Investigating the performance and behavior of reinforced concrete frames with seismic isolators

Dissertation for Master's Degree (Sc.M)

Field: Civil Engineering Major: Structures

Summary:

In recent years, the subject of seismic isolation has been particularly focused on the seismic design of buildings. The main purpose of this work is to isolate the structure from the ground instead of using conventional retrofitting methods. The equipment used to isolate the foundation of the building has two important characteristics: horizontal flexibility and energy absorption capability. The flexibility of the isolator system increases the main period of the structure and leaves it out of the destructive energy range of the earthquake. Recently, in order to investigate the behavior of structures with isolators and with a fixed foundation, after enumerating the types of seismic isolators and the design of the isolator and the provisions of the regulations regarding isolated structures, nonlinear analysis of these types of structures has been done and the behavior of each of the structures has been examined in terms of acceleration-period-shear, etc. 11% and 20% were compared and evaluated, and the results show that with the increase in the cycle time, the acceleration decreases and the increase in damping causes a further decrease in the acceleration in the structure. With the increase in the cycle time, the overall displacement of the structure increases, but the increase in damping controls it to a large extent. In general, comparing the isolated structure with the structure without isolator, the results of the analysis indicate that the acceleration of the floors and the cut of the floors are less in the isolated structure and the lateral displacement is relatively large in the isolated structure, and to solve this problem An energy absorber or damper should be used in the isolation system by increasing the damping, the ability to absorb energy in the structure increases and the decrease in acceleration leads to a decrease in the force applied to the structure. Chapter One: Concepts (images are available in the main file) 1- Introduction: Damages caused to various buildings due to earthquakes are generally caused by two basic factors, which are From:

- The relative drift of the building floors relative to each other

- The acceleration created in the building floors

The change in the shape of the building floors, at different heights, creates relative drift. Since the floors do not move at the same time and at the same speed, during an earthquake, a relative horizontal displacement occurs between them. Sometimes even due to the change of the direction of the force on the building, due to the non-uniformity of the power transfer to all the floors, the floors of the building move in different directions, which causes the destruction of internal separating walls, broken windows, and the destruction of the building's service facilities, it deprives the possibility of using it, and causes significant damages. Also, the acceleration caused by the earthquake is transferred to the floors of the building, which are the place where the mass of the structure is concentrated, and an acceleration proportional to its mass occurs on each floor. In special buildings where the use of internally installed equipment is the main purpose of their construction, the damage caused to the above equipment is far more than the damage caused to the main structure. Therefore, the main issue in order to provide a high seismic resistance of a building is how to minimize the change of location between floors and accelerations of floors. It becomes the connector of the floors, which can be reduced by increasing the hardness; But this act strengthens and intensifies the movement of the earth, which in turn increases the acceleration of the floors and leads to damage to sensitive internal equipment. Floor accelerations can be reduced by making the system softer; However, excessive flexibility causes significant changes in the level of the floors and extensive damage caused by it, and improper performance of the structure under the effect of wind force and low-power earthquakes, and on the other hand, it requires additional design and cost to embed the desired softness in the members and joints of the structure. The above limitations clearly show that the existing way of designing buildings against earthquakes will not provide the desired and ideal design of the structures.The above issue is especially true for special structures that expect high productivity in post-earthquake conditions. Therefore, another method that has been proposed since the beginning of this century and has been put into practice in recent decades due to the availability of various facilities both in terms of construction technology and in terms of engineering knowledge regarding analysis, design and implementation to make structures resistant to earthquakes, is isolation against earthquakes or seismic isolation. The main purpose of this method is to prevent the direct transmission of earthquake force from the foundation to the structure. The use of isolators is the only practical way to simultaneously reduce the displacement between floors and the accelerations of the floors, and by reducing the resulting displacements in the level of the isolator, it provides the required softness of the structure.

In other words, seismic isolation is a new method for designing buildings against earthquakes, which is based on reducing the forces entering the structure due to earthquakes, instead of increasing the capacity of the structure to bear lateral loads. The basis of this method is to reduce the responses by increasing the cycle time and damping in the instruments. Also, the use of this method causes the deformation of the structure to remain within the elastic range, which will increase the safety level of the structure. In this method, only to create lateral rigidity of the structure against lateral loads such as wind load and operating loads, a series of lateral load-bearing elements are recommended in the minimum requirement. In this method, since a small share of the earthquake force enters the structure, the following results can be expected: The relative displacement of the floors is reduced.

- There is a noticeable reduction in the acceleration of the floors.

- The structural damages and non-structural damages are significantly reduced.

- Sections with lower capacity are used.

The concept of seismic isolation is a rich source of theoretical research in the field of dynamics of isolated structural systems and In the field of mechanics, the structures themselves have been provided. These theoretical researches, which have been widely published in structural engineering and earthquake journals, have led to the emergence of design recommendations for isolated structures and design criteria for isolators. Today, several countries provide design codes for isolated structures. Countries such as America, Japan, Italy and New Zealand are leaders in this field and each has its own regulations. 1-1- Research Background 1-1-1 Kanko et al. High damping, lead, rubber supports and layered rubber supports with a steel conductor system have been presented.

1-1-2- Also, Tsai and Kelly (1989) have investigated the behavior of a structure on a linear isolator modeled as a base mass and a linear spring in the base and damper, as a disturbance to frequencies and mode shapes in a system with a fixed base.

1-1-3- Tsai and Kelly (1988) have investigated the non-classical behavior of isolated modes and have considered two modes for the non-isolated structure.

1-1-4- Andriano and Carr (1991) have recently conducted a fundamental study related to the distribution of lateral forces in structures with nonlinear isolation.

1-2- Generalities

A question is always raised and that is: Why do we deal with earthquakes?

And the answer given is to prevent the loss of life and money caused by earthquakes. Every day, engineers, designers and scientists are trying to obtain more resistant materials than before and create new ways and designs to prevent these financial losses and lives. Every day, engineers present new regulations for the strengthening of structures and roads so that the structures that are going to be built have the necessary resistance to deal with earthquakes and ultimately have the least damage; But can the structures that humans build in this way (to withstand earthquakes) always and in any type of earthquake be able to withstand various earthquakes or not? But on the other hand, we also know that the live loads in the building cannot be reduced. Therefore, the dead loads of the building should be reduced as much as possible, which means structural loads.