1997 UBC vs. 2006 IBC Concrete Anchor Design


    When the 2007 California Building Code goes into effect in California on Jan. 1, 2008, there will be a drastic change in the way concrete anchors will have to be designed. Currently, under the 1997 Uniform Building Code (UBC), they are typically designed by Section 1923.1 of that code, which contains allowable stress design provisions and allowable service load values set forth in Table 19-D. This approach has been commonly used for years and is easy because it’s a matter of looking up values in a table. This simple approach has disappeared for seismic design in the International Building Code (IBC).

    Substantially the same anchor design provisions as in 1997 UBC Section 1923.1 are found in Section 1911 of the 2006 IBC. However, the 2006 IBC specifically requires that anchors subject to seismic forces in a structure assigned to Seismic Design Category (SDC) C, D, E, or F be designed by the strength design procedure in Section 1912 of the code, which adopts Appendix D of ACI 318-05 by reference.

    Appendix D covers cast-in-place as well as post-installed mechanical anchors, whereas the scope of 1997 UBC Section 1923 is limited to cast-in-place anchors only. Note that there are two amendments to ACI 318-05 in the 2006 IBC—one in Section 1912 itself, and the second in Section 1908 (Section 1908.1.16). The first difference in designing by Appendix D of ACI 318-05 is that it does not include allowable stress design or tabular values, which have served UBC users so well in the past. This will translate into more design time for the engineer.

    Although Appendix D contains a performance statement that presumably permits other design methods as long as they are "in substantial agreement with results of comprehensive tests," there is one "deemed-to-comply method" that is incorporated in Appendix D. This deemed-to-comply method is the so-called concrete capacity design method, which most of Appendix D is devoted to. This is the method the UBC user will most likely end up switching to from Section 1923.1 of the 1997 UBC.

    Something new in Appendix D are the four failure modes for an anchor subject to tension, the last two of which were not explicitly considered in the UBC. They are as follows (see Figure 1):

    anchor steel itself failing (Figure 1a);
    concrete breakout failure (Figure 1b);
    concrete pullout failure (Figure 1c); and
    concrete side-face blowout, which happens only with deep anchors when the edge distance is small (Figure 1d).

    Concrete splitting failure is not one of these four modes of failure because it is precluded by specifying code minimum-edge distance and minimum spacing between anchors (see ACI 318-05 Section D.8).

    For an anchor subject to shear, Appendix D considers the following three failure modes (see Figure 2), the last of which was not explicitly considered by the UBC:

    anchor steel itself failing (Figure 2a);
    concrete breakout failure (Figure 2b); and
    concrete pryout failure, which can happen only with shallow anchors (Figure 2c).

    For anchors subject simultaneously to tension and shear, an interaction relationship is given in ACI 318-05 Section D.7, which is different from those set forth in Section 1923 of the 1997 UBC.

    Appendix D also has specific provisions concerning an anchor subject to seismic forces in a structure assigned to SDC C and higher, which the UBC user will have to contend with. First, the design strength of an anchor will need to be reduced by 25 percent in seismic applications. More importantly, an anchor itself will have to fail in a ductile mode before any of the concrete failure modes can develop. This means that the anchor itself must be made of ductile steel. A ductile steel element is specifically defined in ACI 318-05 Section D.1 as having a minimum elongation of 14 percent and a minimum reduction of area of 30 percent.

    ACI 318-05 itself has one exception to the ductile anchor-failure requirement. A designer can have an attachment (for example, a base plate) yield before the attainment of the anchor design strength; in that case, ductile anchor failure ahead of concrete failure is not a requirement. Section 1908.1.16 of the 2006 IBC added a second alternative. Under this code section, a designer has the option of providing overstrength anchors rather than ductile anchors. If an anchor is designed for two and a half times the design loads, then the anchor qualifies as an overstrength anchor and ductile anchor failure does not have to precede concrete failure.

    Even with these two alternatives to the ductile anchor failure requirement, in many applications the UBC user will be looking for other alternatives, which will hopefully make their way into the code.

    S.K. Ghosh Associates Inc., is a seismic and building code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and dowtyskga@cox.net, respectively, or at www.skghoshassociates.com.