New Site Coefficients and Site Classification System Used in Recent Building Seismic Code Provisions

2000 ◽  
Vol 16 (1) ◽  
pp. 41-67 ◽  
Author(s):  
R. Dobry ◽  
R. D. Borcherdt ◽  
C. B. Crouse ◽  
I. M. Idriss ◽  
W. B. Joyner ◽  
...  
Keyword(s):  
Seismic Hazard ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Classification System ◽  
Site Amplification ◽  
Site Classification ◽  
Site Class ◽  
Site Classes ◽  
Code Provisions

Recent code provisions for buildings and other structures (1994 and 1997 NEHRP Provisions, 1997 UBC) have adopted new site amplification factors and a new procedure for site classification. Two amplitude-dependent site amplification factors are specified: Fa for short periods and Fv for longer periods. Previous codes included only a long period factor S and did not provide for a short period amplification factor. The new site classification system is based on definitions of five site classes in terms of a representative average shear wave velocity to a depth of 30 m (V¯s). This definition permits sites to be classified unambiguously. When the shear wave velocity is not available, other soil properties such as standard penetration resistance or undrained shear strength can be used. The new site classes denoted by letters A - E, replace site classes in previous codes denoted by S1 - S4. Site classes A and B correspond to hard rock and rock, Site Class C corresponds to soft rock and very stiff / very dense soil, and Site Classes D and E correspond to stiff soil and soft soil. A sixth site class, F, is defined for soils requiring site-specific evaluations. Both Fa and Fv are functions of the site class, and also of the level of seismic hazard on rock, defined by parameters such as Aa and Av ( 1994 NEHRP Provisions), Ss and Sl ( 1997 NEHRP Provisions) or Z ( 1997 UBC). The values of Fa and Fv decrease as the seismic hazard on rock increases due to soil nonlinearity. The greatest impact of the new factors Fa and Fv as compared with the old S factors occurs in areas of low-to-medium seismic hazard. This paper summarizes the new site provisions, explains the basis for them, and discusses ongoing studies of site amplification in recent earthquakes that may influence future code developments.

An Empirical Geotechnical Seismic Site Response Procedure

2001 ◽  
Vol 17 (1) ◽  
pp. 65-87 ◽  
Author(s):  
Adrián Rodríguez-Marek ◽  
Jonathan D. Bray ◽  
Norman A. Abrahamson
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Classification System ◽  
Soil Depth ◽  
Site Response ◽  
Dynamic Stiffness ◽  
Evaluation Procedure ◽  
Site Classification ◽  
Seismic Site Response

A simplified empirically based seismic site response evaluation procedure that includes measures of the dynamic stiffness of the surficial materials and the depth to bedrock as primary parameters is introduced. This geotechnical site classification scheme provides an alternative to geologic-based and shear wave velocity-based site classification schemes. The proposed scheme is used to analyze the ground motion data from the 1989 Loma Prieta and 1994 Northridge earthquakes. Period-dependent and intensity-dependent spectral acceleration amplification factors for different site conditions are presented. The proposed scheme results in a significant reduction in standard error when compared with a simpler “rock vs. soil” classification system. Moreover, results show that sites previously grouped as “rock” should be subdivided as competent rock sites and weathered soft rock/shallow stiff soil sites to reduce uncertainty in defining site-dependent ground motions. Results also show that soil depth is an important parameter in estimating seismic site response. The standard errors resulting from the proposed site classification system are comparable with those obtained using the more elaborate code-based average shear-wave velocity classification system.

scholarly journals Investigation of Soil Characterization in Hatay Province in Turkey by Using Seismic Refraction, Multichannel Analysis of Surface Waves and Microtremor

2021 ◽  
Vol 24 (4) ◽  
pp. 473-484
Author(s):  
Cengiz Kurtuluş ◽  
Ibrahim Sertcelik ◽  
Fadime Sertçelik ◽  
Hamdullah Livaoğlu ◽  
Cüneyt Şaş
Keyword(s):  
Surface Waves ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Seismic Refraction ◽  
Strong Motion ◽  
Site Classification ◽  
Eurocode 8 ◽  
Site Class ◽  
Multichannel Analysis

