Centrifuge modeling of cyclic lateral response of pile-cap systems and seat - type abutments in dry sand

This report presents the results of slow, cyclic, lateral-loading centrifuge tests perfomed on models of pile-cap foundation systems and seat-type bridge abutments in dry Nevada sand of 75

relative density to study the lateral response of these systems. These results are compared with interpretations using simple models, results of nonlinear finite element analyses, established design procedures, as well as other evidence including measurements during earthquakes.(AU)

Site factors and site categories in seismic codes

The report discusses some of the evidence on amplification of earthquake motions due to local soils which culminated in the new definitions of site categories and site coefficients, Fa and Fv, incorporated, first in the 1994 NEHRP Recommended Provissions for Seismic Regulations for New Buildings, and more recently in the 1997 NEHRP and compared to previous code. These site categories and site coefficients are described and compared to previous code provisions. Preliminary results of recent studies are discussed including averages and ranges of site coefficients calculated from recordings of the 1994 Northridge earthquake, which generally validate the 1994 NEHRP values. The possibility of performing similar calculations of site coefficients from available recordings pf the 1995 Kobe, Japan earthquake is also discussed. Use of the low period site coefficient, Fa, in conjunction with de-aggreagated measures of the seismic codes. Areas needing further research are suggested. Finally, it is recommended that the seismic design provisions contained in the 1996 AASHTO Standard Specifications for Highway Bridges be updated to be consistent with the 1994 and 1997 NEHRP and 1997 UBC provisions for site categories and site coefficients

Site factors and site categories in seismic codes : A perspective

The paper discusses the evidence on amplification of earthquake motions due to local soil which culminated in the new definitions of site categories and site coefficients incorporated, first in the 1994 NEHRP Recommended Provisions for Seismic Regulations fo new Buildings, and more recently in the 1997 Uniform Building Code. These site categories and site coefficients are described and compared to previous code provisions. Preliminary results of recent studies are discussed including average site coefficients calculated from recordings of the 1994 Northridge earthquake, which generally validate the 1994 NEHRP values. Areas of further research are suggested. Finally, it is recommended that the seismic design provisions contained in the 1996 AASHTO Standard Specifications for Highway Bridges be updated accordingly. (AU)

Earthquake site response and seismic code provisions

In current versions of guidelines or buildings codes for seismic design, site effects are accounted for through four soil factors which modify the shape of normalized acceleration response spectra recommended for use in design. This basic format was developed as part of an Applied Technology Council study in the early 1970's, and is the basis for the current versions of most seismic design codes. Over the past 20 years, numerous concerns have been expressed about both the limitations and code language dealing with site effects, including the lack of a precise definition of site categories.(AU)

Seismic hazard assessment of the Queensboro bridge

In this demonstration project, a team of NCEER researches and peer reviewers has provided the technical support and expertise necessary to permit a comprehensive evaluation to be made of the seismic vulnerability of the Queensboro Bridge. The results of this study are now being used by Steinman Engineers in the vulnerability assessment of the bridge. Preliminary results from this structural phase indicate that several critical elements in the bridge may require strengthening if it is to meet the seismic perfomance criteria previously defined. These elements include the connections between the superstructure and the masonry piers. The decision to retrofit the bridge has not yet been made but the process will probably extend beyond the seismological and engineering issues described above. Such a decision will likely include the socioeconomic considerations of retrofitting as compared to the consequences of closure and/or the repair/replacement of this critical lifeline for New York City, should an earthquake occur at a future date.(AU)

Construction of stress-strain histories from recorded dynamic response

Instrumental centrifuge laminated box experiments generate acceleration and LVDT displacement histories along the model height. These histories are employed to directly construct the associated dynamic stress-strain histories. Soil shear response at different elevations is explored. Soil strenght during liquefaction and lateral spreading is revealed at different elevations along the model height.(AU)

Centrifuge study of shallow foundation on saturated sand during earthquakes

A series of centrifuge model tests with earthquake shaking simulations were perfomed on the RPI 100-g-ton geotechnical centrifuge to clarify various aspects of the behavior of shallow foundations on liquefiable soil. The tests were conducted with saturated sand of various densities and permeabilities, with and without a fine-grained soil layer on top, and with and without a surface foundation model at the ground surface. Parameters measured included accelerations of soil and structure, excess pore pressures, and settlements of soil and structure. The results indicate a complex interaction between the different aspects of the problem, including formation of a soil water interlayer under the fine-grained soil, and redistribution of total foundation stresses within the sand mass after shaking.(AU)