For seismic design in accordance with the IBC and ASCE 7 provisions, the values of site coefficients, F_a and F_v, have remained the same over the last several editions of the IBC and ASCE 7. They have, in fact, remained the same since they were first introduced in the 1994 edition of the Uniform Building Code. The values were based on studies following the Mexico earthquake of 1985 and the Loma Prieta earthquake of 1989. That changed in the 2018 edition of the IBC and the 2016 edition of ASCE 7, where the values of both coefficients were revised in major ways in order to make them consistent with the seismic maps included in ASCE 7-10 (based on 2008 USGS maps) and ASCE -16 (based on 2014 USGS maps). In the course of making this revision, a significant error also crept in, which eventually had to be corrected in Supplement 1 to ASCE 7-16. In this blog, we will discuss that error and its resolution.

Let’s start with the 2018 IBC

The 2018 IBC incorporated the following revisions to the values of site coefficients F_a and F_v during its update process. These revisions mirrored the same revisions making their way through the update process of the 2016 edition of ASCE 7, which was also ongoing at the time. The numbers relevant to this blog post are highlighted in the F_v table below.

2018 IBC TABLE 1613.2.3(1)
VALUES OF SITE COEFFICIENT, F_a^a

2018 IBC TABLE 1613.2.3(2)
VALUES OF SITE COEFFICIENT, F_v^a

Now let’s see what the F_v table looked like in the final published version of ASCE 7-16

Even though the above mentioned changes in the IBC were supposed to mirror the same changes in ASCE 7-16, that was not how things turned out. ASCE made a last-minute additional revision to the F_v table, as shown below. The F_v values provided for Site Class E where S₁ ≥ 0.2 were deleted, and in their places, references to Section 11.4.8 were inserted. This was done in light of a new provision in Section 11.4.8, which requires a site-specific ground motion hazard analysis to be performed in accordance with Section 21.2 for Site Class E where S₁ ≥ 0.2. This requirement removed the need for any site coefficient for Site Class E where S₁ ≥ 0.2 and thus the corresponding values were deleted.

ASCE 7-16 TABLE 11.4-2 Long-Period Site Coefficient, F_v

This revision made the F_v table in ASCE 7-16 substantially different from the corresponding table in the 2018 IBC. And, this is also where an error crept in.

In making this last-minute change, an exception in Section 11.4.8 was overlooked. Exception 3 to Section 11.4.8 states that, for structures other than seismically isolated structures and structures with damping systems, a ground motion hazard analysis is not required even when the structure is located on Site Class E sites with S₁ ≥ 0.2, provided that period T ≤ T_s and the equivalent static force procedure is used for design. Deletion of the highlighted values above created two problems with this exception:

1. Without a value for F_v, one cannot determine the transition period T_s, and thus cannot check if the exception applies.
2. And, if the exception does apply, one needs a value for F_v to perform the equivalent static force procedure that the exception requires.

In view of the above, further revisions were required, which are provided in Supplement No. 1 to ASCE 7-16.

Changes in the F_v table in ASCE 7-16 Supplement No. 1

The following changes were made to Table 11.4-2. In short, the old deleted values of F_V were brought back with a footnote cautioning the users that a site specific ground motion hazard analysis may be required by Section 11.4.8.

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

These revisions in Supplement No. 1 rectified the error mentioned above. They, however, came with their own problem. An added sentence in Footnote a specifies that the values provided for F_v are to be used “only for calculation of T_S”, presumably just for the purpose of checking if Exception 3 to Section 11.4.8 applies. But what happens when the exception does apply? How is one supposed to obtain the F_v-value for performing equivalent static analysis?

In light of this, two corrected tables are presented below.

VALUES OF SITE COEFFICIENT, F_a^a

VALUES OF SITE COEFFICIENT, F_v^a

More confusion

To compound the confusion discussed above, Supplement No. 1 to ASCE 7-16 is included at the end of the 3rd printing of the ASCE 7-16 Provisions volume. It is logical to expect that it will remain in subsequent printings. Users who have the 1st or the 2nd printing of ASCE 7, however, will need to find the Supplement here: https://ascelibrary.org/doi/10.1061/9780784414248.sup1.

The citation of ASCE 7-16 in Chapter 35 of the 2018 IBC does not specifically indicate that Supplement 1 is adopted. This adds to the murkiness of the situation.

In this particular case, if one takes the view that the IBC takes precedence over its referenced standards as indicated in 2018 IBC Section 102.4.1, and decides to use 2018 IBC Tables 1613.2.3(1) and (2), as printed in the code, that would work out just fine. The last-minute revision to the F_v-table in ASCE 7-16 turned out to have been ill-advised. Supplement No. 1 to ASCE 7-16 has put things back to where they were before the change was made, which takes us back to the F_v-table in the 2018 IBC.