Article Applying Appendix D

Article Applying Appendix D - Applying Appendix D of ACI...

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STRUCTURE magazine January 2008 updates and discussions related to codes and standartds C ODES AND S TANDARDS 21 Applying Appendix D of ACI 318 Sizing Anchor Bolts Makes Me “Sigh” ( ψ ) By Peter Carrato, Ph.D., P.E., S.E. In 2002, the American Concrete Institute’s Building Code Requirements for Structural Concrete (ACI 318 – 2002) for the first time published criteria for Anchoring to Concrete in Appendix D. The current version of Appendix D (ACI 318 – 2005) comprises 26 pages of code and commentary. A first look at this volume of information may appear intimidating to those engineers who are accustomed to designing a base plate in a one page calculation. Appendix D is based on the Concrete Capacity Design (CCD) method that allows the designer to account for many anchorage configurations through the use of a number of psi ( ψ ) factors. The design of concentrically loaded cast- in-place anchor bolts located far from the edge or corner of a slab remains a relatively simple task. Understanding the ψ factors is the key to unlocking Appendix D. Designing anchors bolts is basically a two step process: check the steel design strength then check the concrete design strength. Evaluating the steel has always been easy: take the steel area and multiply by its design strength. Evaluating concrete design strength is not so simple. Depending on the configuration of the anchorage, multiple concrete failure modes are possible, including splitting and cone pullout. These failure modes are influenced by the installation arrangement of the anchors; such as close to the slab edge, near a slab corner, installed in a thin floor, anchors that are close to each other, groups of anchors that are not uniformly loaded, etc. The CCD method uses various ψ factors to help account for all of these potential failure modes and arrangements. Appendix D defines eight ψ factors that are uniquely identified using multiple subscripts; ψ ec,N ψ ec,V ψ ed,N ψ ed,V ψ c,N ψ c,V ψ c,P and ψ cp,N . The subscripts are a guide to the effect addressed by the factor. The first subscript identifies parameters that will modify an ideal concrete stress block that would resist a concentrically applied load on an anchor or group of anchors far from an edge. They include ec for non-uniform loads that would result in ec centric load on a group of anchors, ed for close ed ges, c for c racking and cp anchor that are not c ast-in- p lace. The second subscript covers the type of anchor
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