Dissertation for Master's degree
in the field of civil engineering, structural orientation
Chapter 1: Introduction
-1-Introduction
In recent years, the philosophy of traditional methods that were used in the design of structures against natural hazards has undergone major changes. Extensive destruction of structures designed based on old regulations in recent earthquakes, advances in analysis methods and more complex performance requirements expected by the construction industry have led to the introduction of more effective methods in the design of structures. One of these methods, which exists in many regulations and simplifies the design process, is the equivalent static analysis method, in which the design forces are reduced by the behavior factor. This method is based on the assumption that the resistance of the structure is greater than the value on which the design is based, and in addition, the structure under earthquake absorbs part of the earthquake energy by entering the non-linear stage. The design of optimal earthquakes for the building can be considered to achieve devices with optimal performance, in the sense of the possibility of creating controlled and pre-forecasted damage during an earthquake for the building, while the incorrect estimation of the characteristics of the earthquake and the behavior of the structure and its performance in the face of the earthquake is one of the important causes of severe damage to the structure. In order to better understand these characteristics and features, compared to the conventional prescription methods in the previous regulations that stated the design based on the reduced forces of the earthquake, earthquake design and improvement regulations were presented, according to which the structure's seismic design is suggested by the design method based on performance.
Due to the uneconomical behavior of the structure's elastic behavior under earthquakes, the main goal in the seismic design of buildings is based on the behavior of the building against the force caused by small earthquakes without damage and within the linear range. It remains and withstands structural and non-structural damages while maintaining its overall stability against the forces caused by strong earthquakes. For this reason, the seismic resistance required by the design regulations against earthquakes is generally lower and in some cases, much lower than the lateral resistance required to maintain the stability of the structure in the elastic range, in a strong earthquake. Therefore, the behavior of structures enters the inelastic range when medium and large earthquakes occur, and an inelastic analysis is needed for their design. But due to the high cost of this method and the lack of expansion of non-elastic programs and the ease of the elastic method, common analysis and design methods are generally performed based on the required elastic analysis using resistance reduction coefficients [2]. That is, even though the coefficients of behavior determined in the earthquake bylaws were intended to express hysterical behavior, ductility, increased resistance, damping and energy consumption capacity, the values ??of these coefficients in the earthquake bylaws were based on the observations of the performance of different building systems in past strong earthquakes, based on engineering judgment. Based on this, a lot of research was done in this field to express values ??based on research studies and calculation support in the earthquake regulations, which finally led to the modification of these coefficients based on scientific studies. In this report, the suggested values ??for behavior factor were based on the opinion of groups of expert engineers. For this reason, a specific method for determining its value was not provided. Also, in the NEHRP regulations of 1997 and 2000 (FEMA369 and FEMA303), which were inspired by ATC 3-06, empirical reduction coefficients have been emphasized [11, 13]. In some of the seismic design codes, there is an observation article on the calculation of these coefficients, however, in most of the codes, their values ??are based on engineering judgment, experience and observation of the performance of buildings in past earthquakes and neglecting their increased resistance level [15]. Therefore, considering the above, evaluating the behavior coefficients and checking the relationship between the effective parameters for the structures that are designed according to the regulations is of particular importance. Therefore, in most of the regulations for the design of new earthquakes, the methods of determining it are mentioned.Today, of course, in most regulations, instead of defining a certain value for a type of frames, components of the coefficient of behavior are defined for frames with different ductility and depending on the seismicity of the region, among which we can refer to the Canadian regulations.
In view of the many developments that have taken place in earthquake engineering since the drafting of Iran's regulations on the design of buildings against earthquakes (standard 2800) in 1366 and also despite the wide application of this By-laws in the design of different buildings in the country, knowing the content of this by-law and its concepts is important. Compilation of most of the application regulations of building earthquake planning has been done with the aim of preventing loss of life and possible damages as well as achieving an economic plan for the structure. Among the influential factors in achieving this goal, we can mention the two factors of strength and ductility of the structure. The mentioned factors are among the most important effective parameters in the vibration design of many regulations, including the 2800 standard. The provision of these two parameters in the design method of the aforementioned bylaws is based on the estimation of the goals of these bylaws in mild, moderate and severe earthquakes. These goals have been considered according to the expectations of the behavior of structures during earthquakes that may occur during the useful life of the building, as well as the amount of possible damage to the structure during an earthquake.
In recent decades, by examining the results of previous earthquakes and damages to existing structures, dealing with the concepts of ductility has been more and more the attention of researchers. Among them, after the earthquake in the city of San Fernando in the United States of California in 1971 and due to the many damages caused by this earthquake, many changes were made in the regulations of the earthquake design regulations of the United States and the concepts of ductility were given special attention. Also, investigations on the causes of earthquake damage that occurred in recent years, such as the Northridge (Los Angeles) earthquake in 1993, the 1994 earthquake in Kobe (Japan) and the Rudbar-Manjil (Iran) earthquake in 1369 showed the importance of the structure's malleability in the consumption of earthquake energy, and the design criteria of many earthquake codes with a new approach to provide this parameter in the structure were reviewed and transformed.
In the existing seismic design regulations, using the ability to absorb earthquake energy, taking into account the non-linear behavior of the structure, is one of the main design goals. Since the accurate determination of the deformation capacity of the structure requires non-linear analyzes of the structure, and considering the time-consuming nature of this type of analysis and the ease of linear analyzes compared to non-linear analyses, in these regulations, reduction coefficients known as behavior coefficients (R) have been presented in general for various systems of structures. The determination of the coefficient mentioned in the earthquake design regulations is based on several factors such as structure ductility, additional strength, damping, as well as the reliability coefficients used in the building design criteria. Thus, in these regulations, it is allowed to design the structure for a much smaller force than the actual force of the earthquake by using this structure capability and using the aforementioned coefficients. In these by-laws, these coefficients have been given to the designer according to the characteristics of the desired structure, in terms of the lateral bearing system, and in the form of tables as the force reduction coefficients proposed by the by-laws, in order to determine the earthquake force of the structure design. In this process, according to the actual behavior of the members under the effect of the forces and considering all the influencing parameters, including materials, geometry and characteristics of the members, the ductility corresponding to each effort in each element is estimated. In these bylaws (including the bylaws as well as the instructions for improving the vibrations of existing buildings) in linear analysis, by introducing the "m" parameter, the issue of ductility has been taken into consideration and used as a criterion for evaluating and accepting the performance of each member in the structure.