Seismic Response Of Unanchored And Partially Anchored Liquid Storage Tanks

Author : Miguel Ormeño-Godoy
Publisher :
ISBN 13 :
Total Pages : 290 pages
Book Rating : 4.:/5 (948 download)

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Download or read book Effects of Fixity Conditions on the Seismic Response of Liquid Storage Tanks written by Miguel Ormeño-Godoy and published by . This book was released on 2015 with total page 290 pages. Available in PDF, EPUB and Kindle. Book excerpt: Liquid storage tanks are essential structures and, for this reason, it is important that they remain in operation after a strong earthquake. However, despite the development of design specifications mainly based on numerical studies, strong earthquakes still caused severe damage to storage tanks. The parameter that affects most significantly the seismic behaviour is the fixity of storage tanks. Despite the significance the support condition has not received much attention. The main objective of this doctoral research is to investigate, through physical experiments, the effects of tank support conditions. A good understanding of this support influence will enable more efficient seismic deigns of storage tanks in the future. The first part of the study presents a comparison of two worldwide most commonly used specifications for seismic design of storage tanks. The comparison focused on how both specifications consider the influence of soil-structure interaction and uplift in the design. Values of base shear and overturning moment were compared under different fixity conditions. The comparison reveals that there is no agreement in the current practice with respect to a proper incorporation of the effects of the fixity conditions in the analysis. An aluminium scaled tank model is utilised to investigate the effect of uplift on the seismic response. A comparison of the seismic behaviour of the tank with and without anchorage is described. Recorded ground motions scaled to the New Zealand design spectrum and three tank aspect ratios (liquid-height/radius) were considered. The experiments showed that the tank shell acceleration increased when uplift was allowed. This implies that the inertial forces developed by the tank were higher when the tank was free to uplift. In the next stage a PVC model tank containing water is utilised. The same fixity conditions as the previous experiments and three tank aspect ratios (liquid-height/radius) were also considered. The study confirmed the results obtained in the previous stage regarding to the increase of tank accelerations when the tank was no provided with anchorage. As anticipated the tank displacements increased when uplift was allowed. However, axial compressive stresses, which are a control parameter in the design, decreased by between 35% and 64% with tank uplift. A numerical model confirmed these results. The experimental findings prove that uplift has a beneficial effect on the seismic behaviour of storage tanks by reducing the axial stresses developed in the tank shell. However, it was also proved that accelerations and displacements increased when the tank was allowed to uplift. Large accelerations and displacements may damage the piping connected to the tank due to large relative movement between tank and surroundings. To control excessive relative movements the utilisation of slip-friction connectors is proposed. The movable connectors represent an intermediate condition between an unanchored and a fully anchored tank. A series of shake table experiments on a scale model PVC tank containing water is investigated. A comparison of the seismic behaviour of a fully fixed system (tank with anchorage), a system free to uplift (tank without anchorage) and a partially fixed system (tank with slip-friction connectors) is described. The experiments showed the beneficial effects of slip-friction connectors on storage tanks. In comparison with those of an unanchored tank the uplift displacement reduced. Compared to a fully fixed tank the axial stresses also reduced. A numerical model is proposed which corroborates these results. Finally, the effect of soil-structure interaction is investigated. In all the previous experiments a rigid base was considered. In this stage, a physical model on sand in a box is considered. The sand simulated the soil by providing a flexible base. The experiments were performed using actual records scaled to the New Zealand spectrum for a Wellington site. The results showed that while the top displacement and the tank shell acceleration increased when the model was placed on a flexible base (sandbox), axial compressive stresses decreased in comparison with the case when the model was placed directly on the stiff shake table. The results showed that the effect of soil-structure interaction on the seismic response of storage tanks is similar to that of uplift. In general, a more flexible base, either due to uplift or supporting soil, will reduce the stresses developed in storage tanks. However, it will simultaneously increase the maximum accelerations and displacements.