In this study, shallow seismic surveys, including seismic refraction, Multichannel Analysis of Surface Waves (MASW), Refraction Microtremor (ReMi), and Microtremor measurements were conducted to estimate site characterization at 26 strong-motion stations of AFAD (Disaster and Emergency Management Presidency) in the province of Hatay, situated in one of the most seismically active regions in southern Turkey. The Horizontal to vertical spectral ratio (HVSR) technique was applied, using smoothed Fourier spectra derived from a long duration series to determine dominant frequency values at different amplification levels. Shear wave velocity up to 30 m of the ground was detected with MASW analysis. In the ReMi analysis, up to 80 m was reached with a corresponding average of 650 m/s shear wave velocity. The shear wave velocities estimated by the MASW method up to 30 m were compared with those found by the ReMi method, and they were observed to be very compatible. The province of Hatay was classified according to Vs30 based NEHRP Provisions, Eurocode-8, the Turkish Building Earthquake Regulation (TBDY-2018), and Rodriguez-Marek et al. (2001). The shear-wave velocity (Vs30), Horizontal to Vertical ratio’s (H/V) peak amplitude, dominant period, and site class of each site were determined. The H/V peak amplitudes range between 1.9 and 7.6, while the predominant periods vary from 0.23 sec to 2.94sec in the study area. These results are investigated to explain the consistency of site classification schemes.

Determination of site period for NZS1170.5:2004

2014 ◽  
Vol 47 (1) ◽  
pp. 28-40 ◽  
Author(s):  
Tam Larkin ◽  
Chris Van Houtte
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Closed Form Solution ◽  
Form Solution ◽  
Site Classification ◽  
Lumped Mass ◽  
Mass System ◽  
Site Classes ◽  
Site Period

The fundamental site period, T, is a key parameter for site classification in NZS 1170.5:2004. Many sites in New Zealand will fall into site classes C and D, where the boundary between the site classes is T = 0.6 seconds. NZS 1170.5 offers several methods of determining site classification. The intent of this paper is to expand on NZS 1170.5 and guide practising engineers towards more accurate and efficient methods for determining site period. We review methods to calculate the shear-wave velocity, then give specific examples for calculating the site period for five types of soil profile (uniform layer, shear-wave velocity increasing as a power of depth, shear modulus increasing linearly with depth, two-layer profile and three-layer profile). We find that NZS 1170.5 clause 3.1.3.7 for calculating site period at layered sites is unconservative and inconsistent with two other well-accepted methods for calculating site period. We consider the most accurate and efficient method of calculating site period for layered sites is to represent the profile as a lumped mass system, then calculate the fundamental frequency from the eigenvalues of the system. The successive application of the two-layer closed form solution is also considered an acceptable method.

Shear Wave Velocity- and Geology-Based Seismic Microzonation of Port-au-Prince, Haiti

2011 ◽  
Vol 27 (1_suppl1) ◽  
pp. 67-92 ◽  
Author(s):  
Brady R. Cox ◽  
Jeff Bachhuber ◽  
Ellen Rathje ◽  
Clinton M. Wood ◽  
Ranon Dulberg ◽  
...  
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Site Classification ◽  
Earthquake Hazards ◽  
Site Class ◽  
Geologic Map ◽  
Masw Method ◽  
Elevation Model ◽  
The City

A seismic site classification microzonation for the city of Port-au-Prince is presented herein. The microzonation is based on 35 shear wave velocity ( VS) profiles collected throughout the city and a new geologic map of the region. The VS profiles were obtained using the multichannel analysis of surface waves (MASW) method, while the geologic map was developed from a combination of field mapping and geomorphic interpretation of a digital elevation model (DEM). Relationships between mean shear wave velocity over the upper 30 m of the subsurface ( VS30) and surficial geologic unit have been developed, permitting code-based seismic site classification throughout the city. A site classification map for the National Earthquake Hazards Reduction Program/International Building Code (NEHRP/IBC) classification scheme is provided herein. Much of the city is founded on deposits that classify as either NEHRP Site Class C or D, based on VS30. Areas of the city requiring additional subsurface information for accurate site classification are noted.

