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A Few Things You Need to Know About the New Site Coefficients in ASCE 7-16

Author: Maria Agiplay
Posted Date: 04 Nov 2019

For seismic design in accordance with the IBC and ASCE 7 provisions, the values of site coefficients, Fa and Fv, 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 Fa and Fv 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 Fv table below.

2018 IBC TABLE 1613.2.3(1)
VALUES OF SITE COEFFICIENT, Faa

Site Class MAPPED RISK TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCER)
SPECTRAL RESPONSE ACCELERATION PARAMETER AT SHORT PERIOD
SS ≤ 0.25 SS = 0.5 SS = 0.75 SS = 1.0 SS = 1.25 SS ≥ 1.5
A 0.8 0.8 0.8 0.8 0.8 0.8
B 1.0 0.9 1.0 0.9 1.0 0.9 1.0 0.9 1.0 0.9 1.0 0.9
C 1.2 1.3 1.2 1.3 1.1 1.2 1.0 1.2 1.0 1.2 1.0 1.2
D 1.6 1.4 1.2 1.1 1.0 1.0
E 2.5 2.4 1.7 1.2 1.3 0.9 Note b 0.9Note b 0.9 Note b
F Note b Note b Note b Note b Note b Note b
a. Use straight-line interpolation for intermediate values of mapped spectral response acceleration at short periods, Ss.
b. Values shall be determined in accordance with Section 11.4.8 of ASCE 7.

2018 IBC TABLE 1613.2.3(2)
VALUES OF SITE COEFFICIENT, Fva

Site Class MAPPED RISK TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCER)
SPECTRAL RESPONSE ACCELERATION PARAMETER AT 1-SECOND PERIOD
S1 ≤ 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 = 0.5 S1 ≥ 0.6
A 0.8 0.8 0.8 0.8 0.8 0.8
B 1.0 0.8 1.0 0.8 1.0 0.8 1.0 0.8 1.0 0.8 1.0 0.8
C 1.7 1.5 1.6 1.5 1.5 1.4 1.5 1.3 1.5 1.3 1.4
D 2.4 2.0 2.2c 1.8 2.0c 1.6 1.9c 1.5 1.8c 1.5 1.7c
E 3.5 4.2 3.2 3.3c 2.8c 2.4c 2.4 2.2c 2.4 2.0c
F Note b Note b Note b Note b Note b Note b
a. Use straight-line interpolation for intermediate values of mapped spectral response acceleration at 1-second period, S1.
b. Values shall be determined in accordance with Section 11.4.8 of ASCE 7.
c. See requirements for site-specific ground motions in Section 11.4.8 of ASCE 7.

Now let’s see what the Fv 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 Fv table, as shown below. The Fv 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, Fv

Site Class Mapped Risk-Targeted Maximum Considered Earthquake (MCER)
Spectral Response Acceleration Parameter at 1-s Period
S1 ≤ 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 = 0.5 S1 ≥ 0.6
….. ….. ….. ….. ….. ….. …..
E 4.2 3.3a
See Section
11.4.8
2.8a
See Section
11.4.8
2.4a
See Section
11.4.8
2.2a
See Section
11.4.8
2.0a
See Section
11.4.8
….. ….. ….. ….. ….. ….. …..

This revision made the Fv 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 ≤ Ts 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 Fv, one cannot determine the transition period Ts, and thus cannot check if the exception applies.
  2. And, if the exception does apply, one needs a value for Fv 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 Fv 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 FV 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, Fv

Site Class Mapped Risk-Targeted Maximum Considered Earthquake (MCER)
Spectral Response Acceleration Parameter at 1-s Period
S1 ≤ 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 = 0.5 S1 ≥ 0.6
A 0.8 0.8 0.8 0.8 0.8 0.8
B 0.8 0.8 0.8 0.8 0.8 0.8
C 1.5 1.5 1.5 1.5 1.5 1.4
D 2.4 2.2a 2.0a 1.9a 1.8a 1.7a
E 4.2 3.3a
See Section
11.4.8
2.8a
See Section
11.4.8
2.4a
See Section
11.4.8
2.2a
See Section
11.4.8
2.0a
See Section
11.4.8
F See Section
11.4.8
See Section
11.4.8
See Section
11.4.8
See Section
11.4.8
See Section
11.4.8
See Section
11.4.8
Note: Use straight-line linear interpolation for intermediate values of S1.
*Also See requirements for site-specific ground motions in Section 11.4.8. These values of Fv shall be used only for calculation of TS.

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 Fv are to be used “only for calculation of TS”, 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 Fv-value for performing equivalent static analysis?

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

VALUES OF SITE COEFFICIENT, Faa

Site Class MAPPED RISK TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCER)
SPECTRAL RESPONSE ACCELERATION PARAMETER AT SHORT PERIOD
Ss ≤ 0.25 Ss = 0.5 Ss = 0.75 Ss = 1.0 Ss = 1.25 Ss ≥ 1.5
A 0.8 0.8 0.8 0.8 0.8 0.8
B 0.9 0.9 0.9 0.9 0.9 0.9
C 1.3 1.3 1.2 1.2 1.2 1.2
D 1.6 1.4 1.2 1.1 1.0 1.0
E 2.4 1.7 1.3 1.2c 1.2c 1.2c
F Note b Note b Note b Note b Note b Note b
a. Use linear interpolation for intermediate values of mapped spectral response acceleration at short period, Ss.
b. A site response analysis shall be performed in accordance with Section 21.1, unless exempted in accordance with ASCE 7-16 Section 20.3.1.
c. A ground motion hazard analysis shall be performed in accordance with ASCE 7-16 Section 21.2, unless exempted in accordance with Exceptions 1 to Section 11.4.8.

VALUES OF SITE COEFFICIENT, Fva

Site Class MAPPED RISK TARGETED MAXIMUM CONSIDERED EARTHQUAKE (MCER)
SPECTRAL RESPONSE ACCELERATION PARAMETER AT 1-SECOND PERIOD
S1 ≤ 0.1 S1 = 0.2 S1 = 0.3 S1 = 0.4 S1 = 0.5 S1 ≥ 0.6
A 0.8 0.8 0.8 0.8 0.8 0.8c
B 0.8 0.8 0.8 0.8 0.8 0.8c
C 1.5 1.5 1.5 1.5 1.5 1.4c
D 2.4 2.2d 2.0d 1.9d 1.8d 1.7c,d
E 4.2 3.3d 2.8d 2.4d 2.2d 2.0c,d
F Note b Note b Note b Note b Note b Note b
a. Use linear interpolation for intermediate values of mapped spectral response acceleration at 1-second period, S1.
b. A site response analysis shall be performed in accordance with Section 21.1, unless exempted in accordance with ASCE 7-16 Section 20.3.1.
c. For seismically isolated structures and structures with damping systems, a ground motion hazard analysis shall be performed in accordance with ASCE 7-16 Section 21.2.
d. A ground motion hazard analysis shall be performed in accordance with ASCE 7-16 Section 21.2, unless exempted in accordance with Exception 2 or 3 to Section 11.4.8.

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 Fv-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 Fv-table in the 2018 IBC.