Seismic Hazard Suite

Site Class Effects

The Site Class Effects worksheet modifies the ground motions from legacy datasets for site class effects because they are only available for NEHRP site class boundary B/C.

Site Class

In step 1, use the drop-down list to select the NEHRP site class according to ASCE/SEI 7-16 [?] as Figure illustrates. The RMC Site Classification Toolbox, part of the RMC Seismic Hazard Suite, helps assess the site class. Table provides the available site classes for the general procedures for site class adjustment in ASCE/SEI 7-16 used to modify the ground motions for site class effects. Ensure the selected NEHRP site class is consistent with the value selected on Modern NSHM worksheet.

Figure: Step 1 of Site Class Effects worksheet: Site class input.

Table: Site classification (ASCE/SEI 7-16 [?]).

Site Class	$\overline{v_s}$ (ft/s)	$\overline{N}$ or $\overline{N_{ch}}$ (bpf)	$\overline{s_u}$ (ft/s)
A. Hard rock	$\overline{v_s}$ > 5,000	N/A	N/A
B. Rock	2,500 < $\overline{v_s}$ ≤ 5,000	N/A	N/A
C. Very dense soil and soft rock	1,200 < $\overline{v_s}$ ≤ 2,500	$\overline{N}$ or $\overline{N_{ch}}$ > 50	$\overline{s_u}$ > 2,000
D. Stiff soil	600 ≤ $\overline{v_s}$ ≤ 1,200	15 ≤ $\overline{N}$ or $\overline{N_{ch}}$ ≤ 50	1,000 ≤ $\overline{s_u}$ ≤ 2,000
E. Soft clay	$\overline{v_s}$ < 600	$\overline{N}$ or $\overline{N_{ch}}$ < 15	$\overline{s_u}$ < 1,000
	Any site profile with more than 10 feet of soil with: Plasticity index, PI > 20 Moisture content, w ≥ 40 percent Undrained shear strength, su < 500 psf

Adjusted Ground Motions

In step 2, the general procedures in ASCE/SEI 7-16 [?] are used to adjust the NEHRP site class boundary B/C hazard curves for the user-specified site class.

If a short-term NSHM is selected for the model on the Legacy NSHM worksheet, the return period to truncate the short-term hazard curves is input as Figure illustrates. Otherwise, conditional formatting applies a gray background to this input. The short-term (one-year) forecasts must not be applied for very low rates of exceedance because hazard estimates from induced earthquakes are not compatible with estimates of long-term seismic hazard caused by tectonic processes. Therefore, the mean seismic hazard curves are truncated at this user-specified return period.

Figure: Step 2 of Site Class Effects worksheet: Return period input for short-term NSHM.

The general procedures in ASCE/SEI 7-16 are used to adjust the NEHRP site class boundary B/C hazard curves for USGS legacy datasets. The ground motions for PGA, 0.2-second SA (S_S), and 1-second SA (S₁) are multiplied by the site coefficients in ASCE/SEI 7-16 [?] to adjust for the user-specified site class to obtain the modified PGA (PGA_M), 0.2-second SA (S_MS), and 1-second SA (S_M1) used on the Hazard Curves worksheet, as shown in Equation, Equation, and Equation.

$$PGA_{M} = F_{PGA}PGA$$

$$S_{MS} = F_{a}S_{S}$$

$$S_{M1} = F_{v}S_{1}$$

where:

F_PGA = PGA site coefficient
F_a = short-period site coefficient
F_v = long-period (1-second SA) site coefficient

The coefficients to adjust for site class effects in ASCE/SEI 7-16 [?] depend on the ground motion levels for soil sites because of non-linear response at higher ground motions. Therefore, site class coefficients are linearly interpolated for intermediate ground motion levels as indicated in ASCE/SEI 7-16. Table through Table provide the site coefficients from ASCE/SEI 7-16 [?].

Table: Site coefficient, F_PGA

Site Class	PGA (g)
	≤ 0.1	0.2	0.3	0.4	0.5	≥ 0.6
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	1.2	1.2	1.2	1.2	1.2
D	1.6	1.4	1.3	1.2	1.1	1.1
E	2.4	1.9	1.6	1.4	1.2	1.1

Table: Short-period site coefficient, F_a.

Site Class	$S_S$ (g)
	≤ 0.25	0.5	0.75	1.0	1.25	≥ 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	1.2*	1.2*	1.2*

1. ^*Values for Site Class C were used for Site Class E with S_S ≥ 1.0g, as indicated by the exception in Section 11.4.8 of ASCE/SEI 7-16 [?].

Table: Long-period site coefficient, F_v.

Site Class	$S_1$ (g)
	≤ 0.1	0.2	0.3	0.4	0.5	≥ 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	2.2	2.0	1.9	1.8	1.7
E	4.2	4.2*	4.2*	4.2*	4.2*	4.2*

1. ^*Values for Site Class E with S₁ ≥ 1.0g are not provided in ASCE/SEI 7-16 [?]. Use with great caution.

ASCE/SEI 7-16 [?] does not provide F_v values for Site Class E with S₁ greater than 0.1g. The toolbox uses the F_v value of 4.2 for S₁ less than or equal to 0.1g for S₁ greater than 0.1g to perform the calculations. Conditional formatting applies an orange background to cells for F_v and S_M1 for S₁ greater than 0.1g as a warning.

Figure is an example of the ground motions modified for site class effects for legacy short-term datasets.

Figure: Step 2 of Site Class Effects worksheet: Modified ground motions.

Plots of GM as a function of AEP for the user-specified site class are at the bottom of the worksheet as illustrated in Figure.

Figure: Step 2 of Site Class Effects worksheet: Hazard curve plot.