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About Anchor Strength Design for International Building Code (IBC)
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 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?
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
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:
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