In our last and rather long blog post, we discussed how Supplement No. 1 to ASCE 7-16 made an important correction to Table 11.4-2, Long-Period Site Coefficient, F_v. We also discussed how that correction introduced a new problem. In this blog post, we would like to say a few more things on the latter item because, as a result of that, we are continually receiving emails from designers who, under certain circumstances, are now receiving some very confusing output from various online tools that provide mapped and design ground motion parameters as well as the Seismic Design Category (SDC) of a structure.

Please do read the earlier blog for a detailed description on how we arrived at this situation.

The problem is related to the highlighted portions of Table 11.4-2 shown below:

ASCE 7-16 Supplement No. 1 Table 11.4-2 Long-Period Site Coefficient, F_v

Per Footnote a, for Site Classes D and E and for S₁ ≥ 0.2, the F_v values given in the table are only to be used for the purpose of determining T_S, which is needed to see if the structure qualifies for one of the exemptions included in Section 11.4.8 from the requirement of site-specific ground motion analysis. However, no guidance is provided on what to do if a structure does qualify for an exemption. Some value of F_v is still required for determining S_D1 not only for calculating the design base shear but also for determining the SDC of the structure. Since ASCE 7-16 literally states that the values of F_v in Table 11.4-2 are for calculation of T_S only, the online ground motion tools are simply not providing any value of S_D1 and SDC, leaving the users confused and frustrated.

The good news is, a Supplement No. 3 to ASCE 7-16 is in the works to mitigate this issue, and also to incorporate some more improvements in the relevant provisions. The bad news is, it will still take a few months for that supplement to go through the balloting and approval processes and be released. While we wait for that Supplement to be published, here is a quick fix for the issue, which will likely be the resolution the supplement will offer: simply revise Footnote a as shown below:

“These values of F_v shall only be used for calculation of T_S and for the calculation of S_D1 when following the exceptions of Section 11.4.8.“

Unfortunately, the online tools will very likely wait for the Supplement to be published before making any changes to their algorithm. But designers can still utilize the above suggestion to manually determine S_D1 and SDC for their structures.