Here are the numerous organizations that helped write and continue to endorse Standard Plan A. 

This video was created to explain the practical principles of residential seismic retrofitting to the scientist on the ICC 1300 code committee. This webpage cannot be understood without watching this video first.

Standard Plan A, A Vision For Renewal. 

Standard Plan A was published in 2006 and revised in 2008. It is based on these calculations made by Jim Russel P.E. in 2004.  Since its publication it has been widely used by the CEA EBB grant program as well as all the building departments in the San Francisco Bay Area.  Based on this wide scale acceptance it will certainly be used for many years to come.  The purpose of this article is to look at this 20 year old guideline and see if it needs to be modernized in light of current hardware types and retrofit strategies.   

How Much Money Has Been Spent On Standard Plan A Retrofits?

The California Earthquake Authority EBB program has as of January 2025 provided $3,000 grants for approximately 12,000 Standard Plan A retrofits for a total of $36,000,000 dollars in taxpayer money.  The program is in still in effect and many more Standard Plan A retrofits will be paid for in the future.

$3,000 pays for approximately 1/3 of the average CEA retrofit so homeowners have contributed another $72,000,000 for a total of $108,000,000.  The Cities of Berkeley and El Cerrito also require its use in their transfer tax retrofit grant programs. These Cities and their citizens have probably spent  another $10,000,000 on Standard Plan A retrofits.  Standard Plan A retrofits done by homeowners independent of these programs will also account for untold millions of dollars more. At stake here is something like $150,000,000 that has been, and will be, used to promote public safety. This should give you some idea why it must be as cost effective as possible.    

The REINFORCEMENT SCHEDULE

 

The REINFORCEMENT SCHEDULE is the bedrock of every Standard Plan A retrofit.  Let me try and explain how it works.  In this sample case the REINFORCEMENT SCHEDULE tells us a 1200 square foot light construction house requires a minimum of 14’8″ of plywood bracing, (7) 1/2″ bolts, and (13) L70s on each wall line in order to meet Standard Plan A’s cripple wall retrofit requirements. 

Light construction means the exterior walls are wood with drywall on the interior walls.  According to Standard Plan A’s calculations a single story light construction house weighs 34 lbs. per square foot.   If you want to know the weights of houses with different configurations go here.  

How Is Earthquake Force Measured?

Just imagine a man weighing 300 lbs. suddenly jumps on a bathroom scale.  The speed of his body as it falls on the scale is its velocity.  Velocity is defined as the change in position divided by the time of travel.  Terms like miles per hour are a measure of velocity.  The velocity x 300 lbs. is the amount of vertical force that will be hitting the top of the scale measured in pounds of force.   When this same force is directed sideways  you have a lateral force also measured in pounds of force. 

Earthquake resisting hardware is measured by its capacity to resist lateral earthquake forces. Imagine a piece of hardware nailed to a to a 2 x 4.  If a lateral force of 300 lbs. is applied to the hardware and it bends or the nails pull out of the 2 x 4  we say this hardware has failed and has a capacity of 300 lbs.    If the nails are the first to fail, we say the nails are the failure mode.  If the hardware bends first, the hardware is the failure mode. 

The Standard Plan A CONNECTOR CAPACITY TABLE

The CONNECTOR CAPACITY table above is in Standard Plan A and shows the earthquake-resisting capacities of various types of hardware. The numbers on the left represent the capacity of the hardware while on the right it tells us the name of the hardware, in this case the Simpson L70.  For example, in the left column we are told the L70 has a capacity of 458 lbs. and on the right column we are told it must be installed with eight (8)-10d nails x 1 1/2″ long.  

Why Is The CONNCTOR CAPACITY Table Important?

The CONNECTOR CAPACITY Table has a direct correlation to the retrofit designs found in the REINFORCEMENT SCHEDULE.  The best way to explain this is by looking at the sample design we created in the REINFORCEMENT SCHEDULE above.  Standard Plan A’s engineering calculations tell us this house will be subjected to 3,794 lbs. of lateral force on each wall line.  Further, it tells us the plywood has a capacity of  380 plf (pounds of resistance per linear foot).  If we install 14’8″ of plywood as shown on the the plywood column, it has the capacity to resist 5,570 lbs. of earthquake force.  Likewise, a 1/2″ bolt has a capacity of 820 lbs. such that the 7 bolts bolts shown in the bolt column have an earthquake-resisting capacity of 5,740 lbs.  Finally, the L70 framing anchor has a capacity of 458 lbs.  If we install 13 of these as required by the REINFORCEMENT SCHEDULE the earthquake resisting capacity equals 5,940 lbs. Each earthquake resisting component, plywood, bots, and framing anchors can resist more than the 3,794 lbs. of force the calculations told us this house will need to resist which makes this an effective retrofit.  

