Seismic Response of Tank-Fluid Systems: State of the Art Review and Dynamic Buckling Analysis of a Steel Tank with the Added Mass Method

If we want to make a direct comparison between the Eurocode and the US standards, we must consider the simplified design procedure given by the Eurocode itself. In fact, the rigorous procedure is based on a different and more sophisticated analogue. The great difference between the Eurocode simplified method and the US Standards methods is that in the former the pseudo-accelerations are obtained from the elastic response spectrum, while in the latter they are obtained from a design spectrum, which is the elastic one suitably reduced by a behavior factor. This fact leads to lower seismic forces in the US Standards than in the Eurocode, which seems to be the most conservative. One of the most important consequence of this fact lays in the impossibility to carry out the uplift analysis according to the EC8 tabular method.
Furthermore, the seismic force levels and response modification factors included in API 650 and ACI 350.3 provide results at allowable stress level. So, when comparing these provisions with other documents defining seismic forces at strength levels, the seismic forces obtained according to API or ACI should be increased by the applicable factors to derive comparable forces at strength levels.
Another reason of the Eurocode higher loads is in the adopted combination rule, the absolute sum, in contrast to the SRSS rule adopted by API 650 and ACI 350.3.
One important point about the simplified design method adopted by the variuos code and based on the Malhotra’s work, is that the overturning moments are always determined using the analogue for rigid anchored tanks, also in the API 650 where only steel tanks (which are very flexible) are referred to. Then, in API 650, a lateral force coefficient is specified to represent the amplified tank acceleration as a ratio of the gravity acceleration g; it does not take into account explicitly the effects of shell flexibility, site conditions or support conditions. Each of these factor may individually amplify or reduce the tank acceleration. When the value of a “moment parameter” (the so called anchorage ratio) exceeds 1.54 the tank is deemed to have overturned, and accordingly, it must be anchored.