Investigating the behavior of cross-shaped buckling braces

Master's Thesis

Emran-Zelzaleh

Abstract

Using braces as a lateral resistance system in steel structures is one of the most common methods to withstand the forces caused by earthquakes. One of the weak points of this system is the compression resistance of the bracing member and its buckling as a result of reducing the member's load. Today, this problem has been solved with the advancement of technology and the appearance of braces resistant to buckling. These frames are a new type of convergent bracing systems (CBF) and are mostly used diagonally and chevron (7 and 8) and are not used crosswise. For this reason, in this research, an idea to use this system cross-wise and compare it with the single-diameter bracing system in two frames of one opening with the same dimensions of beam and column in the ABAQUS finite element software (ABAQUS) has been modeled for both forms of bracing and a laboratory sample has been simulated to validate the software. The results show that the use of the cross system shortens the length of the compression brace. As a result, the dimensions of the sheath become smaller compared to the single-diameter one and under cyclic loading it has more energy absorption than the one-diameter one.

Key words:

Cross buckling-resistant brace, convergent brace, lateral resistance system, cyclic loading, energy absorption

Chapter 1 Introduction

1 Introduction

To deal with the lateral force caused by Earthquake can use different systems. Some of them that are used in normal buildings are:

Reinforced concrete shear walls[1]

Steel shear walls[2]

Bending frames (concrete and steel)[3]

Bracing system[4]

1.1- Concrete shear wall system

As well as structural beams and columns Concrete has the ability to bear gravity loads but is vulnerable under seismic loads, adding a shear wall causes the lateral seismic force to be absorbed by these walls and prevents the application of seismic forces and deformations to beams and columns. As a result, adding only two or four shear walls reduces the vulnerability of all beams and columns. Of course, it should be noted that due to the high hardness of shear walls, usually a lot of forces are created in the foundation under them, which requires strong strengthening of the existing foundation or adding piles at the foot of the shear walls. The connection of the shear wall to the structure should be in a way that can transfer the force of the floor to the wall so that the wall can absorb the force of the earthquake and reduce the lateral deformation of the building with its hardness. For this purpose, at the level of the roofs, proper connections should be established by planting bolts between the shear wall and the slab. It is also possible to establish a good cohesion between the wall and the existing structure by planting bolts in the beam and column and covering these elements in shear wall concrete.

1.2 Steel shear wall system

This type of system resistant to lateral loads, which has become popular especially in the last three decades, is used to build structures and retrofit and strengthen old buildings. The positive point of this system in strengthening old buildings is the possibility of servicing the building due to its quick and easy installation. Its advantages include saving steel consumption up to 50%, simplicity of implementation, installation speed and lower cost. This system can be used in both steel structures and concrete structures. Its shear strength is higher than other lateral resistant systems such as bracing system and concrete shear wall and its energy absorption is better. Shear walls are not only suitable for dealing with risks such as explosions and storms. It is also considered the best system for resisting the forces caused by the strongest earthquakes. One of the biggest and most important features of using this system is the considerable saving of space due to the reduction of thicknesses. The economic analysis of the system shows that its use, especially instead of bending frames, leads to considerable savings in steel consumption. 1.3 Bending frame system This term refers to a frame that does not have joints in the nodes in the frame, and instead, the nodes are completely rigid and resist rotation. They are usually statically indeterminate. Members of bending frames behave like beams.Of course, the entire rotation of a node is done by maintaining the state and angles of the connected members, and the amount of rotation and anchor distribution among the members depends on the stiffness of the connected members. Clamping the columns reduces their effective length and allows the columns to be thinner. Due to the rigidity of the connections, the anchors and rotations of the beams are also reduced. Flexural frames are very sensitive to settlement, because it intensifies the strains in the frame and changes the stress distribution in them.

     Connections in steel flexural frames are such that the wings of the members are completely connected to the wings of other members. This can be done by welding or bolting sheets. And in the concrete bending frame, the connections are executed integrally with the continuous bending reinforcements. The cutting is controlled by using the necessary restrictions. 1.4 Steel bracing system Steel bending frames experience many displacements due to strong ground movements, and three important problems follow, i.e. damage to non-structural members, intensification of the ?-? effect, as well as cracking and failure of the beam-to-column connection in buildings. For this reason, engineers have resorted to using braced frames more. Bracing in a single form usually has a low plasticity capacity. The hysteretic behavior of braces in tension and compression is not symmetrical and they usually lose a lot of resistance during one-way loading. Because of this complex behavior, the actual distribution of internal forces and deformations is contrary to what conventional design methods predict. Design simplifications and implementation considerations usually cause bracing of some floors to be much stronger than what is required and in some floors to have results close to the design goals. All the aforementioned issues, in addition to considering the loss of bracing strength after buckling, cause us to face some weak floors in a building, where damage caused by earthquakes and other lateral loads is concentrated in them, which, if it does not lead to total damage, will cause damage to non-structural members. will follow Therefore, since 1990, the design criteria of braces have changed, and of course, a lot of research has been done to improve the above situation, especially in the case of converging braces. In fact, the bracing system consists of beams and columns that are connected with steel members and joint joints to resist lateral forces. 1.4.1 Types of braces Generally, braces can be divided into coaxial braces, in which only the brace members are subjected to tension or compression, and off-axis braces, in which parts of the structure are subjected to shear or compression. They are bent and involved in the load, divided. As for its various forms that are common, it can be divided into knee or chevron braces, cross braces, and diagonal braces.  Buckling-resistant frames [5] (BRBF) are a new type of bracing system. Due to the prevention of brace buckling, as shown in Figure 1-1, they have a symmetrical cycle of energy.

(Diagrams and pictures are available in the main file)

1.1 Common non-buckling brace components

Enclosed flowing part: this part can be rectangular, cross and. be Because this part is designed to flow under cyclic load, mild steel that shows more ductility is used more.

Enclosed elastic part: This part usually has a larger area than the central core to ensure its elastic response. This can be achieved by widening the steel core. It is also possible to use hardener welded to the steel core to increase the area. This part is not present in some non-bending braces.

•        Non-enclosed elastic piece: This piece is the continuation of the enclosed elastic piece that connects the brace to the connecting plate (gast plate). In the design of this piece, attention should be paid to its local buckling and facilitating the installation and removal of this brace. Separating and expansion materials: sliding materials that effectively eliminate or minimize the transfer of shear between the steel core and the mortar. Materials such as rubber, polyethylene, silicone oil, mastic tape, etc. These are among these cases. 1.2 Objectives and scope of the research The examination and review of articles and reports shows that non-buckling braces are mainly used in diagonal and chevron shapes.