Understanding Standard Plan A

This article was written by Howard Cook, one of the founding members of the international code Council that wrote Standard Plan A.

 
Standard Plan A is a regional guideline used by the CEA Bolt and Brace Program and San Leandro’s Retrofit Workshop.  It is highly recommended that you attend this workshop. Hardware, methods, and engineering knowledge can change so fast that  once Standard Plan A is updated, such as the last update in 2oo8, this workshop is the only place where you will learn about modern developments.  This can save you a lot of time and money and is well worth the $35 they charge.

These calculations are the basis of Standard Plan A and are the best and most exhaustive calculations available for home retrofit designers.  As a committee member I saved these calculations for posterity sake.  We use these calculations in every job we do.

Critique and Possible Improvements

 

  • Standard Plan A is one way of retrofitting  a house.  The other way is to custom design a retrofit using its engineering basis. No matter which system is used, the object is to resist the anticipated ground acceleration predicted for a large earthquake on the Hayward Fault. We will be looking at how best to use modern hardware, modern methods, and its engineering basis to provide the same protection as Standard Plan A.

 

Standard Plan A has some built-in inefficiencies such as the following:

 

    1. It uses the dated 2001 California Building Code
    2. Standard Plan A omits three types of floor to foundation connectors that in many situations are the only ones that will work. (You will be able to learn about this at the Retrofit Workshop)
    3. It only allows 2 types of shear transfer ties, the Simpson L90 and the H10. Recent tests have shown the L90 is stronger than originally tested and the H10 is weaker than originally tested. Using inaccurate values throws the entire system out of balance.
    4. Standard Plan A specifies the wrong nails for this same hardware.  This is because the hardware recommended has been tested with different sized nails.
    5. Standard Plan A based its plywood strength on a table in the 2001 California Building Code which is no longer valid.
    6. The values specified for foundation bolts is based on the 2001 California Building Code.  Recent testing has shown  bolts are stronger than the values in Standard Plan A
    7. Bolts, plywood, and other hardware are actually much stronger than assumed in Standard Plan A. If this is taken into consideration, retrofits will be cheaper and more efficient to resist the anticipated ground acceleration.
    8. Properly installed plywood is twice as strong as that found in Standard Plan A.
    9. If interpreted literally the standard only applies to a very limited number of houses.  The Retrofit Workshop will teach you how to be flexible.
    10. This paint-by-numbers approach relies on a theoretically built house. Actual houses do not look like this.
    11. Retrofits that use the base shear formula  are more effective and much less costly.

 

A General Analysis of the Technical Problems

The cost of a designed retrofit compared to a base shear formula retrofit.

Chart Showing the Difference in Cost between Standard Plan A retrofit and Designed RetrofitSome cities or programs such as the Bolt and Brace Program Bay require us to use this standard.  When this happens we improve on the standard by considering these factors.  All of our retrofits adhere to the state of the art engineering calculations behind Standard Plan A .

Scientists measure the earthquake resistance of plywood in terms pounds (of resistance) per linear foot (plf).  For example, plywood rated at 500 plf can resist 500 pounds of earthquake force per each linear foot. The plf plywood has depends on the installation method.  Below is the plywood requirement of Standard Plan A found in the California Building Code.

Highlighted in green Standard Plan A specifies plywood nailing with 8d nails  4″ apart at “panel edges”.  An 8d is a type of nail.  The table shows 6d, 8d, and 10d nails in the Nail Size column.   These nails are all used in plywood nailing.   Standard Plan A uses RATED sheathing (sheathing is another name for plywood).  It uses the nailing and 380 plf earthquake resistance  shown by the blue arrow. Standard Plan A earthquake resistance compared to other nailing patterns is very weak.

APA Plywood

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.

 There was some discussion of using even weaker plywood nailing due to fear the 2 by 4s would split.

The committee never checked to see how difficult it is to split a 2 by 4 when nailed into the narrow edge.  If they had, this would not have been a concern.

Using weaker plywood nailing patterns from a defunct building code makes using more 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.

2 by 4 block with plywood

 

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.

WARNING: This can only be done if the Nailed Blocking Method found in Standard Plan A is not used.  The Nailed Blocking Method found in Standard Plan A can split the wide face of the 2 by 4 blocking.

To sum up: 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

The Nailed Blocking Method of Shear Wall Construction

The aforementioned Nailed Blocking Method illustrates another weakness found in Standard Plan A and Appendix Chapter A3.

Blocking

As mentioned earlier, the strength of a shear wall is dependent on the spacing of the nails on the edges of the plywood.  The main outer edge is the upper top plate. This nailing on the edges is called edge nailing. 

Standard Plan A’s illustrates its understanding of edge nailing below.  The blue arrow points at the lower top plate which is NOT an outside edge.Oouter boundary framing member (the upper top plate) is.

In an earthquake, the floor slides on top of the upper top plate, conveying forces 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 lower top plate which serve no useful purpose.

Standard Plan A staggers the nails 4 inches apart along the upper top plate.  It also puts one nail into the upper top plate and and 4 inches away puts another nail in the lower top plate.  The outer boundary nailing on the upper top plate becomes 8 inches apart, and not 4 inches as it should be. This creates  a shear wall that can only resist 190 pounds or earthquake resistance per linear foot.   Proper edge nailing creates plywood earthquake resistance with 870 pounds of resistance per linear foot.

