INFLUENCE OF SOIL SOFTENING AND LIQUEFACTION ON RESPONSE SPECTRA FOR BRIDGE DESIGN

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INFLUENCE OF SOIL SOFTENING AND LIQUEFACTION ON RESPONSE SPECTRA FOR BRIDGE DESIGN

Author: Youd, T L Carter, B | Size: 18.50 MB | Format: PDF | Quality: Original preprint | Publisher: Brigham Young University | Year: 2003 | pages: 159

The purpose of this investigation is to assess the adequacy of seismic bridge design criteria and to suggest modifications to account for the influence of soil softening and liquefaction. To define the influence of soil softening on response spectra and assess the adequacy of LRFD seismic criteria, records are analyzed from five sites underlain by soils that liquefied. Findings are: (1) Where pore water pressures rose early during ground shaking, soil softening reduced short period (<0.7 sec) spectral accelerations. (2) Where soil softening did not occur early, softening has little influence on short period (<0.7 sec) ground motions. (3) Soil softening usually causes enhanced long-period (>0.7 to 1.0 sec) spectral values due to the onset of ground oscillation that persisted after strong ground shaking ceases. (4) For short fundamental periods (< 0.7 sec), LRFD acceleration coefficients, A, of either 0.60 g or 0.30 g and Code Soil Profile Types (CSPT) III or IV, elastic seismic design coefficients, Csm, conservatively envelope calculated spectra, indicating that criteria in the LRFD code are adequate for liquefiable sites. (5) For structures with fundamental periods >0.7 sec, an A of 0.60 g and a CSPT IV generates Csm, that conservatively envelope the calculated actual response spectra. (6) For design at liquefiable sites, increased ground deformation within the liquefied zone must be considered.

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