Notice how the 5,570 lbs. of plywood lbs., the 5,740 lbs. of bolt capacity , and the 5,940 lbs. of framing clip capacities are almost equal.  This balance is the hallmark of a cost-effective retrofit because any capacity that exceeds the weakest capacity of any component is redundant and wasteful. 

The problem with all of this is the fact that all the capacities listed in the CONNECTOR CAPACITY table are wrong and therefore the REINFORCEMENT SCHEDULE is inaccurate. 

The Actual Capacity of the L70

Table 2 below shows the actual capacity of the Simpson  StrongTie L70 from an ICC Evaluation Report.  Notice it has a capacity of 740 lbs., not the 458 lbs. shown in the CONNECTOR CAPACITY table. This is nearly a 40% difference. 

  TABLE 2

Let’s assume the REINFORCEMENT SCHEDULE requires (20) L70s with a capacity of 458 lbs. on each wall line for a total capacity of 9,680 lbs. If the house has 4 sides we need a total of 80 L70s for a cost of $4,000 if the installed price of an L70 is $50 each.  

If we use the 740 lbs. capacity found in the ICC report just (14)-L70s provide us with 10,360 lbs. of capacity on each wall line which is more than the 9,680 we need. If the house has 4 sides we need a total of 56 L70s are required for a cost of $2,800.  In other words, if ones uses the actual capacity of the L70 rather than the inaccurate capacity found in the CONNECTOR  CAPACITY table you get more earthquake resistance with less hardware with a cost savings of $1,200.   

The Simpson Strong-Tie L90

The same is true with the Simpson L90.  In the CONNECTOR CAPACITY table a L90 can has a capacity of 600 lbs. TABLE 2 from the Simpson Catalog gives us the actual capacity of 925 lbs.  This is a 35% difference.  Use of the 600 lb. capacity found in the CONNECTOR CAPACITY Table instead of its actual 925 lb. capacity results in waste similar to that found when using the Simpson L70. 

  TABLE 2

PAGE FROM THE SIMPSON STRONGTIE CATALOG SHOWING THE CAPACITIES OF THE SIMPSON L70 AND L90

Consequences

Standard Plan A is more expensive than it needs to be because the CONNECTOR CAPACITY table has inaccurate capacities. Using accurate capacities will reduce the cost and at the same time provide for an equal, if not greater, quality retrofit.  

 

The H10 Anchor

THE CAPACITY AND NAILING OF THE H10 HARDWARE

                          

The CONNECTOR CAPACITY Table contains a 505 lbs. capacity for an H10 framing anchor.  H10 framing anchors are no longer made and have been replaced by the 565 lb. H10A hardware for post ~1950 construction and the 490 lbs. capacity H10AR hardware for pre ~1950 construction.  Pre ~1950 homes always use the H10AR because it fits full sized framing that is 2″ thick compared to the H10A which fits modern lumber that is only 1 1/2″ thick.  The 505 lb. H10 capacity  in the CONNECTOR CAPACITY Table does not equal the capacity of the H10A or the H10AR hardware and is inaccurate under all circumstances.  This is one more error in Standard Plan A’s CONNECTOR CAPACITY table which can reduce effectiveness and increase cost.   

   Using The Wrong Nails

 The CONNECTOR CAPACITY table tells us the H10 hardware needs  eight (8) – 0.131 (diameter) x 1 ½ (long) nails.  Neither the H10A nor H10AR hardware uses this size nail.  Instead, they use (9) .148 x 1 1/2″ nails at the bottom and (9) nails on the side for a total of EIGHTEEN nails, as shown in the green and red boxes from the Simpson Strong-Tie Catalog below. This is contrary to the EIGHT nails shown in the CONNECTOR CAPACITY table. When hardware uses less than half the required nails in addition to the incorrect sized nails, the manufacturer rates the hardware at zero capacity.   

 

                                         PHOTOGRAPH OF AN H10AR

Standard Plan A Bolting Hardware

The CONNECTOR CAPACITY table rates the earthquake resisting capacity of a 1/2″ bolt at 820 lbs.   According to the American Wood Council connector calculator the actual capacity is 1038 lbs..

Similarly, the CONNECTOR CAPACITY Table rates the capacity of a 5/8 bolt at 1170 lbs., while the actual capacity is 1484 lbs.  

