June 2, 2010
Steel Industry Anchorage Research Produces Immediate Results At ICC Hearings

Seismic Force Resisting Systems (SFRS) in Cold-Formed Steel (CFS) light-framed buildings are typically comprised of shear walls with anchor bolts.  In recent editions of the American Concrete Institute’s Building Code Requirements for Structural Concrete ACI 318 (ACI, 2005 and ACI, 2008, Appendix D) the design capacities of anchors for breakout strength in shear are significantly reduced when compared to historical values found in legacy code requirements in Seismic Design Categories C, D, E, and F. 

The wood industry responded to the new ACI requirements by conducting tests and, in October 2009, proposing an exemption to the requirements in the ICC International Building Code for conventional wood framing, leaving CFS at a competitive disadvantage.  In response, the steel industry united behind a quick response effort involving SFA, AISI, SSMA and various manufacturer partners to conduct a test program with the intention of expanding the code exemption to include CFS.

ACI 318 requires a reduction in concrete break-out design capacity unless connections are ductile.  Until recently, there has been no specific test data available to substantiate the need for the reduced designed capacities for anchors in concrete for a typical CFS track to concrete connection loaded parallel to the edge (per ACI 318, Appendix D).

To characterize a typical anchor bolted connection through experimental testing the AISI/SFA/SSMA research team undertook a study with the following goals –

  • Establish test data for the connection capacity when loaded parallel to the edge of slab
  • Determine whether the connection exhibits ductile steel behavior
  • Propose rational design capacities for the connection based on test results

A total of 21 tests were performed. All tests were 5/8-inch diameter single-bolt tests in 33-mil or 68-mil CFS tracks connected to concrete with post-installed steel anchor bolts. Concrete specimens were tested “as-cast,” without the intentional creation of cracks in the test specimen, which was consistent to similar tests previously conducted on wood sill plates.

Materials for the testing were provided by Dietrich Metal Framing.  Laboratories and supervision for the testing were provided by Hilti North America. Testing was conducted between February and April 2010.

Monotonic tests were run at a loading rate of between 1 and 3 minutes to failure. Cyclic tests were conducted with a load-based cyclic testing protocol (sine wave) at a frequency of 0.1Hz (1 cycle every 10 seconds).

Research Findings –

The findings of this research demonstrated that “bearing” of the CFS track was the first and only material limit. The connection assembly exhibited the following behavior phases:

  • Initial take-up and displacement (connection assembly gets “seated”)
  • Elastic bolt bending combined with elastic deformation of the CFS track due to bolt bearing
  • Inelastic deformation of the CFS track due to bolt bearing

Various levels of cyclic load levels helped determine whether the connection exhibited ductile steel behavior. The connections tested exhibited significant deformation capacity and ability to sustain loads. The bolt showed no permanent deformation. The bolt/concrete interface showed no damage. The behavior observed was ductile.

This experimental testing demonstrated that actual capacities of the track-to-concrete anchor bolt connection for CFS tracks ranging from 33 mil up to 68 mil and with edge distances as low as 1.75” exceed those historically used for design, supporting the use of AISI bolt-bearing design values for the connection in lieu of those determined in accordance with ACI 318, Appendix D.

The experimental data demonstrated that ductile steel failure modes rather than concrete failure modes limit the capacity of the connection. This finding demonstrated that there is no need to reduce the capacity of the connection based on concrete strength as specified in ACI 318.

Based on these findings, public comments were submitted by AISI and SFA to the ICC to obviate the calculation required in ACI 318, Appendix D for CFS. 

The public comment was heard by the ICC membership in May 2010 at the Dallas Final Action Hearings.  Despite an uphill battle to modify a previously-approved proposal, the steel industry was successful in obtaining an exemption for CFS in the International Building Code.

According to Mark Nowak, President, Steel Framing Alliance,“The result of this testing has equipped us with data that was previously unavailable to better understand the need to reduce designed capacities for anchors in concrete. The research team should be commended for bringing this work together in an extremely short time frame before the submission due date of the ICC.  The help from Hilti North America and Dietrich Metal Framing demonstrates how valuable our industry sponsors are to creating changes in the codes that level the competitive playing field for CFS.” To read the research paper, click here.

Editor, Framework Online

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Brought to you by the Steel Framing Alliance (SFA) on the first Wednesday of each month, February through December. Framework Online arms you with important news and commentary on the cold-formed steel (CFS) framing and construction industries.