The results of subsurface exploration and laboratory testing indicate a site class C. The structural engineer has reported that the preliminary category for the proposed structure is Seismic Design Category D. The engineering geologist has determined that the probabilistic MCEG peak ground

Question

The results of subsurface exploration and laboratory testing indicate a site class C. The structural engineer has reported that the preliminary category for the proposed structure is Seismic Design Category D. The engineering geologist has determined that the probabilistic [latex]MCE _{ G }[/latex] peak ground acceleration with a 2 percent probability of exceedance within a 50-year period is 0.55g and the largest [latex]84^{ ext {th }}[/latex] percentile geometric mean peak ground acceleration for characteristic earthquakes on all known active faults within the site region is 0.35g. Using the map titled “Maximum Considered Earthquake Geometric Mean [latex]\left( MCE _{ G } ight)[/latex] PGA, %g, Site Class B for the Conterminous United States” in ASCE Standard 7-10 (2010), the value is 0.45g. Determine the site-specific [latex]MCE _{ G }[/latex] peak ground acceleration that should be used in the geotechnical earthquake engineering analyses for liquefaction, slope movement, and so forth, if the governing building code is the 2012 edition of the International Building Code.

Accepted Answer

Since the structural engineer has determined that the proposed structure will be Saeismic Design Category D, from Section 1803.5.12 of the International Building Code (2012), the maximum considered earthquake geometric mean [latex]\left( MCE _{ G } ight)[/latex] PGA must be determined. ASCE Standard 7-10 (2010), that is titled “Maximum Considered Earthquake Geometric Mean [latex]\left( MCE _{ G } ight)[/latex] Peak Ground Acceleration,” states:

“21.5.1 Probabilistic [latex]MCE _{ G }[/latex] Peak Ground Acceleration: The probabilistic geometric mean peak ground acceleration shall be taken as the geometric mean peak ground acceleration with a 2 percent probability of exceedance within a 50-year period.”

This value has been determined by the engineering geologist and is 0.55g.

“21.5.2 Deterministic [latex]MCE _{ G }[/latex] Peak Ground Acceleration: The deterministic geometric mean peak ground acceleration shall be calculated as the largest [latex]84^{ ext {th }}[/latex] percentile geometric mean peak ground acceleration for characteristic earthquakes on all known active faults within the site region. The deterministic geometric mean peak ground acceleration shall not be taken as lower than 0.5 [latex]F _{ PGA },[/latex] where [latex]F _{ PGA }[/latex] is determined using Table 11.8-1 [Table 14.12] with the value of PGA taken as 0.5g.”

The largest [latex]84^{ ext {th }}[/latex] percentile geometric mean peak ground acceleration for characteristic earthquakes on all known active faults within the site region has also been determined by the engineering geologist and it is 0.35g. Entering Table 14.12 with site class C and at the PGA ≥ 0.5 column, the value of [latex]F _{ ext {PGA }}[/latex] = 1.0. Then 0.5 [latex]F _{ ext {PGA }}[/latex] = 0.5(1.0) = 0.50g. Hence, the value of the deterministic [latex]MCE _{ G }[/latex] peak ground acceleration = 0.50g, which is higher than the value determined by the engineering geologist.

“21.5.3 Site-Specific [latex]MCE _{ G }[/latex] Peak Ground Acceleration: The site-specific [latex]MCE _{ G }[/latex] peak ground acceleration, [latex]PGA _{ M },[/latex] shall be taken as the lesser of the probabilistic geometric mean peak ground acceleration of Section 21.5.1 and the deterministic geometric mean peak ground acceleration of Section 21.5.2. The site-specific [latex]MCE _{ G }[/latex] peak ground acceleration shall not be taken as less than 80 percent of [latex]PGA _{ M }[/latex] determined from Eq. 11.8-1.”

[latex]q_{ ult }=\frac{Q_{p}}{B^{2}}=\sigma_{v}^{\prime} N_{q}[/latex] (11.8)

As indicted above, the probabilistic [latex]MCE _{ G }[/latex] peak ground acceleration = 0.55g and the deterministic [latex]MCE _{ G }[/latex] peak ground acceleration = 0.50g, with the lesser value of 0.50g governing. The last step is to determine the 80 percent value of [latex]PGA _{ M }[/latex] determined from Eq. 11.8-1, which uses the PGA from the map in ASCE Standard 7-10 (2010) titled “Maximum Considered Earthquake Geometric Mean [latex]\left( MCE _{ G } ight)[/latex] PGA, %g, Site Class B for the Conterminous United States,” or:

[latex]PGA _{ M }=\left( F _{ PGA } ight)( PGA )=(1.0)(0.45 g)=0.45 g[/latex]

Taking 80% of this value = (0.8)(0.45g) = 0.36g. Thus for this example problem, the governing value to be used in the geotechnical earthquake engineering analyses is peak ground acceleration = 0.50g.

Table 11.8 Summary of Row Reduction Factors From Lateral Load Tests on Pile Groups
Spacing in pile diameters	Row reduction factors				Main soil type	Reference
Spacing in pile diameters	Row 1	Row 2	Row 3	Row 4	Main soil type	Reference
2.8	0.6	0.4	0.4	___	Clay	Rollins et al. 1998
3	0.9	0.5	___	___	Clay	Meimon et al. 1986
3	0.7	0.6	0.5	___	Clay	Brown et al. 1987
3	0.8	0.4	0.3	___	Sand	Brown et al. 1988
3	0.8	0.7	0.3	0.3	Sand	Ruesta &Townsend 1997
3.3	0.82	0.61	0.45	0.45	Clay	Rollins et al. 2006
4.4	0.90	0.80	0.69	0.73	Clay	Rollins et al. 2007
5.7	0.95	0.88	0.77	___	Clay	Rollins et al. 2008

TABLE 14.12 Site Coefficient [latex]F_{P G A}[/latex]
Site class	Mapped maximum considered geometric mean [latex]\left(M C E_{G} ight)[/latex] peak ground acceleration
Site class	PGA ≤ 0.1	PGA = 0.2	PGA = 0.3	PGA = 0.4	PGA ≥ 0.5
A	0.8	0.8	0.8	0.8	0.8
B	1.0	1.0	1.0	1.0	1.0
C	1.2	1.2	1.1	1.0	1.0
D	1.6	1.4	1.2	1.1	1.0
E	2.5	1.7	1.2	0.9	0.9
F	See Sec. 11.4.7 of ASCE Standard 7-10 (2010)
Note: Use straight-line interpolation for intermediate values of PGA. Source: Table 11.8-1 from ASCE Standard 10-7 (2010).

Question 14.9: The results of subsurface exploration and laboratory testing...

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