Demystifying ACI 318 Appendix D

November 2010 » Features » CONCRETE
A practical guide for anchoring to concrete using strength design
Nicholas F. Ciminello, P.E.

By now, many designers may have heard the buzz about strength design and the new code requirements affecting anchor design, including “cracked concrete.” In 2003, the International Building Code (IBC) introduced state-of-the-art requirements for testing, evaluation, and design of anchors in concrete, including anchors post-installed in hardened concrete. The 2003, 2006, or 2009 IBC is now adopted in nearly all United States jurisdictions, making these changes applicable nationwide.

How has anchor testing andevaluation been affected?
Post-installed mechanical anchors qualified for strength design are required to be tested by an independent laboratory, according to ACI 355.2, Qualification of Post-Installed Mechanical Anchors in Concrete. Performance is tested over a wide range of concrete compressive strengths (2,500 to 8,000 psi) and in a wide range of installation conditions, which may include sensitivity to drill hole diameter, reduced and excessive torque, and for in-service applications such as cracked concrete and seismic loading. Additionally, the design values are now based on statistical analysis of independent test results that ensure more reliable anchor performance. And, after mechanical anchors have been tested and evaluated according to ACI 355.2, the manufacturer may obtain an ICC-ES Evaluation Service Report (ES Report), which provides evidence of code compliance to the building official.

How has anchor design been affected?
Section 1913 of the 2003 IBC and Section 1912 of the 2006 and 2009 editions of the IBC, respectively, state that anchors resisting seismic forces and post-installed anchors such as expansion anchors and undercut anchors installed in hardened concrete shall be designed in accordance with the strength design procedure that references Appendix D of ACI 318. Appendix D requires post-installed anchors to be tested and qualified according to ACI 355.2 (which is supplemented by ICC-ES Acceptance Criteria AC193). ACI 355.2 introduced several new tests including the performance of anchors in cracked concrete conditions.

What is cracked concrete? When should I account for it in my design?
The term “cracked concrete” relates to the likelihood of the concrete base material developing cracks in the region of anchorage at (or below) the expected service load level. Factors that may contribute to concrete cracking, and must be considered by the design professional, include the effects of restrained shrinkage, flexure in tension zones, seismic activity, temperature fluctuations, soil pressure, differential settlement, et cetera.

Only post-installed mechanical anchors that have met the requirements for use in cracked concrete in accordance with ACI 355.2 are permitted for use in applications where crack development is expected, and/or in a geographic region of moderate or high seismic risk. In geographic regions of moderate or high seismic risk (Seismic Design Categories C , D, E, or F), anchors resisting earthquake forces must also pass the Simulated Seismic Test in accordance with ACI 355.2.

Note that in lieu of extensive analysis to prove that the concrete will remain uncracked throughout its service life, and to minimize the risk of the unqualified anchors being used in a cracked concrete application, many design professionals choose to always assume “cracked concrete” and only specify those anchors that have been tested and qualified accordingly.

What happened to the allowable stress design (ASD) tables? Can Istill use them for anchor design?
The allowable stress design (ASD) method of designing concrete anchorage is still in Section 1911 and Table 1911.2 of the 2006 and 2009 IBC. However, the ASD method of designing concrete anchors and applying spacing and edge reduction factors to determine an allowable load is no longer applicable under the International Building Code for post-installed anchors in hardened concrete and anchors resisting earthquake forces. Only strength design is recognized as code compliant for these anchors and only concrete anchors qualified to the new standards are permitted for these conditions.

So what is strength design?
Strength design is a design procedure used by engineers to ensure acceptable levels of safety and the proper functioning of a structure for its expected lifetime. Strength design is based on statistics, engineering principles, and a general knowledge of material behavior. Section 1602 of the IBC defines strength design as, “A method of proportioning structural members such that the computed forces produced in the members by factored loads do not exceed the member design strength.” When applied to anchorage to concrete, strength design requires the design resistance (anchor capacity) to be greater than the factored load (load demand). This is simply stated as the following:

A practical guide for anchoringto concrete using strength design
The flowcharts — general (figure 1), shear (figure 2), and tension (figure 3) — found on these pages serve as a practical guide for the design community. They are intended to give the reader a general understanding of the strength design method as it applies to post-installed mechanical anchors in concrete.

Figure 1: Use this flowchart for understanding the general requirements and code checks for post-installed mechanical anchors.
Figure 2: Use this flowchart for analyzing and designing post-installed mechanical anchors in shear.
Figure 3: Use this flowchart for analyzing and designing post-installed mechanical anchors in tension.

Although the information provided should not be considered a substitute for ACI 318 Appendix D, it will provide a resource for concrete anchor design using the strength design method.

Design examples — Two design examples can be viewed by clicking here. They contain an example for calculating the factored resistance design strength in tension and in shear, as well as the equivalent allowable stress design load.

Is there software available to help?
Yes. Many anchor manufacturers have recognized the complexity of the new code requirements and have developed software packages capable of conducting anchor design utilizing the strength design method according to ACI 318 Appendix D. In many cases the software can be downloaded for free from the manufacturer’s website.

Acknowledgements
The author wishes to acknowledge John R. Henry, P.E., principal staff engineer with the International Code Council (ICC), and Brian Gerber, S.E., principal structural engineer with ICC Evaluation Services, for reviewing this article and making valuable comments and suggestions.

References

  • American Concrete Institute (ACI) 355.2-07 – Qualification of Post-Installed Mechanical Anchors in Concrete and Commentary
  • American Concrete Institute (ACI) 318-08 (Appendix D) – Building Code Requirements for Structural Concrete and Commentary (Anchoring to Concrete)
  • International Code Council (ICC) – 2003, 2006, 2009 International Building Code
  • International Code Council Evaluation Service (ICC-ES) AC 193, February 2010 – Acceptance Criteria for Mechanical Anchors in Concrete Elements
  • Powers Power-Stud+ SD2 Anchors in Cracked and Uncracked Concrete, ESR-2502, May 1, 2010, ICC Evaluation Service, Inc.

Nicholas F. Ciminello, P.E., is a senior staff engineer for Powers. For more information contact the Powers Technical Support Hotline a 1-888-745-2633 or engineering@powers.com.

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