Out-of-Plane Wall Anchorage Design

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.

Figure 1 Out-of-Plane Wall Anchorage Assembly

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.

– Paul

Paul McEntee

Author: Paul McEntee

A couple of years back we hosted a “Take your daughter or son to work day,” which was a great opportunity for our children to find out what their parents did. We had different activities for the kids to learn about careers and the importance of education in opening up career opportunities. People often ask me what I do for Simpson Strong-Tie and I sometimes laugh about how my son Ryan responded to a questionnaire he filled out that day:

Q.   What is your mom/dad's job?
A.   Goes and gets coffee and sits at his desk

Q.   What does your mom/dad actually do at work?
A.   Walks in the test lab and checks things

When I am not checking things in the lab or sitting at my desk drinking coffee, I manage Engineering Research and Development for Simpson Strong-Tie, focusing on new product development for connectors and lateral systems.

I graduated from the University of California at Berkeley and I am a licensed Civil and Structural Engineer in California. Prior to joining Simpson Strong-Tie, I worked for 10 years as a consulting structural engineer designing commercial, industrial, multi-family, mixed-use and retail projects. I was fortunate in those years to work at a great engineering firm that did a lot of everything. This allowed me to gain experience designing with wood, structural steel, concrete, concrete block and cold-formed steel as well as working on many seismic retrofits of historic unreinforced masonry buildings.