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Seismic site coefficients and acceleration design response spectra based on conditions in South Carolina : final report.
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2014-11-15
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[PDF-26.07 MB]
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Edition:Final report
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Abstract:The simplified procedure in design codes for determining earthquake response spectra involves
estimating site coefficients to adjust available rock accelerations to site accelerations. Several
investigators have noted concerns with the site coefficients recommended in current codes, herein
called the 1994 National Earthquake Hazard Reduction Program (NEHRP) site coefficients,
including (1) the suitability of the 1994 NEHRP coefficients for conditions different from Western
United States geology; (2) the appropriateness of using a single coefficient for a wide range of soil
stiffnesses; and (3) the suitability of using coefficients that are independent of depth to top of rock.
This report describes a systematic ground response study to determined site coefficients (F)
appropriate for South Carolina’s geologic and seismic conditions. The study involves assuming
conditions at seven locations in the Atlantic Coastal Plain and four locations in the South Carolina
Piedmont. Over 60,000 total stress, one-dimensional equivalent linear (SHAKE2000) and nonlinear
(DMOD2000) ground response simulations are conducted using numerous representative shear wave
velocity profiles and a suite of over 130 synthetic rock outcrop motions generated with the computer
program Scenario_PC assuming return periods of 475 and 2,475 years.
Results of the ground response analyses are compiled into over 400 plots of computed values of
F versus average shear wave velocity in the top 100 ft (VS100ft) grouped by site location, depth to top
of soft rock (HB-C) or hard rock (HHR), spectral period (T), and spectral acceleration of the rock input
motion (Soutcrop). In nearly all the plots, the following three distinct features can be seen—(1) an
increasing trend in F as VS100ft increases from zero; (2) a zone of peak values of F; and (3) a
decreasing trend in F as VS100ft increases to the velocity of the reference rock.
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