4.5 NEHRP (2001)

1) Reference

Building Seismic Safety Council, 2001, NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, 2000 edition, Part 1: Provisions (FEMA 368).

Building Seismic Safety Council, 2001, NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, 2000 edition, Part 2: Commentary (FEMA 369).

 

2) Amplification factor

NEHRP (National Earthquake Hazards Reduction Program) classified the ground to six site classes from A to F. The amplification factor of acceleration response spectrum for each site classes are provided as Table 5.5.1 and Table 5.5.2. The site class B is the rock and the amplification of other site classes were defined comparing to the site class B. The Ss and S1 in Table 5.5.1 and Table 5.5.2 means the spectral response acceleration value in (g) at 0.2 sec and 1.0 sec of site class B respectively.

 

Table 4.5.1  Amplification factor for acceleration response spectra at 0.2 sec

Table 4.5.2  Amplification factor for acceleration response spectra at 1.0 sec

 

Site class was defined as follows;

 

Site Class A: Hard rock with measured shear wave velocity, VS > 5,000 ft/sec (1500 m/s)

Site Class B: Rock with 2,500 ft/sec < VS £ 5,000 ft/sec (760 m/s < VS £ 1500 m/s)

Site Class C: Very dense soil and soft rock with 1,200 ft/sec < VS £ 2,500 ft/sec (360 m/s < VS £ 760 m/s) or with either N > 50 or Su > 2,000 psf (100 kPa)

Site Class D: Stiff soil with 600 ft/sec < VS £ 1,200 ft/sec (180 m/s < VS £ 360 m/s) or with either 15 £ N £ 50 or 1,000 psf £  Su £  2,000 psf (50 kPa £  Su £  100 kPa)

Site Class E: A soil profile with VS < 600 ft/sec (180 m/s) or with either N < 15 and Su £  1,000 psf or any profile with more than 10 ft (3 m) of soft clay defined as soil with PI > 20, w ³ 40 percent, and Su < 500 psf (25 kPa)

Site Class F: Soils requiring site-specific evaluations:

1. Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive clays, and collapsible weakly cemented soils.

Exception: For structures having fundamental periods of vibration equal to or less than 0.5 second, site-specific evaluations are not required to determine spectral accelerations for liquefiable soils. Rather, the Site Class may be determined in accordance with Sec. 4.1.2.2 and the corresponding values of Fa and Fv determined from Tables 4.1.2.4a and 4.1.2.4b.

2. Peats and/or highly organic clays (H > 10 ft [3 m] of peat and/or highly organic clay where H = thickness of soil)

3. Very high plasticity clays (H > 25 ft [8 m] with PI > 75)

4. Very thick soft/medium stiff clays (H > 120 ft [36 m])

VS: Average shear wave velocity in top 100 ft (30 m)

N: Average field standard penetration test for the top 100 ft (30 m)

Su: Average undrained shear strength in top 100 ft (30.5 m)

PI: Plasticity index

w: Moisture content (in percent)

H: Thickness of soil