Correlation Study of Shear Wave Velocity in near Surface Geological Formations in Anchorage, Alaska

1997 ◽  
Vol 13 (1) ◽  
pp. 55-75 ◽  
Author(s):  
S. K. Nath ◽  
D. Chatterjee ◽  
N. N. Biswas ◽  
M. Dravinski ◽  
D. A. Cole ◽  
...  
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Correlation Study ◽  
Surface Measurement ◽  
Depth Range ◽  
Site Class ◽  
Near Surface ◽  
Surface Method ◽  
Site Classes

In the summer of 1995, surface measurements of shear wave velocity (β) was conducted at thirty six sites, approximately, in the 0-50 m depth range. Of these, at seven sites values of β, soil log and blow count (N) from borehole measurement were available from previous investigations by others. Using these seven sites for calibration, we compared the velocity profiles yielded by the surface and borehole measurements for these sites. The results show broad similarities. Using the soil logs and shear wave velocity variations at the seven sites, four site classes (SC-Ic, SC-II, SC-III and SC-IV) could be identified. The surface method corresponding to the mean value of β tends to underestimate β between about 1 and 18 percent for site classes SC-Ic, SC-II and SC-III compared to the downhole method. For SC-IV, β is overestimated by 11 percent using surface method. Moreover, the blow count (N) data for each site class shows a linear relationship with β obtained by the surface measurement.

scholarly journals Spectral Estimation of Site Characteristics of an Earth Quake Prone Region

2021 ◽  
Author(s):  
T. Seshunarayana ◽  
N. Sundararajan
Keyword(s):  
Seismic Hazard ◽  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Earth Quake ◽  
Ambient Noise ◽  
Site Amplification ◽  
Time Data ◽  
Ground Shaking ◽  
Site Characteristics

Abstract The site amplification characteristic is an important part of evaluation of seismic hazard since much of the damage caused by earthquakes may be attributed directly to the ground shaking. Identifying ahead of time such areas which are prone to amplified ground shaking due to earthquake could greatly aid seismic hazard evaluation as well as improved hazard mitigation effects. It is also a well known fact that in most cases site amplification/ shaking is stronger in low shear wave velocity areas. The objective of the present study is to estimate site characteristics of as many as 116 sites in an area of approximately 35 sq.km comprising various geological units including soft alluvial deposits which not only tend to amplify certain frequencies of ground motion but also extend the duration of earth quake that may cause further damage. The basic idea of this study is to decipher the natural ground response during quite period as well as triggered response by spectral analysis. The methodology adopted for the study is modified micro tremor which is based on ambient noise as well as triggered response. Further, the spectral ratio of H/V of ambient noise and triggered data by Nakamura method with short duration data were also estimated. In addition, based on some empirical relations, the site response frequency and amplification were also computed using the shear wave velocity obtained up to a depth of 30 m (VS30) by the method of multichannel analysis of surface wave (MASW) and depth to bed rock estimated through refraction seismic studies. The results of the study has shown that a simple comparison of spectra of quiet time data and triggered time data revealed that in alluvial soils with thick overburden, the signal amplification is more at low frequency (< 10 Hz), whereas in thin overburden the amplification was found to be low at low frequencies. The salient features of the study with merits are presented herein.

Estimates of Site-Dependent Response Spectra for Design (Methodology and Justification)

1994 ◽  
Vol 10 (4) ◽  
pp. 617-653 ◽  
Author(s):  
Roger D. Borcherdt
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Strong Motion ◽  
Response Spectra ◽  
Building Code ◽  
Ground Shaking ◽  
Geological Conditions ◽  
Site Classes ◽  
Code Provisions

Recent borehole-geotechnical data and strong-motion measurements constitute a new empirical basis to account for local geological conditions in earthquake-resistant design and site-dependent, building-code provisions. They provide new unambiguous definitions of site classes and rigorous empirical estimates of site-dependent amplification factors in terms of mean shear-wave velocity. A simple four-step methodology for estimating site-dependent response spectra is specified herein. Alternative techniques and commentary are presented for each step to facilitate application of the methodology for different purposes. Justification for the methodology is provided in terms of definitions for the new site classes and derivations of simple empirical equations for amplification as a function of mean shear-wave velocity and input ground-motion level. These new results provide a rigorous framework for improving estimates of site-dependent response spectra for design, site-dependent building-code provisions, and predictive maps of strong ground shaking for purposes of earthquake hazard mitigation.

