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

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 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, 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.