When Does 450=840? Why Not Fix It? SBCA Tried...

Originally published by the following source: SBCAOctober 15, 2015


SBCRI's goal is to ask: Is this true or not? SBCRI provides the facts and lets the reader decide.

As discussed in the first part of this series, the facts that SBCRI has available to it leads to the conclusion that the IRC code-compliance math assigns, by law, the following OSB design values:

  • 350 plf = 600 plf for isolated braced wall panels without gypsum wallboard
  • 450 plf = 840 plf for isolated braced wall panels with gypsum wallboard

This codified math provides a significant competitive advantage for the OSB products of APA-The Engineered Wood Association members. 

Why are these design values used?

Why is this math acceptable to the ICC, building code officials and the engineering community? Perhaps more importantly, why does APA not correct this in the IRC? They have never disputed SBCRI’s testing by providing real-building test data using ASTM E564 techniques (i.e. trusses on top of the walls versus a steel beam, etc.). The 2007 APA Building Seismic Safety Committee (BSSC) report also states that the SBCRI testing provides a reasonable result. Both the APA BSSC report and SBCRI testing confirm that a change to the design values should be made.

Did SBCA Take Appropriate Action? If Not Why Doesn’t It?

SBCRI undertook proprietary testing which was performed in a manner that replicates precisely the IRC isolated braced wall panels in a braced wall line in a real building as the following photos illustrate:

The ultimate strength values generated by SBCRI are based on tests that were reported to APA and AWC in 2013. Subsequently, test data has been added to the SBCRI OSB benchmark database as new benchmark testing is undertaken. 

The SBCRI testing confirms the APA BSSC report ultimate strength values and reflect real building performance as follows:

  • 350 plf for isolated braced wall panels without gypsum wallboard
  • 450 plf for isolated braced wall panels with gypsum wallboard

This work was provided to the American Wood Council (AWC), the International Code Council (ICC), APA and also publicly submitted as IRC code change proposal B308-13, as follows:

This item is on agenda for individual consideration because a public comment was submitted.

Public Comment:

Larry Wainright, Qualtim, representing Structural Building Components Association, requests Approval as Modified by this Public Comment.

Modify the proposal as follows:

TABLE R602.10.4.4

Bottom plate connection to foundation Fastener Fastener Spacing Any Species Stud Framing
Tested Capacity System Effects Factor IRC Lateral Design Capacity
3/8", 7/16" or 15/32" WSP @16" and 24" o.c framing Anchor bolts in accordance with code requirements 6d (2" x 0.113" nails) or 8d (2 1/2 x 0.131") 6:12 335 350 1.80 600
3/8", 7/16" or 15/32" WSP @16" and 24" o.c framing (with 1/2" gypsum on interior face of wall. Anchor bolts in accordance with code requirements 6d (2" x 0.113" nails) or 8d (2 1/2 x 0.131") nails and Types S or W drywall screws 6:12 WSP & 16:16 for GWB 465 450 1.80 840

a. The lateral design capacity of braced wall panels is based on full scale wall assembly tests using the minimum restraint provisions of the IRC, further adjusted by the partial restraint/systems effect factor.

Note: the change from 335 to 350 plf for the walls without GWB changed the system effect factor to 1.7 and similarly for walls with GWB so that a common “standard equivalency factor” could be used. This change accurately reflected tested performance well.

Table 1: Proposed 2013 IRC Code Change Proposal table showing “standard equivalency factors”, which APA acknowledges exist in APA’s bracing spectrum analysis which can be read in the APA-BSSC report.

The above code change proposal renders both the SBCRI and APA BSSC report data accurately and transparently defines the implied system factor used in the IRC. The history behind the IRC braced wall design properties can be found in “The Story Behind IRC Wall Bracing Provisions, Parts 1 & 2 - Crandell/Martin [pdf].”

APA and AWC both worked to disapprove this code change proposal. The question is: why?

The SBCRI-SBCA goal is to provide a set of facts that have test data behind them, provide all appropriate references and undertake common-sense analysis. From there, and based on the merits, the reader can decide. We appreciate any comments, data and new analysis that will add to our knowledge. For more in-depth knowledge on this topic, please review this collection of published articles


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