Increasing the ductility of coaxial braced frames by using low yield stress steel in the braces

Master's Thesis

Civil Engineering - Earthquake Engineering

1391

Abstract

Frame structures with coaxial braces are among earthquake resistant systems. These structures are widely used in short buildings to tall buildings for reasons including relatively calculations. It is simple, easy to implement and has low related costs. However, due to the presence of members prone to buckling in the harness, it leads to low ductility and a significant drop in hysteresis loops after several loading cycles. To prevent this undesirable situation from occurring, many researches have been presented in the field of providing a malleable member in restraining coaxial straps. The idea of ??this project is to use a member prone to flow in a part of the windbreak, which is called a fuse. In this study, steel with low yield stress and aluminum are used in a part of the member to prevent buckling of the member and lead to full hysteresis cycles, ductility and energy decay capacity. Against the side load, braced frames are coaxial.  If the connection  If this type of frame is jointed between different members, it is called a simple frame (jointed frame) with coaxial bracing. In coaxially braced frames, the axis of beams, columns and bracing members intersect at a common point. The resistant core of the truss or the resistant network of the truss is the stability factor of the simple frame system. The requirements for forming a truss network are vertical elements or columns, diagonal diagonal elements or braces and beams, if the bracing system has braces that interact with the inner beam, such as the seven and eight bracing system. In this type of frames, the members resistant to lateral loads transfer the lateral force to the ground through the axial function of the members, and because of this axial function, the members of this type of frames have high stiffness and low displacement. For appropriate seismic responses, these restraints must be designed in such a way that they achieve appropriate resistance and ductility. In a way that is compatible with the philosophy of seismic resistant design, inelastic responses are expected in restraining coaxial girders during severe earthquakes. The design of coaxial diagonal braces should be able to withstand plastic deformation and the ability to dissipate energy in a stable state in the form of buckling in compression and yielding in tension. The correct design is to ensure that the deformation of the plastic occurs only in the restraint of the straps and that the connections do not suffer any damage. This allows the structure to withstand strong earthquakes without losing its gravity resistance. Past earthquakes have shown that the described behavior will not be realized unless the bracing frames and their connections are designed correctly.

According to Iran's 2800 Code [1], the use of coaxial bracing is allowed in buildings up to 50 meters in height, in frames with joint connections, and for taller buildings in mixed frames. Ease of implementation and low costs of joint connections  compared to the gripper connections  It has led to more use of this type of bracing as elements resistant to lateral loads.  Low consumption of steel per square meter of these structures compared to bending structures is another advantage of them. In addition, the lower sensitivity of these frames to the amount and quality of welding is one of the advantages of these frames compared to the bending frame structures.

Besides the advantages of the structures with coaxial bracing, their low plasticity should be mentioned as the main weakness of this type of frames compared to the bending frame structures. Coaxial bracing system changes its shape with lateral stimulation. In this case, one of the diameters is under pressure, and during an earthquake, with the increase in the energy level on the system, when the force in a diameter under pressure reaches about R equal to the design time force, this element undergoes yielding and buckling.. With the formation of a plastic joint on this member, its hardness and load capacity decrease, when  In the next cycles, this element is put under pressure again due to the damage caused earlier and the reduction of hardness and resistance caused by it, it undergoes more buckling and the deformation of the dough in the place of the created plastic joint is intensified. Based on the seismic loading tests of coaxial bracing systems, it is said that the hardness and resistance of these bracings decreases to 25% to 35% of the initial conditions after several heavy cycles of seismic loading. Referring to the above, although coaxial braces show good resistance under small loads and small displacements, they are prone to buckling and instability under heavy seismic loads. In other words, their seismic reliability is not suitable. Ductility results from the inelastic behavior of structural members and connections. The inelastic behavior in bending structures starts with the yielding of the farthest web section and continues until the complete formation of the plastic joint. The formation of plastic joints in different parts of the structure absorbs energy and provides the structure's plasticity. On the other hand, in structures with coaxial bracing, the lateral force is transferred through the axial force of the members. The axial force causes buckling of compression straps before it reaches the inelastic stage and absorbs energy. As a result, the hysteresis curve of the structures with coaxial bracing is narrow and their degree of plasticity is low.

Reconstruction speed is one of the important factors in the evaluation of earthquake-resistant systems, which has received a lot of attention in recent years. Structures with coaxial bracing, due to the limitation of their structural damage to the members and connections of the braces, compared to bending structures, have a high speed of reconstruction and a low cost. As a summary of the merits of structures with coaxial braces, it can be stated as follows:

The reconstruction and repair of these structures is generally limited to the members and joints of the braces, and as a result, their reconstruction areas are limited, low cost, and the speed of reconstruction is high. Comparison with less bending structures and their exploitation will be possible immediately after the earthquake.

Strengthening of steel or concrete structures with coaxial bracing is always considered as a simple and economical solution. The abundance of these structures has made the experience of construction contractors more in building these structures. The lower amount of steel consumption per square meter of these structures compared to the bending frame structures is also one of the positive points of these structures.

Despite the many merits of structures with coaxial braces, their low plasticity has caused their use to be limited. Obviously, any modification that leads to an increase in the plasticity of these structures will provide the possibility of benefiting from their positive features. In the last two decades, extensive researches have been carried out in order to increase the ductility of coaxial braces. In each of these researches, an effort has been made to increase their ductility by modifying connections or embedding malleable members in restraining coaxial straps. Briefly, the research conducted in this field can be divided into three categories: A- The use of friction joints in coaxial braces. B- The use of sheaths to increase the buckling load of the brace. So that the research in some methods has reached industrial production and they have been used in the retrofitting of a number of buildings. However, most of these methods require special expertise and skills that are not within the capacity of construction contractors, and in addition, some of these methods have limitations that limit their use to a specific type of strapping, which is why they have not been welcomed. The purpose of this thesis is to introduce the ductility member, which increases their ductility by embedding in the restraining of coaxial straps. while possessing the following features:

1- Easy installation and replacement in all types of coaxial harnesses.

2- Proper formability

3- No need for special materials and technology.