Photo

Damage Caused by Over-Sized Bolt Holes

 

Floor to Cripple Wall Connections

As shown by the red arrow, Standard Plan A puts “angles” (shear transfer ties) above shear walls between joists.  The shear wall is going to take all of the force.  Properly designed retrofits concentrate bolts and shear transfer ties at shear wall locations.



This leads us to the following construction detail:

STT Sixteen o c

 

Fasteners in Pressure Treated Wood

Standard Plan A recommends galvanized nails in ACQ pressure-treated lumber.  Tests done in New Zealand showed hot dipped galvanized nails deteriorate in ACQ lumber.

Presssure treated

 

Galv Nalls Corroded

 

Comparison of Standard Plan A as written compared to a Standard Plan A Retrofit using updated information

Component requirements for a 1500 square foot single story stucco house on each foundation line.

 

Let’s deconstruct this chart and discover the engineering behind it.

Standard Plan A uses rated plywood nailed with 8d nails 4″ apart on the edges which can resist 380# per linear foot.  22′-8″ of plywood nailed like this can resist 8,645#.  This means the committee concluded a house built like this must resist 8,645# of earthquake force on each side and the bolting and shear transfer ties connections should all resist this same amount of force at a minimum.

10 linear feet of Structural 1 plywood nailed with 10d nails 2″ apart can resist 8,700 pounds of force.  A difference of 12′-8″ of plywood required.

In other words, on each side Standard Plan A requires 12′ 8″ more plywood than needed on each side.  Putting up plywood is much more involved than nailing up plywood.  A great deal of labor-intensive framing is also required.  This added plywood adds greatly to the expense of a retrofit.  The 2″ nailing compared to 4″ nailing represents an insignificant increase in cost.

Consequently, Standard Plan A requires 50′-8″ more plywood than required (12’8″ x 4 sides)

L90 and H10 (H10AR) hardware.

Standard Plan A requires 15 pieces L90 hardware on each side.  When using a capacity of 600 pounds of resistance per piece Standard Play A this connection will resist 9,000# of force.

When using the actual capacity of 925# we only need 10 pieces and achieve 9,345# of resistance on each side.

Consequently, Standard Plan A requires 20 more L90s than required.  (5 pieces x 4 sides)

H10 (H10AR) hardware.

Standard Plan A requires 15 pieces H10 (H10AR) hardware.  Using the 505# value this hardware can resist 7,575# IF THE CORRECT NAILS ARE USED, which is actually less than the 8,645# value we need.  For the system to work we should add 3 more pieces of H10 hardware.

The actual value for modern H10AR hardware (which is what a homeowner or contractor would end up buying) is 490#.  If the correct nail is used, 15 of these can resist 7,350# which is even less than the 8,645# value we need.  To get the 8,645# we also need to add 3 pieces of H10.

Bolting

Standard Plan A has unnecessary bolting.  The provision below found in Standard Plan A is the exact requirement for new construction.

Bolts outside of panels.JPG

 

 

In Conclusion

  • It uses the outdated 2001 California Building Code.

 

  • 4 feet is the maximum cripple wall height it addresses.  The laws of physics and science make no distinction between cripple walls 10″ or 10′ tall.

 

  • Standard Plan A omits a type bolting hardware of that in many situations is the only ones that will work.

 

  • It only allows 2 types of shear transfer ties, the Simpson L90 and the H10. The L90 is stronger, and the H10 is weaker than the value used when creating Standard Plan A.

 

  • Two other shear transfer ties, the LTP4 and A23 are often the only option.  Standard Plan A does not allow for this hardware, such that many contractors end up doing nothing at all when this hardware is required.

 

  • Standard Plan A specifies the wrong nails for this H10 hardware.

 

  • Standard Plan A does not account for balloon framing, a common construction method in older homes.

 

  • Standard Plan A bases its plywood strength on a table in the 2001 California Building Code which is no longer valid.  This value is 1/2 that of the current building code.

 

  • The strength values specified for foundation bolts are inaccurate.  They are actually much higher.

 

 

  • Standard Plan A bases its plywood strength on a table in the 2001 California Building Code which is no longer valid.

 

  • This 3-inch-thick framing requirement is no longer part of the building code.  This change means retrofit shear walls can now be built according to code using standard 2-inch-thick framing with a resistance of 870 pounds of force per each linear foot of plywood.  In other words, with the current code one can put in half the plywood and get over twice the strength.

The cost of a designed Plan Set A retrofit compared to custom designed retrofit using the base shear formula.

              • Standard Plan A uses outdated and weak bolting hardware.
              • Standard Plan A leads to a greater expense.
              • It allows plywood not made for earthquake resistance.
              • The nailing of the plywood is insufficient and weak.
              • It uses shear wall construction methods that have not been tested by the leading research laboratory.
              • Hardware is specified with the wrong nails.
              • It omits 3 types of commonly used floor connections (shear transfer ties).