Stresses in Storage Tanks Caused by Differential Settlement

Abstract

Differential settlement around the circumference of the foundation below the shell of a vertical cylindrical storage tank is a particularly severe form of settlement, and can cause distortion and stressing of the tank. The resulting deflections and stresses in the shell and primary wind girder of a tank with a floating roof can be analysed, assuming the shell to behave as a membrane. The analysis is reviewed and the results are applied to present a new design chart for predicting the maximum shell membrane stress caused by settlement. For unanchored tanks, the stiffness of a tank may be such that the shell can ‘bridge’ across localized soft spots in the foundation. A simplified analysis of bridging is presented and a criterion for predicting the onset of bridging is proposed, supported by model test results. Expressions for the associated shell membrane stresses are derived. For unanchored floating-roof tanks, it is concluded that the occurrence of bridging will always limit the maximum shell membrane stress to an acceptable value. However, the bending stresses in the shell-to-base joint of a bridging tank require separate assessment in all cases. The settlement criteria contained in the current British and American codes of practice for in-service inspection of tanks are reviewed in the light of these findings.