Conflict between ACI 318-11 and ASCE 7-10: Application of Overstrength Factor in Anchor Design
Application of an overstrength factor is an important issue in designing anchors in concrete in accordance with Appendix D of ACI 318-11. A primary purpose of the seismic design provisions of ACI 318-11 Sections D.3.3.4 and D.3.3.5 is to ensure that anchors resisting significant seismic forces do not undergo sudden brittle failure under the design loads. One way to achieve this is to amplify the anchor design forces by an overstrength factor.
When it comes to anchorage of nonstructural components, in order to help engineers comply with the overstrength requirements of ACI 318 Appendix D, Supplement 1 to the 2010 edition of ASCE 7 has introduced values of overstrength factors for different types of architectural, mechanical, and electrical components in Tables 13.5-1 and 13.6-1. However, this has also given rise to a conflict with ACI 318 Appendix D regarding how the overstrength factor should be applied, as expressed in the following question.
Q: We are dealing with anchoring an electrical component hanging from the concrete ceiling. When we went to ASCE 7-10 Table 13.6-1 to look up its overstrength factor, we noticed Footnote c to the table which says: “Overstrength as required for anchorage to concrete. See Section 12.4.3 for inclusion of overstrength factor in seismic load effect”. The relevant load combinations in ASCE 7-10 Section 12.4.3 applies the overstrength factor to the horizontal seismic load effect (QE) only, and not to the vertical seismic load effect (EV = 0.2SDSD). On the other hand, ACI 318-11 Section 3.3.4.3(d) says: “The anchor or group of anchors shall be designed for the maximum tension obtained from design load combination that include E, with E increased by Ωo”. It seems to me that the seismic load effect E includes effects from both horizontal and vertical seismic forces, and both are required to be amplified by Ωo. This is different from what ASCE 7-10 seems to require. Please advise.
A: What you have pointed out is a disconnect between how the overstrength factor is handled in ASCE 7-10 and in ACI 318-11 Appendix D. ASCE 7-10 Section 12.4.3 is very specific that the overstrength factor, when required, should be applied to the effects of the horizontal seismic forces (QE) only, and not to the effects of vertical seismic forces (EV). ACI 318-11, on the other hand, does not make any such distinction. ACI 318-11 Sections D.3.3.4.3(d) (for tension) and D.3.3.5.3(c) (for shear) require the overstrength factor to be applied to E, which one could reasonably think includes both QE and EV. We have mentioned this issue in one of our blog posts some time ago. The figure below illustrates the differences in the forces that need to be considered for anchor design of a nonstructural component.
Comparison of application of overstrength factor for designing component anchorage,
where Fp is the component design force from ASCE 7-10 Section 13.3.1 and Wp is the component weight
It is our opinion that, since ASCE 7-10 is the standard adopted by the 2012 IBC for providing the demand side of a building design, its provisions related to determining force demand on anchors should govern over those from ACI 318-11, which is adopted by the 2012 IBC to provide the strength side of the design of concrete buildings. So, it should be all right to apply the overstrength factor only to QE to meet the requirements of ACI 318-11 Sections D.3.3.4.3(d) and D.3.3.5.3(c).
