Presenting a new model of braces resistant to buckling and investigating its seismic behavior

Master's thesis in the field of structural engineering

Chapter one: Introduction

1-1.  General

One of the most important natural events that has always changed people's lives and sometimes destroyed human civilizations by destroying buildings is earthquake. Therefore, humans have always tried to identify and deal with the dangers caused by earthquakes and still have not succeeded in fully controlling this huge energy. Now, even though many researchers have conducted valuable research in the field of safe and appropriate construction, a large number of inhabitants of this planet are still buried under the debris caused by earthquakes every year, and many structures lose their efficiency or collapse after the earthquake.

Iran is located in the seismically active region of the world, and according to documented scientific information and observations of the 20th century, it is one of the most dangerous regions in the world due to earthquakes. It is considered powerful. Currently, Iran is at the top of the countries where earthquakes are associated with high casualties, and in recent years, on average, an earthquake has occurred every five years with very high casualties and financial losses in some parts of the country. Although it is very difficult to completely prevent the damages caused by severe earthquakes, but by increasing the level of information regarding the seismicity of the country, identifying and studying the vulnerability of buildings, securing and retrofitting them properly and in principle, it is possible to reduce the losses and damages caused by future earthquakes to the optimum extent.]1[

In order to identify and curb this phenomenon, researchers have always tried to prepare many regulations around the world for the calculation and construction of earthquake-resistant structures and many methods for Calculate this force and design structures against it. After calculating the earthquake force, methods for designing earthquake-resistant buildings are proposed, and these methods can be divided into two classical (traditional) and modern categories. Each and every component of the structure is designed based on the ultimate strength or maximum force method. But in modern methods, the stability of the structure with the design method based on performance has also been proposed.]2[

In instrument systems, usually two factors are very important for designers. The first is the safety of the structure and the second is the comfort of the residents against external loads such as wind and earthquakes. To achieve this goal, two factors of displacement and absolute acceleration are effective and must be controlled. In this regard, various systems have been presented that generally change the behavior of the structure in such a way that the incoming energy of the earthquake does not damage the main components of the structure. Due to the fact that non-earthquake-resistant structures are found in our country and due to the fact that the use of additional systems reduces the dynamic response of structures in a very favorable way, the use of these systems is important in our country.

Although the dynamic loads on structural systems may be caused by various factors such as the effect of wind and waves and the movement of cars, there is no doubt that one of these types of dynamic loads is the most important for structural engineers. It is the stimulation caused by earthquakes. Of course, the importance of the earthquake problem is partly due to the harmful results that an earthquake leaves in a densely populated area. Since the design of economic structures with various architectures and dimensions that are able to withstand the forces resulting from a strong earthquake requires a high ability in the art and science of engineering, it seems logical that the field of earthquake engineering as a framework in which the theories and techniques presented in the dynamics of structures and It will be displayed and used.

The ability of conventional methods of designing and building existing structures is very limited and does not meet the ever-increasing needs of designing new structures. For example, increasing the height of buildings due to the lack of land in big cities and meeting the needs of new architecture with unusual forms are among the problems that show the need for new technologies in construction in our country.

1-2..  Necessity of conducting the present research: Various structural systems have been used to deal with the lateral forces caused by earthquakes in steel buildings, which can be referred to as the resistant bending frame system, the convergent braced system and the divergent braced system. Each of these systems, in turn, has its own advantages and disadvantages, which has been the subject of earthquake engineering research in recent years.

In Iran, the use of convergent bracing systems is very common among structural engineers. Therefore, addressing this issue and stating the disadvantages of these systems and providing practical solutions in the field of eliminating these disadvantages can greatly help the development of Iran's construction industry in order to make buildings safer. One of the types of convergent brace systems is the buckling resistant brace system or BRB for short This system is one of the most powerful systems available to control the adverse vibrations of structures against lateral forces, and today in most parts of the world, this system is widely used to dissipate the energy caused by earthquakes.

In this type of braces, the purpose of the brace is to reach the yield point under compressive axial load by preventing the buckling of the member, which is done by an external mechanism. Therefore, the brace will yield both in tension and compression without buckling. Also, since the buckling of braces is not desirable for energy consumption, this system, which has elastoplastic behavior, is very effective in consuming earthquake energy.]3 [

In the present research, studies have been conducted on braces resistant to buckling as a system resistant to the forces caused by earthquakes. Since the installation of earthquake-resistant systems from the point of view of economics and strengthening helps the proper behavior of the structure against dynamic loads, research on these systems is of great importance. Buckling-resistant braces have many advantages over conventional convergent braces, and in terms of structures, they also show favorable behavior against lateral forces. In addition to these advantages, a series of disadvantages have been stated for these braces, which are mentioned below:

The construction of BRB braces is somewhat complicated and expensive and requires modern technology.

Due to the complexity of the construction, the production of BRB braces is the monopoly of certain companies.

In case of using steel with a wide yield range as the cross-section core, additional forces will be applied to the structure. 3[

1-3.    Research Objectives

The main objective of this study is to research a type of buckling-resistant brace with a new design that eliminates the disadvantages mentioned for BRB braces in this design. The design of this brace is actually derived from the shape of the brace resistant to buckling proposed by Sridhara[2]. [16] The new brace has a simple manufacturing technology and does not require complex manufacturing technologies. Also, with the modifications considered, this new brace will maintain the stability of the structure more than the current brace in more severe earthquakes. Also, the carrying capacity of the used materials will be used more than the existing models.

In this design, the cross section core is made of thin wall steel The sheath is also designed in such a way that in severe earthquakes, when the reduction in length caused by the compressive force in the core exceeds a certain limit, the section of the sheath can withstand a percentage of the compressive forces of the core as an auxiliary compressive member. Intertrib, which creates a gap between the connection and the sheath, so that a percentage of the compressive force is tolerated by the sheath in changing the desired shape by sticking the connection and the sheath together. Also, the design of the sheath should be such that the cross-section of the sheath yields under compressive forces, in other words, lateral buckling of the sheath should be prevented. In this case, energy consumption will be higher than the current buckling-resistant brace, and as a result, the structure will be more stable against lateral forces. Therefore, in this study, according to the forces acting on the core, an optimal cross-section for the core will be designed first, and in the next step, the sheath will be designed so that it can act in more severe earthquakes and maintain the stability of the structure.