While the Simpson Strong-Tie Tye Gilb R&D lab in Stockton is a large testing facility, the world’s largest R&D lab is Mother Nature herself. Natural disasters such as earthquakes or storms put our engineering designs to the test. In this week’s blog post, I’ll be turning attention to wall anchorage for out-of-plane forces and the lessons we have learned from Mother Nature so far.
The 1979 building code incorporated many of the lessons learned from the 1971 San Fernando earthquake. In 1994, Mother Nature put the 1979 building code to the test with the January 17 Northridge earthquake. The Northridge earthquake showed that some of the increased design and detailing requirements in the 1979 building code worked well to improve performance over what was observed in 1971. However, it also revealed to researchers that acceleration at the roof level of single story warehouse buildings were three to four times the ground acceleration. The combination of higher than expected acceleration and excessive deformation of the wall anchorage assembly caused many wall anchorage failures.
Several changes in the design forces used for wall anchorage and additional detailing requirements were incorporated in the 1997 Uniform Building Code. The requirements have been refined with each new building code, but overall the requirements and design forces have remained about the same under the current International Building Code. Wall anchorage design is governed by ASCE 7-05 and ASCE 7-10 Section 12.11. These provisions aim to mitigate the brittle wall anchorage failures observed in past earthquakes by increasing the design force and in Seismic Design Categories C through F, requiring:
- continuous ties,
- limiting the maximum sub-diaphragm aspect ratio,
- increasing the design force for the steel elements of the wall anchorage system,
- precluding the use of toenails or nails in withdrawal or cross-grain bending or cross-grain tension,
- embedded straps be attached or hooked around reinforcing steel,
- considering effects of eccentricity on system,
- considering additional wall anchorage force at pilasters.
A material overstrength factor of 1.4 is used for the steel elements of the anchorage system to ensure sufficient nominal overstrength for the entire wall anchorage system. This 1.4 factor applies to all the steel components in the wall anchorage system, including the sub-diaphragms and continuity ties. In addition, edge nailing of the roof or floor sheathing to a wood framing member may not be considered to provide out-of-plane capacity. Also note that the wall anchorage design equations for wall anchorage have been updated in ASCE 7-10 to account for the span length of diaphragms. This may help reduce design forces where diaphragm spans (defined by distance between walls) are less than 100 feet.
It’s worthwhile to point out 2009 & 2012 IBC Section 1908.1.9 recognized ACI 318 Section D.3.3.4 and D.3.3.5 ductility requirements for concrete anchorage should not be applied on top of the ASCE 7 wall anchorage system requirements. Since the ASCE anchorage design forces have already been factored to protect against brittle failure, applying the additional ductility requirements from ACI 318 Appendix D was overly conservative.
The 1999 SEAOC Blue Book by the 1999 Seismology Committee and the September 2008 SEAOC Blue Book Tilt-up Buildings discusses much of the philosophy for the current code wall anchorage design provisions. In addition, Simpson Strong-Tie has a panelized roof system Technical Bulletin (T-PRS12) with several wall anchorage design examples and details. Woodworks also provide resources and case studies of panelized roof systems. Some building jurisdictions have their own requirements in addition to those in the IBC, such as the City of Los Angeles. The designer should check with the Building Department with jurisdiction over the project to determine if they have any additional wall anchorage design requirements.
What lessons do you expect Mother Nature to reveal to the industry in the next “Big One”? Let me know in the comments.
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