Seismic improvement of elevated concrete water storage tanks using FRP sheets

Master thesis in the field of civil engineering - hydraulic structures

Abstract

Aerial liquid storage tanks are used not only to store water, but also to store chemicals and toxic substances in different forms. Considering the use of these structures in civil and urban development and industrial networks, their importance is determined before and after the earthquake. The importance of these structures comes from the fact that an important task such as water supply is the responsibility of these reservoirs. Also, during an earthquake, if a failure occurs in chemical and toxic storage tanks, it will cause environmental and natural losses.

In this thesis, the failure mechanism of these structures has been determined by examining the behavior of high concrete water storage tanks and using finite element software, ANSYS. Then, in order to strengthen the earthquakes of these tanks using FRP composite, they have been controlled and investigated under dynamic loads by re-modelling. The failure mechanism of these tanks was found to be a bending failure type. The location of this failure is at the beginning of the concrete support shaft. After that, by selecting 9 models, nonlinear static analysis of increased pressure and dynamic time history analysis were performed on them. Then, by obtaining the plasticity coefficients and maximum stresses, the results have been compared. According to the results, it is clear that FRP sheets have the ability to improve the studied tanks. In the end, the best models for improvement are introduced. It was also found that if there are other factors in the presentation of the improvement plan (including the position of the connecting pipes, etc.), the length and thickness of the FRP sheets used will change. 

Key words: liquid concrete tank, overpressure analysis, seismic strengthening chart, FRP

Chapter 1

Introduction

Air tanks for liquid storage are used not only to store water, but also to store chemicals and toxic substances, in different forms. Considering the use of these structures in civil and urban development and industrial networks, their importance is determined before and after the earthquake. The importance of these structures comes from the fact that an important task such as water supply is the responsibility of these reservoirs. Also, during an earthquake, if a failure occurs in chemical and toxic storage tanks, it will cause environmental and natural damage.

Due to the complexity of the behavior of these structures, the need for more investigations and studies is felt in this type of tanks. These complications are mostly related to the interaction between water and the structure. A number of these tanks collapsed or were damaged in the 1990 Manjil earthquake (in Rasht city and Bandar Anzali) and in 2003. When the liquid storage tank vibrates, hydrodynamic forces are created on the contact surface between the water and the walls of the structure; The amount of these forces depends on the acceleration that the tank receives from the ground. In high tanks, in addition to these forces, the mass of the tank, which is located in the upper part of the supporting structure, transfers an anchor to the foot of the supporting structure. Deterioration in elevated tanks is mainly due to the anchor created at the foot of the supporting structure. This anchor causes local or overall deterioration of the system.

Having the characteristics of the high concrete tank structure, which was damaged in the 1990 Manjil earthquake (in Rasht city) and in 1990 Manjil, we model it. The forces that we have to apply to the structure include static forces and dynamic forces. Static forces include the weight of the structure and the weight of the water inside the tank. dynamic forces, whose origin is the movement of the earth; It includes the forces caused by the mass of the structure, the mass of static water and the mass of moving water.

The application of hydrodynamic forces to the structure can be done in two general ways:

1- Statically (using different regulations)

2- Dynamically

After the forces are applied to the structure, the stresses and internal forces of the structure are examined and finally the failure mechanism of the structure is detected.

Failure The structure is of two types:

1- Flexural failure: Flexural failure is usually associated with rebar yielding. In this case, the reduction of resistance in the residual rings is not seen, but the reduction of hardness due to the yielding of the rebars is evident. In the condition that the wall is subjected to compressive force, bending failure is associated with crushing of compressive concrete; In this case, in addition to reducing the hardness, there is also a reduction in resistance..

2- Shear failure: Walls that have a small ratio of dimensions (height to length) suffer from shear failure, in this case the walls suffer from diagonal cracks. In this case, the failure mode occurs in brittle form at the base of the wall.

By identifying the type and manner of failure, the discussion of improving the tank is raised.

Today, the maintenance and restoration of structures has become very important due to the high costs of their construction, for this reason and due to the increasing need of engineers and experts in the construction industry to strengthen, repair and improve concrete structures, various and numerous methods have been proposed for this issue. One of the methods of strengthening concrete structures against vibrations is the use of FRP composites. Among the advantages of these materials are the simplicity of implementation at the same time as the high speed of operation, low weight, high tensile strength of the sheets, resistance to corrosion, absorption of vibrations and increase in the strength and strength of the structure (especially against dynamic loads).

Compared to other methods of strengthening, it is possible to point out the appropriate performance of its implementation facilities and devices.

In this research, we want to determine the behavior of elevated concrete water storage tanks using finite element software, mechanism Identify the failure of these structures. Then, in order to strengthen the tremors of these tanks using FRP composite, we will remodel it and control and investigate it under dynamic loads.

Chapter Two

Overview of past research

The construction of elevated tanks (composite) started for the first time in the late 1970s in Canada. In the late 1980s, America began to build these types of tanks. The topics of elevated water storage tanks are the continuation of the topics raised in connection with ground water storage tanks. The direct connection between them makes the history of ground reservoirs, which is a large part of the history of elevated reservoirs, to be examined first.

In the topic of reservoirs, one of the most important factors influencing the design are hydrodynamic forces. The first major research to obtain earthquake forces in circular tanks containing liquids was done by Jacobsen in 1949 [1]. He used Bessel's function, he solved Laplace's equation for fluid motion, then he obtained an expression for pressure and hydrodynamic forces acting on a rigid cylinder. As mentioned, the tank is considered rigid, so the flexibility of the walls is neglected. Housner, in 1957 [2], calculated the hydrodynamic pressures on the tank walls by analytical method. Then, in 1963, he modeled water pressure with a spring and a mass. Veletsos (Veletsos), in 1974 [3], considered the flexible solution of the tank; He obtained an expression for the rigid rocking pressure in the preliminary limit of vibration. In this work, he assumed that the system is a degree of freedom. Fisher (Fischer), in 1979 [4], hydrodynamic pressures, taking into account the flexibility of the tank walls, does not have a significant effect on the formation of surface waves. But he expressed his reason based on the low impact of the waves and structure modes. Haroun and Housner (Haroun & Housner), in 1981 [5], presented the original Housner model, considering the elastic behavior of the reservoir in the preliminary case. Haroun in 1985 [6], by modeling a separate spring and mass to consider the effects of waves in hydrodynamic forces, provided a more complete model; In this model, the flexibility and mass of the structure is considered by adding another spring and ram. He also examined the rotational excitation in his model. In all models, only the first wave mode was considered. It should be noted that only the first mode is the basis of modeling and the following modes are not important. Maheri and Severn (Maheri & Severn), in 1988 [7], investigated the effect of flexibility on hydrodynamic pressures, in this work, it has been tried to propose a method based on the experiments, which can obtain hydrodynamic pressures by considering the flexibility of the reservoir.

In the sudden doubt test, we see large differences in the values ??obtained compared to the values ??obtained by Jacobsen relations, which shows the effect of flexibility Flexibility in the emergence of hydrostatic forces