Seismic Site Classifications and Site Amplifications for the Urban Centres in the Shallow Overburden Deposits

2012 ◽  
Vol 3 (1) ◽  
pp. 86-108 ◽  
Author(s):  
P. Anbazhagan ◽  
M. Neaz Sheikh
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Classification System ◽  
Site Response ◽  
The United States ◽  
Site Classification ◽  
Soil Thickness ◽  
Soil Layers ◽  
China And India

This paper presents seismic site classification practices for urban centres in Australia, China, and India with special emphasis on their suitability for shallow soil sites. The geotechnical aspects of seismic site classifications play a critical role in the development of site response spectra, which is the basis for the seismic design of new structures and seismic assessment of existing structures. Seismic site classifications have used weighted average shear wave velocity of top 30 m soil layers, following the recommendations of National Earthquake Hazards Reduction Program (NEHRP) or International Building Code (IBC) site classification system. The site classification system is based on the studies carried out in the United States where soil layer may extend up to several hundred meters before reaching any distinct soil-bedrock interface. Most of the urban centers in Australia, China, and India are located on distinct bedrocks within few meter depth of soil deposits. For such shallow depth soil sites, NEHRP or IBC site classification system is not suitable. A new site classification based on average soil thickness, shear wave velocity up to engineering bedrock is proposed. The study shows that spectral value and amplification ratio estimated from site response study considering top 30 m soil layers are different from those determined considering soil thickness up to engineering bedrock.

scholarly journals Multi-method site characterization to verify the hard rock (Site Class A) assumption at 25 seismograph stations across Eastern Canada

2021 ◽  
pp. 875529302110010
Author(s):  
Sameer Ladak ◽  
Sheri Molnar ◽  
Samantha Palmer
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Hard Rock ◽  
Site Characterization ◽  
Eastern Canada ◽  
Site Class ◽  
Paleozoic Rock ◽  
Rock Types ◽  
Site Period

Site characterization is a crucial component in assessing seismic hazard, typically involving in situ shear-wave velocity ( VS) depth profiling, and measurement of site amplification including site period. Noninvasive methods are ideal for soil sites and become challenging in terms of field logistics and interpretation in more complex geologic settings including rock sites. Multiple noninvasive active- and passive-seismic techniques are applied at 25 seismograph stations across Eastern Canada. It is typically assumed that these stations are installed on hard rock. We investigate which site characterization methods are suitable at rock sites as well as confirm the hard rock assumption by providing VS profiles. Active-source compression-wave refraction and surface wave array techniques consistently provide velocity measurements at rock sites; passive-source array testing is less consistent but it is our most suitable method in constraining the rock VS. Bayesian inversion of Rayleigh wave dispersion curves provides quantitative uncertainty in the rock VS. We succeed in estimating rock VS at 16 stations, with constrained rock VS estimates at 7 stations that are consistent with previous estimates for Precambrian and Paleozoic rock types. The National Building Code of Canada uses solely the time-averaged shear-wave velocity of the upper 30 m ( VS30) to classify rock sites. We determine a mean VS30 of ∼ 1600 m/s for 16 Eastern Canada stations; the hard rock assumption is correct (>1500 m/s) but not as hard as often assumed (∼2000 m/s). Mean variability in VS30 is ∼400 m/s and can lead to softer rock classifications, in particular, for Paleozoic rock types with lower average rock VS near the hard/soft rock boundary. Microtremor and earthquake horizontal-to-vertical spectral ratios are obtained and provide site period classifications as an alternative to VS30.

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