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Eccentric Holdowns in Light-Framed Shearwall Systems

This is an increasingly expansive and complex topic whereby one ultimately asks "Do post failures in field-built shear walls occur due to eccentric type holdowns?" Based on years of light-framed shear wall testing, we find no evidence to support this contention. In addition, we know of no documented evidence attributing shear wall failures to post failures caused by eccentric holdowns, in either single or multi-story structures, during past seismic events.

Designing Shear Wall Posts for Eccentricity

The internal post stresses calculated in a 'bare post' approach are very conservative and unwarranted at this time. This type of simplified static analysis of a single component in a shear wall system, with numerous elements that interact in very complex ways, will be highly inaccurate. Testing has verified that post failures rarely occur even at ultimate wall strength levels, although calculations by the "bare-post" method often produce ultimate demand-to-allowable stress level capacity (D/C) ratios in the range of 5 to 9. Any wood member with a D/C ratio over 3 would be expected to fail.

Also see: The Effect of Eccentric Overturning Restraint in Complete Shear Wall Assemblies

ICBO Position

ICBO issued a document, dated January 17, 2000, which states, "While there is agreement that a bare (unsheathed) post loaded by an eccentric connection can be adequately evaluated using a simple free body diagram, the same cannot be said for a member that is braced by sheathing/fasteners applied to one or both sides of the post."

Conclusion

A desire for safer structures is the common goal for all involved in the building industry. However, premature design recommendations for eccentricity should not be included in this process. The Building Seismic Safety Council (BSSC) Technical Subcommittee 7 has been tasked to study this issue. The BSSC is contracted by FEMA to write the NEHRP provisions, which serve as the basis for seismic provisions in the IBC, IRC, ASCE 7, and NFPA 5000 codes. The task of this committee will be to determine the appropriate design procedures for shear wall boundary members due to several different limit states. As a manufacturer of both eccentric and concentric type holdowns, we support the efforts of this committee to ultimately determine if the eccentric holdown issue is a concern and, if so, what rational based recommendations should be implemented.