Beginning with the 2005 edition of the Building Code Requirements for Structural Concrete, also known as ACI 318, significant changes for design of anchors to concrete foundations were introduced for light frame construction. With adoption of ACI 318 in the 2006 International Building Code (IBC), these requirements are now part of the building code in most jurisdictions across the United States.

Engineers, builders, and specifiers are quickly finding that applications of some of the code requirements to light-frame construction require significant change from past practices that have performed well. They now see that the anchorage of wood or steel sill plates in homes or low-rise construction requires very tight spacing of anchor bolts in moderate- to high-seismic areas.

There remains significant debate over the appropriateness of the new ACI 318 requirements. In his August, 2008 STRUCTURE Magazine article For What Planet Is This Code Written?, Richard Hees raises several concerns, and noted that some of the requirements in ACI 318 appendix D are not appropriate for ANY light-frame construction: wood or steel.

In the past, the American Concrete Institute (ACI) and others have used what was called the “45-degree cone method” to determine anchor bolt capacity. Developed in the 1970s, this worked well in many instances, but concerns were raised about the ductility of concrete connections in seismic events. Test at the University of Stuttgart led to the development of the “Kappa method” (K), which was eventually refined at the University of Texas Austin to the “Concrete Capacity Design (CCD)” method used in the current version of ACI 318 Appendix D.

With anchor bolt connections, the strength depends on the capacity of the anchor bolt itself, and the capacity of the concrete. The lesser of these two gives the eventual value used by designers. The capacity of the steel is easy to calculate. The capacity of the concrete depends upon several factors in addition to the strength of the concrete: length the bolt is embedded, configuration of the end of the anchor (hooked, headed, expansion, etc.), distance of the bolt from other bolts, and distance of the bolt from the edge or corner of the slab. Where there is the biggest concern about capacity reduction is at slab edge conditions, since there is already a capacity reduction due to edge distance, and this is where most anchor bolts are used in light-frame construction. The concern by ACI is a non-ductile “breakout” of the concrete at the edge of the slab in a seismic event.

The Structural Engineers Association of Northern California (SEAONC), in conjunction with Simpson Strong-Tie and the American Forest & Paper Association, initiated a testing program to evaluate the capacity and ductility of anchors in wood sill plates. Using 5/8” anchor bolts in 2x4 and 3x4 and wood sill plates, researchers loaded the bolts to failure, with in-plane (horizontal) forces along the length of the sill. The research showed that capacities were much higher that those permitted in ACI 318 Appendix D, and that a ductile failure in the bolt and bolt/wood connection could be achieved before breakout of the concrete. This satisfied one of the key issues in ACI 318 section D3.3.5: that the anchor/structure connection “undergo ductile yielding at … forces no greater than the [anchor] design strength.” If ductile yielding cannot be demonstrated, section D3.3.6 requires anchor capacities to be multiplied by 0.4 – reducing capacity by 60%¹ .

At code hearings of the International Code Council in October, a code change was approved that exempted wood framing from the ACI 318 anchor requirements based on the SEAONC testing program. Despite attempts by SFA and AISI, steel was not given the same exemption. However, the code change leaves the door open for the steel industry to provide data permitting exceptions for steel framing at the final action hearings in 2010.

AISI and SFA are working closely with practitioners and researchers, and have developed the initial requirements for the next phase of tests. The current schedule is for testing to be completed in February, 2010, to meet the deadline for public comments and final actions for the 2012 International Building Code.

Once the final report of the upcoming testing is issued, SFA will provide results on their website, and if applicable, CFSEI will provide design and detailing alternatives allowing greater concrete capacity for connections to steel framing.

What does this mean for designers of steel-framed systems in the interim? For now, all of the provisions of ACI 318 appendix D still apply, unless building officials or local ordinances provide specific exceptions for their jurisdiction. The capacity of the steel framing – typically the bottom track – is still the same: based on the bolt bearing equations in Chapter E of AISI North American Specification for the Design of Cold-Formed Steel Structural Members (S100). However, the reduced concrete capacities mandated by ACI for the concrete side of the connection in seismic design categories C, D, E and F still greatly reduce the capacity for the connection.

Don Allen, Technical Director, Steel Framing Alliance