Question 14.9: The results of subsurface exploration and laboratory testing...
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 MCE _{ G } peak ground acceleration with a 2 percent probability of exceedance within a 50-year period is 0.55g and the largest 84^{\text {th }} 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 \left( MCE _{ G }\right) 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 MCE _{ G } 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.
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 \left( MCE _{ G }\right) PGA must be determined. ASCE Standard 7-10 (2010), that is titled “Maximum Considered Earthquake Geometric Mean \left( MCE _{ G }\right) Peak Ground Acceleration,” states:
“21.5.1 Probabilistic MCE _{ G } 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 MCE _{ G } Peak Ground Acceleration: The deterministic geometric mean peak ground acceleration shall be calculated as the largest 84^{\text {th }} 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 F _{ PGA }, where F _{ PGA } is determined using Table 11.8-1 [Table 14.12] with the value of PGA taken as 0.5g.”
The largest 84^{\text {th }} 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 F _{\text {PGA }} = 1.0. Then 0.5 F _{\text {PGA }} = 0.5(1.0) = 0.50g. Hence, the value of the deterministic MCE _{ G } peak ground acceleration = 0.50g, which is higher than the value determined by the engineering geologist.
“21.5.3 Site-Specific MCE _{ G } Peak Ground Acceleration: The site-specific MCE _{ G } peak ground acceleration, PGA _{ M }, 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 MCE _{ G } peak ground acceleration shall not be taken as less than 80 percent of PGA _{ M } determined from Eq. 11.8-1.”
q_{ ult }=\frac{Q_{p}}{B^{2}}=\sigma_{v}^{\prime} N_{q} (11.8)
As indicted above, the probabilistic MCE _{ G } peak ground acceleration = 0.55g and the deterministic MCE _{ G } peak ground acceleration = 0.50g, with the lesser value of 0.50g governing. The last step is to determine the 80 percent value of PGA _{ M } 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 \left( MCE _{ G }\right) PGA, %g, Site Class B for the Conterminous United States,” or:
PGA _{ M }=\left( F _{ PGA }\right)( PGA )=(1.0)(0.45 g)=0.45 gTaking 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 | |||
| Row 1 | Row 2 | Row 3 | Row 4 | |||
| 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 F_{P G A} | |||||
| Site class | Mapped maximum considered geometric mean \left(M C E_{G}\right) peak ground acceleration | ||||
| 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). |
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