TopPlate

 

Hardware used in Standard Plan A

This chart found in Standard Plan A gives the earthquake-resisting capacities of various types of hardware measured in pounds of resistance. For example, according to this chart, an L70 shear transfer tie resists 458 pounds of earthquake force pushing against it before it fails.

                                                                          Table 1

Table

 

The L70

In Table 1, an L70 can resist 458 lbs. of force.  Table 2 below shows the actual value found in the manufacturer’s catalog which is 740 lbs, a difference of 282 lbs

The L90

In Table 1, an L90 shear transfer tie can resist 600 lbs. of force. In Table 2 it can resist 925 lbs of force, a difference of 325 lbs.

Inaccurate values for hardware increases the cost retrofits.  This consideration should lead building officials to review this part of Standard Plan A.

 

                                                                         Table 2

L90 Table

Simpson StrongTie L90

Simpson StrongTie L90 found in Standard Plan A

Standard Plan A Table

Table 1

The H10A and H10AR Shear Transfer Ties

Table 1 mentions an H10 above.  There is no such thing as H10 hardware.   There is H10A hardware and there is H10AR hardware, but no H10 hardware.  For the H10 hardware Table 1 specifies (8) – 8d x 1 ½” 8d nails.  Neither the H10A or H10AR hardware use this size nail.  This is the wrong nail and given there is no such thing as H10 hardware we are not even sure what they are talking about.

Let’s assume the H10 is supposed to represent either type of hardware.  As shown in Table 3, the correct nails are 10ds and not 8d nails for both types of hardware.

The H10A and the H10AR are very different in terms of how much earthquake force they can resist.  Assuming the nail size in the table is ignored, and the correct nails are used, the H10A can resist resist 590 pounds of force.  If the correct nails are used the H10AR can resist 590 pounds of force.  Of course, because Standard Plan A specifies the wrong nails in either case we do not know for sure.

The H10A is designed for newer lumber and the H10AR is designed for older full-dimension lumber.  We see both newer and older types of lumber and Standard Plan A should reflect this.  If the hardware values are downgraded and the wrong nails used the entire system is thrown out of balance.

                                                                                                       Table 3

H10 Values

 

Standard Plan A Bolting Hardware

Table 1 rates the capacity of a half-inch bolt as being 820 lbs.  In fact it is 1037 pounds according to recent tests.  Table 1 gives the 5/8 bolt  1070 lbs. while the actual value is 1555 lbs.  These actual higher values mean Standard Plan A requires more bolts than necessary.  This increases cost.   The values below were found using the connector calculator found on the American Wood Council website.

This is the earthquake resistance of 1/2 inch bolts.

Here is the Value for 5/8" Bolts

This is the earthquake resistance of /8″ Bolts

 

Retrofits without cripple walls use bolt substitutes for standard bolts. Standard Plan A specifies the Simpson UFP10 for this bolt substitute.  The obsolete UFP10 resists 1340 lbs of force as shown in Table 1. The URFP and  FRFP which have values of 1530# and 1810# respectively superseded it. Modern and stronger bolting hardware requires less hardware overall.  Naturally, this will have a big impact on cost.

The Simpson URFP

The Simpson URFP

This is the strongest Foundation Anchor made.

The Simpson FRFP

 

 

 

 

 

 

 

 

 

What do the Engineers say?

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?

ANSWERS:

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.

 

Bolt Placement

This means only the bolts located where the plywood are located actually do anything. This why the bolts or UFP anchors must be installed within the plywood braced panels.

Bolts only at panels

 

 

 

This is reinforced in this requirement.

Bolts on each end

 

 

 

 

 

Bolts outside of braced panels.  These bolts don’t do anything and increase cost.

On the other hand elsewhere it states

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.  The carpenters first drove very large nails through the wide 4″ face of the mudsill.  Two inches of nail would then be protruding out trhough the mudsill.  They then pushed the mudsill and nails into the wet concrete.  Mudsills are firmly attached to their foundation even if you do not see any bolts.  This mudsill does not have any bolts and did not slide.

Blue House With Arrows

 

Or this house.

Mudsill did not move

 

Existing Bolts

 

 

 

 

This is absolutely correct. Rather than using existing bolts that always have over-sized holes in the mudsill, use new bolts.

Oversized Hole No Washer

MSP

Oversized 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 she shear walls between joists. The shear wall is going to take all of the force. Properly designed retrofits concentrate bolts at shear wall locations.

 

Table S1

This leads us to the following construction detail:
STT Sixteen o c

 

 

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

Plywood Installation

The red arrow shows the plywood nailed 4 inches apart in the upper top plate. It shows the same nailing of 4 inches apart in the lower top plate.  Edge nailing into the upper top plate 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.

Standard Plan A specifies 4 inches apart nails in both the upper and lower top plates.   As shown below where it says “Staggered Spacing” the upper top plate receives a nail every 8 inches (not 4 inches) so the plywood connection  is only half as strong as it should be.

Staggered

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

In Conclusion

              • 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.
              • Hardware is specified with the wrong nails.
              • It omits 3 types of commonly used floor connections (shear transfer ties)

 

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