IMPLICATIONS OF RECENT NONLINEAR ANALYSES ON SEISMIC STANDARDS OF LIQUID STORAGE TANKS

摘要

Possible areas of modifications to current seismic codes and standards of liquid storage tanks are highlighted based on recent advances in the nonlinear analysis of these critical structures. Use of state-of-the-art finite element codes and correlation of the results with observations from past earthquakes have pointed out to a few fundamental differences between the real behavior of tanks and that stipulated in current standards. On the demand side, nonlinear time-dependent uplift analyses of unanchored tanks showed inconsistencies with current standards in expected values of exerted hydrodynamic overturning moments and of developed axial shell stresses, leading to further inconsistencies as to the onset of global overturning of the tank. On the capacity side, allowable shell stresses adopted in seismic standards are largely based on buckling stress values which are inconsistent with the elephant-foot bulges noted in reality and predicted by state-of-the-art finite element programs. It was found that hoop tension near the tank base can play a significant role in producing this elastic-plastic buckling mode, yet such an effect is totally ignored in the two most commonly used standards for seismic design of tanks. It is clear that more realistic seismic provisions must be integrated in current design standards, both on the demand and capacity sides.