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Concern

In areas already susceptible to damage, APA's narrow braced wall system will reduce the performance of current minimum code compliant braced walls by at least 20%

Supporting Evidence

  1. History shows site built narrow panels, particularly at garages, don't perform well in high wind or earthquake events
  2. Baseline wall testing performed by APA to justify their system was not built to current code requirements
  3. APA's proposed method assumes the return wall always provides adequate overturning restraint
  4. APA's method is prone to installation errors and can result in a 50% reduction in performance according to test data
 
     

Understanding Wall Bracing Solutions

Building Methods:

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a) History shows site built narrow panels, particularly at garages, don't perform well in a high wind or earthquake events

Narrow braced wall panels have historically not performed well during wind and seismic events. Particularly, problems have been observed at garage fronts where often the garage extends out from the house and is required to resist significant lateral loads. These garage fronts may often have living space above and very small wall areas on either side of the large garage opening. Garages have been observed to fail typically by racking as these small wall sections on either side of the garage opening are not capable of resisting the lateral loads due to wind or seismic activity.

 
Seismic: BEFORE   Seismic: AFTER
     
 
Wind   Wind
 
     
 
 

b) Baseline wall testing performed by APA to justify their system was not built to current code requirements

APA's system was approved to the 2004 IRC supplement based on testing that showed comparative performance to a continuously sheathed 4:1 aspect ratio benchmark wall, which is allowed in the 2003 IRC. However, review of the test data indicates that this testing was performed on a benchmark wall which was not properly built to code. The code-required gypsum in Figure R602.10.5 was not installed.


   

IRC Figure R602.10.5
 
APA Tested Assembly

Testing performed by Simpson Strong-Tie of APA's system versus a baseline wall built properly to code reveals shortcomings in the performance of APA's system. The extent of the shortcomings is based on the claims that APA made on page 308 of the 2005 Final Action Agenda. Here they have multiple statements indicating that the goal with their system is to provide walls “equivalent to the code required minimums” or “that are at least as stiff and strong as wall fabricated in accordance with the wall bracing Table R602.10.1”, or provide a modified portal frame “tested to have the strength and stiffness (overall performance) at least equal to that of the 4:1 aspect ratio panels”. Simpson testing indicates that the APA frame is actually 20% less stiff and thus doesn't meet APA's performance goals.

Testing of APA's system versus a baseline wall built properly to code reveals shortcomings in performance of APA's system. Testing performed by Simpson Strong-Tie on a properly constructed 4:1 benchmark wall compared to APA's 6:1 wall with gypsum, indicates that there is an approximate 20% reduction in performance of the walls.

Contact Simpson Strong-Tie for Third-Party test reports.

 Technical Details: Analysis of APA Testing Methodology

     
 
 
 

c) APA's proposed method assumes the return wall always provides adequate overturning restraint

The APA method assumes that during all events the return wall provides appropriate overturning restraint. The system relies on the attachment of the side wall sheathing and the weight of the framing elements to resist the narrow portal from overturning. This is questionable, especially during a high wind event, because the same forces which are producing the lateral loads on the structure are also attempting to lift the roof off the structure. This uplift force reduces the ability of the return wall to resist the overturning force.

      

 Technical Details: On Overturning

     
 
 
 

d) APA's method is prone to installation errors and can result in a 50% reduction in performance

APA's narrow braced wall panel has multiple framing requirements necessary to achieve the required performance. The framer must take special care to insure that all of the requirements are met. The following list provides some common areas of possible misinstallations.

Likely Misinstallations:

  • Improper Framing Assembly
    • Double 2X studs required on each end of the panel
    • Outside stud continuous (king stud)
    • Minimum Dbl 2X12 Header framed over the entire wall
    • Plate between jack studs and header
  • Omitted washers from Anchor Bolts
    • 2X2X3/16" washer required
  • Wrong Fasteners Used
    • 8d common required
  • Wrong Fastener Spacing
    • Nails at 3" o.c. to each framing member in the panel
  • Overdriven Sheathing Fasteners
    • APA recommends one additional nail for every two overdriven
  • Strap omitted from inside of wall
    • 1000 lb strap required
  • Electrical holes cut in sheathing

Misinstallation Testing:

APA has performed testing on their 6:1 aspect ratio wall, which examines the effects of common installation errors on wall performance. APA's testing indicated little or no reduction in stiffness when their system is misinstalled, and their field representatives have been communicating this in presentations around the country. Review of APA's test data indicates that they only tested with a high capacity holdown, which is the reason APA has been reporting minor performance differences.

Simpson started testing APA's system as our experience with narrow panels led us to believe that this could not be true. Figures 2 and 3 illustrate the results of testing conducted by Simpson Strong-Tie. The results indicate that the 6:1 walls have a significant reduction in capacity with minor reductions in the nailing pattern when holdowns aren't used – resulting in low end-restraint. When a holdown is installed (i.e. high end-restraint) the reductions are much smaller.

Figure 2

Figure 3

  1. High End Restraint percentage from APA report T2004-54 Table 3; equals (6:1 with holdown) / (4:1 baseline with holdown, no gypsum)
  2. Low End Restraint percentage from Simpson Strong-Tie testing; equals (6:1 no holdown) / (4:1 baseline no holdown, no gypsum)
  3. Simpson Strong-Tie testing was performed at its IAS accredited laboratory and was independently witnessed

 Technical Details: Implications of Misinstallations

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