For example, let’s say the REINFORCEMENT SCHEDULE  requires (10) bolt for a capacity of 14,840 lbs. on each wall line for a total of 40 bolts if the house has 4 sides.  If we use the old inaccurate 5/8″ bolt capacity of 1170 lb., we will need 13 bolts (14,840/1170 = 12.6 bolts rounded up to 13) on each wall line for a total of 52 bolts. If we use the actual  bolt capacity of 1,484 lbs., we only need 10 bolts on each wall, for a total of 40 bolts.  If bolts cost $100 each we save $1200 on this retrofit if we use the correct bolt capacity. 

The Substitution of the UFP10 With The URFP

The CONNECTOR CAPACITY table uses a low clearance bolting hardware called the UFP10 with a capacity of 1340 lbs.  The Simpson UFP10 is no longer available.   Instead contractors use, and building departments accept,  the Simpson URFP which has a higher capacity of 1530lbs.  This is a capacity difference of 13%.  The cost of retrofits that use the URFP can be reduced by 13% if the correct capacity is used.  The labor and material cost for each product is the same. 

Below you can see Standard Plan A CONNECTOR CAPACITY table compared to the table on the right that meets the building code.  

CONNECTOR CAPACITIES IN STANARD PLAN A

CONNECTOR CAPACITIES PER CODE



                 

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

This video is probably the easiest way to understand this concept but if you prefer written explanations see the text below. 

 

 

Shear wall boundary nailing mistake

CONSTRUCTION DETAIL FROM STANDARD PLAN A- INCORRECT NAILING.

                 STANDARD PLAN A PLYWOOD NAILING PATTERN

These two images are from Standard Plan A. The blue arrow on the left figure points to the lower top plate which is NOT the plate where the plywood is supposed to be nailed.  The green arrow points to the upper top plate which IS the edge where the edge nailing should be.   The red arrows point at the misplaced nails in the lower top plate. 

The figure on the right shows Standard Plan A’s requirement to stagger the plywood nails into the t0p and lower top plates.  The result is nails 8″ o.c. edge nailing in the upper top plate.  This edge nailing is supposed to be 4″ o.c., not 8″ o.c. 

The load path works like this:

  • The end joist slides on the upper top plate and the 1 1/2″ nails attaching the framing anchors to the top plate transfer this load exclusively into the upper top plate (the 1 1/2″ long nails in the L90 or L70 framing anchors are not long enough to penetrate into the lower top plate).
  • The plywood is nailed to the upper top plate 8″ o.c. which at 8″ o.c. can only resist 190 lbs plf .  The load is finally transferred into the bolts through the plywood. The nails in the lower top plate do not do anything because they are not attached to the framing anchors. 

Remember, the REINFORCEMENT TABLE is based on 380 Lbs. per linear feet which is only achieved if the nails are 4″ o.c. in the upper top plate, not the 190lbs provided by 8″ o.c. top plate nailing provided for in Standard Plan A.

The load then transfers from the plywood, into the bolts, and finally into the foundation. 

 

The Mudsill Connection

The Problem

 

Screenshot at Sep 07 18-40-55

 

There are four ways to modify the framing so the plywood is attached to the mudsill 

 

Diagram of plywood nailed to mudsill using the flush cut method

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 second method, called the reverse block method attaches a 2 by 4 to the plywood and then the 2 by 4 is nailed to the mudsill.

 

.

NAILING INTO REVERSE BLOCK SHEAR WALL

                            NAILING REVERSE BLOCK SHEAR WALL ONTO MUDSILL.

 

 

Stapled blocks are a third method. This is similar to the reverse block method except blocks are stapled and not nailed.

 

 

               STAPLED BLOCK WITH STRENGTH OF 50 NAILS AND NO SPLITTING.

 

The Nailed Blocking Method

 

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 to the blocks.

Shear Wall Blocks being Installed

                 TECHNICIAN NAILING A BLOCK ONTO THE MUDSILL

 

Standard Plan A uses the Nailed Blocking Method

 

The Problem With Nailed Blocks Is That They Split

Another Split Block on a Cripple Wall Retrofit

The senior engineer at the America Plywood Association evaluated all these methods and came to the following conclusion:

 

Conclusion

For our sample house when we use the REINFORCEMENT SCHEDULE we need 29′ 4″ of 190plf plywood, (7) 1/2″ bolts and (13) l90 framing anchors on each wall line for a total cost of $19,284.

If we use code values for this same house we need 16 linear feet of 380 plf plywood, (6) 1/2″ bolts, and (7) L90s on each wall line for a total of $11,344