In this article, site response analyses for 124 sites collected in Korea were performed to evaluate earthquake ground motions in regions of shallow bedrock in Korea. Based on the results, a new two-parameter site classification system was developed for use in regions of shallow bedrock. The system incorporates depth to bedrock and mean VS of soil above bedrock as parameters for site classification instead of VS30. Soil sites were divided into seven site classes and the corresponding site coefficients were proposed for each site category. Some verification work demonstrates the superiority of the proposed system over the current seismic codes.

Development and Verification of a New Site Classification System and Site Coefficients for Regions of Shallow Bedrock in Korea

The Standard Penetration Test (SPT) is increasingly used to generate shear wave velocity (VS) profile of a site for seismic hazard assessment. However, it is difficult to perform site investigation using the SPT for weathered rock and bedrock even at shallow depths in most cases. Consequently, the stiffness of bedrock (VSb) is assumed to be 760 m/s, and only partial thickness of weathered rock is used for site response assessment in Korea. The increase in VSb and thickness of weathered rock (Hw) may affect the overall seismic response of a site, even though the site response is mostly governed by the upper soil layers. In this study, the effect of VSb and Hw on response spectrum in Korea was investigated by performing equivalent-linear site response analyses on three representative sites each of SC and SD soil categories as defined in the Korean seismic code based on the average shear wave velocity of top 30 m of a site (VS30). The SC and SD categories are defined similar to that in the International Building Code (IBC). Further, the effect of increase in VSb and Hw on the selection of design response spectrum based on seismic codes was also investigated. An increase in VSb resulted in an increase in spectral amplification, while the effect of increase in Hw was not substantial for the representative sites considered in this study. This means that the VS of bedrock needs to be assessed properly. On the other hand, it is possible to get a reliable seismic response even if the Hw cannot be determined accurately. The incorporation of VSb in VS30 for shallow bedrock sites (bedrock depth < 30 m) fails to consider the effect of increase in seismic amplification at large VSb. Hence, it is preferable to consider the effect of VSb separately on the design response spectrum. In addition, the effect on soil non-linearity due to variation in the VSb and Hw was found to be insignificant.

/pdf/effect-of-bedrock-stiffness-and-thickness-of-weathered-rock-35riqut20n.pdf

Effect of bedrock stiffness and thickness of weathered rock on response spectrum in Korea

Recent studies have shown that the site coefficients obtained from site response analyses of Korean soil sites are significantly different from those specified in the current Korean seismic code, especially in the short-period portion of site response. This difference is mainly attributed to the shallow bedrock conditions (bedrock depth usually less than 30 m) in Korea. This study proposes a new site classification system and site coefficients for shallow bedrock sites based on site response analyses of more than 300 sites. The proposed site classification scheme and site coefficients proposed in this study are an improvement over the current seismic code and previous studies.

Site Classification System and Site Coefficients for Shallow Bedrock Sites in Korea

Interaction between a well foundation (caisson) and the surrounding soil during earthquake shaking involves complicated material and interface nonlinearities such as soil inelasticity, separation, 

Significance of Interface Nonlinearity on the Seismic Response of a Well-Pier System in Cohesionless Soil:

Development of Site Classification System and Seismic Site Coefficients for Korea

New Guidelines will soon replace the 1998 New York City Department of Transportation (NYCDOT) Seismic Design Guidelines for Bridges in the New York City (NYC) region. A hazard study determined new region specific motions at very hard rock sites (Uniform Hazard Spectra, UHS, and associated acceleration time histories). A study of site effects for conditions representative of NYC followed in order to recommend: (1) Generic rock and soil design spectra and (2) Guidelines for site-specific studies. The site effects study scope included: (i) Collecting subsurface information and developing generalized soil profiles; (ii) Performing site response analyses to obtain UHS (horizontal and vertical) at soil sites; and (iii) Evaluating the effects of softer rock on top of the Very Hard Rock. A fully probabilistic approach was used to develop horizontal and vertical UHS at soil sites. It accounted, in a rigorous probabilistic manner, for variations and uncertainties in: (1) Soil stiffness, stress-strain nonlinearity (G/Gmax), and material damping; (2) Depth of soil to rock; (3) Rock stiffness under the soil. Current building and bridge codes specify spectra as a function of only the Soil Class. In the new Guidelines, the soil generic spectra are a function of three parameters: (i) Soil Class; (ii) Depth to Rock; and (iii) Rock Class Under the Soil. Soil Class is defined by averaging only soil vs without including rock, down to a maximum depth of 100 ft if the rock is deeper than 100 ft. Rock Class is classified as a function of the average Vs of the top 20 ft of the rock. Vertical design spectra and design V/H ratios needed to obtain site-specific vertical design motions, are also provided at soil sites as function of the same three parameters. Issues included in the Guidelines but not discussed in this paper are liquefaction and site-specific studies.

2008 New York City DOT Seismic Design Guidelines for Bridges Considering Local Site Conditions

New York City's Department of Transportation is in the process of evaluating and, if necessary, rehabilitating its many important bridges to meet seismic guidelines. As part of a comprehensive 2-part evaluation of the Brooklyn Bridge that used the latest modeling techniques, this article describes the basis for engineers' determination that the bridge's foundations have the ability to withstand a 2,500-year event without any sliding or separation at their bases, obviating the need for retrofits that might alter the architectural form of this unique crossing.

Appraising the Brooklyn Bridge

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2008 New York City DOT Seismic Design Guidelines for Bridges Considering Local Site Conditions

Appraising the Brooklyn Bridge