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Publication# Experimental Determination of the Capacity of Wall-to-Base Connections in Storage Tanks

Résumé

Earthquakes around the world have affected many unanchored tanks used for storage of gasoline. When these unanchored tanks are subjected to strong motion, the moment generated at the base, by the mass of liquid contained in the tank, will cause partial uplift of the base (i.e. rocking of the tank), inducing cycles of rotation at the wall-to-base connection. Actual codes of standard practice used in Europe and New Zealand limit the rotation that may be allowed at this connection to 0.2 radians. This limit was based on a series of assumptions since no realistic tests had been conducted. This paper presents the experimental research conducted to determine the real rotational capacity of these connections. The tests were carried out considering the bending stresses and the membrane stresses that develop at the base plate when uplift occurs. Specimens from an old existing tank (dismantled) were used. New specimens were also fabricated and used to allow for a more comprehensive study. Results from this research are presented in the form of plots of maximum rotation vs. number of cycles to failure, called RN curves. These curves revealed that the current Eurocode limit of 0.2 radians is overly conservative. Based on the limited data, a limit of 0.4 radians would be more realistic.

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Méthode expérimentale

Les méthodes expérimentales scientifiques consistent à tester la validité d'une hypothèse, en reproduisant un phénomène (souvent en laboratoire) et en faisant varier un paramètre. Le paramètre que l

Liquide

vignette|L'eau est une substance abondante sur la surface terrestre, se manifestant notamment sous forme de liquide.
vignette|Diagramme montrant comment sont configurés les molécules et les atomes pou

Storage tank

Storage tanks are containers that hold liquids, compressed gases (gas tank; or in U.S.A "pressure vessel", which is not typically labeled or regulated as a storage tank) or mediums used for the shor

Chargement

Gustavo Cortes, Alain Nussbaumer

When an unanchored tank is subjected to strong motion, the impulsive mass of the contained liquid generates a moment which if strong enough could cause partial uplifting of the base plate. As the tank undergoes uplifting, the shell-base welded connection is subjected to cycles of rotation which could lead to its failure. To address this issue, the Eurocode and the New Zealand’s recommendations limit the rotation amplitude that the shell-base connection of a tank may undergo to 0.2 radians. This limit is obtained from the following two assumptions: (1) a plastic hinge with a length equal to twice the thickness of the base plate forms at the base plate, next to the connecting weld, and (2) the maximum strain that can be sustained by the base plate is 5%. While these two assumptions are reasonable and have been the state of practice for many years, no research work is believed to exist backing these two assumptions and therefore the 0.2 radians limit. For this reason, an experimental research was conducted to determine the real rotational capacity of shell-base welded connections found in tanks. A total of 24 shell-base connections were tested considering the bending and membrane stresses that develop at the base plate when uplift occurs. Constant amplitude tests were carried at different amplitudes of rotation in order to create curves of rotation versus number of cycles to failure. The main finding from this investigation is that the current limit of 0.2 radians is very conservative and that a limit of 0.4 radians would be more realistic.

2011