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Standard Plan A versus a Designed Retrofit

Standard Plan A Compared to an Engineered Retrofit

This article was written by a former member of the regional International Code Council committee that wrote Standard Plan A.  Most of this material was taken from a presentation he gave to the Berkeley Building Department urging them NOT to adopt standard because of its many deficiencies.  It is a paint by numbers approach which relies on a theoretically built house rather than the way they are actually built.

It is always less effective and more expensive than one designed specifically for an individual house that uses something called the base shear formula.   The base shear formula is a  formula used for seismic design found in the building code.  The table below is from a presentation the author gave to the Berkeley Disaster Commission in 2006.  It shows the cost differential between a designed retrofit using the base shear formula and one using Standard Plan A.  Prices have changed proportionally since then.  As you can see, a Standard Plan A retrofit is far more expensive than one that uses the base shear formula.

Base Shear

 

 

 

 

 

 

 

 

 

 

 

Subsequent research and code changes has eclipsed much of the information found in Standard Plan A.    For example, as shown in the table below, Standard Plan A recognizes 1/2 inch diameter bolts as having 820 pounds of earthquake resistance, and a 5/8 inch diameter bolts as having 1170 pounds.   What this means is that  if an earthquake pushed on an 1/2 inch bolt with 820 lbs of force, the bolt would fail.  If it pushed on a 5/8 bolt with 1170 pounds of force,  it would fail.  This is no longer true according to the values published by the American Wood Council, which by reference is part of the building code.  According to their research a 1/2 inch bolt has a resistance of 1,037 pounds.   If you add a Mudsill Plate this same bolt can resist 1,340 pounds while a 5/8 bolt can actually resist 1,485 pounds.  Mudsill plates do not make any difference in 5/8 bolt performance.

 

Bolt Capacity

If Standard Plan A requires ten 1/2-inch bolts be used, the bolts will be able to resist 8200 lbs. of force.  You can get the exact same resistance using six bolts with Mudsill Plates.  In other words, you achieve the same amount of resistance at close to half the cost.

Standard Plan A Plywood is Half as Strong as it could be if the Retrofit were Designed.

The earthquake resistance of plywood is measured in pounds of earthquake force it can resist per linear foot of plywood.  This is expressed with the abbreviation plf, meaning “pounds per linear foot.”  The plf that plywood can resist depends on how it is installed.  Below is the plywood requirement of Standard Plan A and is also found in the California Building Code.Plywood Capacity

 

 

As highlighted in green, Standard Plan A specifies the plywood be nailed with 8d nails (this is a type of nail that has a certain diameter and length)  4″ apart on all the edges into 15/32 inch thick APA RATED SHEATHING.  (Sheathing is another name for plywood.)   When this is done, the plywood can resist 380 pounds of earthquake force per linear foot according to the building code.  (Rated plywood is not actually made specifically for shear walls; Structural 1 is.)

The reason this low capacity nailing was selected is because at the time the standards was put together the code said that if a shear wall were to be nailed in such as way that it could resist over 350 pounds per linear foot then the sill would need to be 3by framing.  Given the fact that retrofit sills are less than 3by this was the highest value the committee could accept.   

This is no longer part of the code and 2by sills are now allowed for shear walls over 350 pounds per linear foot.  It is quite likely that if the committee were in existence today the standard would be based on much higher plywood nailing capacities.

2 by 4 block with plywood

 

As shown by the red arrow below, if 10d nails are nailed into APA STRUCTURAL 1 plywood 2 inches apart on the edges, the plywood can resist 870 pounds per linear foot, DOUBLE the capacity of the plywood used in Standard Plan A.

APA Plywood

In order to comply with Standard Plan A we must use 8d nails 4 inches on edges as mentioned earlier.  If we double the number of 8d nails we can provide you with a retrofit that is almost twice as strong (730 pounds per linear foot versus 380 plf).  If we use 10d nails and space them 2″ apart on the edges we can get and even stronger 870 pounds per linear foot.  WARNING: This can only be done if the Nailed Blocking Method found in Standard Plan A is not used.  If this method is used there is a very serious concern that the wide face of a block will split.   The wall framing behind the plywood must be also installed in such a way that close nailing patterns can be used.

To sum up: as illustrated in the chart below, if you have a 1200 square foot stucco house where Standard Plan A and requires 20 linear feet of plywood bracing, you can get the same strength with less than half this amount of plywood if the nails are closer together and the Nailed Blocking Method is not used and use Mudsill Plates on the bolts.  Designed or engineered retrofits are more effective, cost less for labor, and less for materials.  In short, you can get a retrofit that will perform identically to Standard Plan A at half the cost.  If you would like to know more about the earthquake resistance of plywood, see  this webpage.

Plywood Bracing

 

 

 

 

 

 

 

 

The Nailed Blocking Method of Shear Wall Construction

The aforementioned Nailed Blocking Method is another weakness found in Standard Plan A and Appendix Chapter A3.  According to the author of this article (and he was a member of the Standard Plan A Committee), it was determined the Nailed Blocking Method in Standard Plan A has a lot of problems.

Blocking

 

 

 

 

 

 

 

Using the Wrong Nails

H10

 

 

 

 

 

 

Here you see that Standard Plan A specifies 8d nails 1 1/2 inches long.  The manufacturer specifies 10d nails 1 1/2 inches long be used.  This can only be interpreted as sloppy work and a sign of minimal experience on the part of the people who put Standard Plan A together.

As mentioned earlier, the strength of a shear wall is dependent on the spacing of the nails on the edges of the plywood into the outer boundaries of the 2 by 4 framing behind it.  This is known as edge nailing.   In the Standard Plan A construction detail below, the blue arrow points at the lower top plate which is NOT an outer boundary framing member (the upper top plate is).  The floor slides on top of the upper top plate, into the nails attaching the plywood to the upper top plate, and then down into the bolts.  The house does not slide on the lower top plate which makes any nails into the lower top plate redundant.  The red arrow points at a row of nails in this bottom plate.  According to this construction detail, if nails are staggered 4 inches apart between the upper and lower top plates as shown, the outer boundary nailing will be 8 inches apart on the outer boundary upper top plate, creating a shear wall that can only resist 190 pounds per linear foot.   Given that horizontal siding can resist 100 pounds per linear foot, one wonders if spending the money on this retrofit is worth it at all.

 

TopPlate

Standard Plan A in Summary

  • Its engineering basis is outdated
  • It is more expensive than it needs to be.
  • It allows plywood that is not made specifically for shear walls.
  • The nailing of the plywood is pathetically weak.
  • In at least one case it recommends the wrong nails.
  • You are better off hiring a good engineer or someone who knows how to design a retrofit.

 

 

 

 

 

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