We have recently received questions regarding the overstrength factor as it relates to the design of anchorage to concrete of nonstructural components and nonbuilding structures addressed by ASCE 7-10 Chapters 13 and 15. This topic is important and deals with some coordination issues between the 2012 IBC, ASCE 7-10 and ACI 318-11. John Silva of Hilti North America has provided us with the following answers that we hope will provide the information needed to apply the overstrength factor correctly.

Q. I am designing the attachment of a heavy suspended light fixture to the soffit of a concrete slab and am required to use the seismic provisions included in ACI 318-11 Section D.3.3 for the design of the anchors. Because the slab depth does not permit me to satisfy the ductile anchor option of D.3.3.4.3(a), I would like to design the anchors for the calculated seismic force amplified by the overstrength factor [ACI 318-11 Section D.3.3.4.3(d)]. I am wondering what overstrength factor I should use. Is it the overstrength of the structure found in ASCE 7-10 Table 12.2-1?

A. No. The overstrength factor should be associated with the nonstructural component being anchored, in this case the light fixture. The seismic provisions of ACI 318-11 Sections D.3.3.4 and D.3.3.5 intend to ensure that anchors resisting significant seismic forces do not undergo sudden brittle failure. One way to achieve this is to add sufficient extra strength to the anchor by design, so that the failure must occur elsewhere in the system. Increasing the earthquake inertial load with an overstrength factor has been included in the ACI 318-11 standard as one of four possible options for preventing premature anchor failure.

For anchors attaching a column of a building to the foundation, the structural overstrength factor obtained from ASCE 7-10 Table 12.2-1 is to be used. For a nonbuilding structure designed in accordance with Chapter 15 of ASCE 7-10, the overstrength factors provided in that chapter are applicable. For anchoring a nonstructural component, the overstrength factor should be associated with the nonstructural component. In any case, the overstrength factor applies only in those cases where the provisions of ACI 318-11 Sections D.3.3.4.3(d) or D.3.3.5.3(c) are invoked.

Q. Okay, but where do I go to get the overstrength factors for nonstructural components. I am using ASCE 7-10, and I cannot find them anywhere. Am I missing something?

A. The overstrength factors for various nonstructural components are given in ASCE 7-10 Tables 13.5-1 [Coefficients for Architectural Components] and 13.6-1 [Seismic Coefficients for Mechanical and Electrical Components]. However, if you are using the first or second printing of ASCE 7-10, you won’t find them in those tables. This is because the overstrength factors were first introduced in ASCE 7-10 via a supplement, Supplement No. 1, which, in turn, was incorporated in the third printing of the standard. Some important discussion on the differences between the first, second, and third printings of ASCE 7-10 can be found here.

You can download Supplement No. 1 from here.

Q. From your blog post linked above, I understand that ASCE 7-10 Supplement No. 1 is adopted only in the 2015 IBC and not in the 2012 IBC. The governing building code in my jurisdiction is 2012 IBC. So, if the new overstrength factor values were added only in Supplement No. 1, can I still use them?

A. You are correct that the new overstrength factors are technically not part of the 2012 IBC. However, absent the supplement, there is no other guidance for applying the provisions of ACI 318-11 Sections D.3.3.4.3(d) [for tension] and D.3.3.5.3(c) [for shear], and these provisions are definitely adopted by the 2012 IBC. As a result of significant synchronization issues in the 2012 IBC cycle, some adjustments are required to achieve compatibility between the 2012 IBC and ACI 318-11. This is discussed in detail here and here.

Some state code adoption agencies (such as the State of California) have made the necessary adjustments to rectify these issues during their code adoption process. When in doubt, you should clarify the position of the relevant jurisdiction before proceeding. From a technical standpoint, it is our opinion that, where applicable, the overstrength factors provided in ASCE 7-10 Supplement No. 1 are correctly applied with the seismic provisions of ACI 318-11 Sections D.3.3.4.3(d) and D.3.3.5.3(c).

Q. ASCE 7-10 Section 15.7.5 requires that for tanks and vessels assigned to SDC C or higher, anchors are to be developed in concrete for the steel strength in tension as determined by ACI 318 Appendix D Equation D-3. Should that be Equation (D-2) instead? Equation (D-3) is for concrete breakout. Also, in this case, are we required to amplify the seismic tension by the overstrength factor of the tank/vessel, or do we just use the tension force as obtained from structural analysis?

A. Yes, the reference should be to Eq. (D-2), which is the equation for calculating the steel strength of the anchor. To our knowledge, this correction has not yet been included in the errata for ASCE 7-10.

Where anchors are designed to satisfy the ductile anchor element provisions of D.3.3.4.3(a), there is no need to apply the overstrength factor. Note, however, that the anchor must still be designed for the externally applied tension loads in accordance with D.3.3.4.4, not D.4.1.1. The reference in the original printing of ACI 318-11 incorrectly gives the required section for the anchor design as D.4.1.1. The corrected reference to D.3.3.4.4 is included in the errata for ACI 318-11.