Performance evaluation of non-buckling brace and determination of seismic performance parameters used in Iran standard 2800

Dissertation for Master's Degree in Civil Engineering

Structural Orientation

February 2013

Although the construction and design methods of structures have expanded over many years, the effect of earthquakes is still one of the most important problems in designing buildings in seismic areas. Conventional wind braces undergo a lot of lateral deformations against the lateral loads of earthquake or wind force, and if these deformations exceed a certain limit, it causes structural and non-structural damage and the safety and integrity of the structure is endangered. To overcome the mentioned problems, new types of windbreakers have been developed for the first time in Japan about 30 years ago. These braces are designed in such a way that they are resistant to buckling and as a result have symmetrical curves under tensile and compressive cyclic loads resulting from the impact of earthquake forces, and also improve the behavior of the structure in terms of stability and energy absorption.

Structures armed with non-buckling braces are among the few earthquake-resistant structures that have two characteristics of high stiffness and energy consumption at the same time. In addition to new buildings, these structures are widely used in the improvement of existing steel and concrete structures. Considering the seismicity of the country and the increasing use of these structures in the world, the use and localization of this type of bracing system in Iran is inevitable, and the entry of this lateral bearing system into the design regulations, especially the Iranian earthquake regulations (2800), is mandatory.  In the meantime, determining the seismic performance parameters such as additional resistance coefficient, ductility coefficient, ductility-dependent resistance reduction coefficient and behavior coefficient of these structures is essential for the design of these systems.

Keywords: torsional buckling braces, ultimate strength, ultimate displacement, ductility, behavior coefficient

Chapter One

  Introduction

-1-Introduction

Earthquake is one of the destructive natural phenomena that has left a lot of life and financial losses in the last half century alone, and considering the earthquake proneness of our country and its location on the Alpine-Himalaya belt, it is necessary that we deal more with the methods of dealing with this phenomenon, including making structures resistant to earthquakes. Therefore, seismic evaluation of structures and strengthening of existing buildings is considered as a necessity. In structures whose design is based on the force of an earthquake, a resistant system must always be designed to deal with the lateral force of an earthquake on the structure, which includes the use of various metal braces, but the main defect in conventional braces is the difference between the tensile and compressive capacity of these braces and the deterioration of their resistance in cyclic loading, so in order to achieve an elastoplastic behavior. Ideally, a suitable mechanism should be used to prevent compression buckling of braces. The considered method is to enclose a malleable metal core in the middle of a volume of concrete that is itself covered by a metal membrane, which is called a torsional buckling brace (BRB) and consists of a bearing element and a side support element. The bearing element carries the axial loads in each of the two tension and compression modes, which are transferred to the BRB. The supporting elements provide lateral support for the load-bearing elements to prevent the buckling of the BRB when the BRB is loaded under pressure, and the BRB will be able to increase the strength, ductility and energy dissipation capacity of the steel elements that are made for load-bearing. They are made of steel and concrete, so that they can be used to solve the problem of buckling of ordinary wind braces. We examine the performance of these windbreaks in reinforced concrete structures in three different heights (3, 6, and 12 floors). The initial design of all the mentioned structures was done using Etabs software, and then nonlinear analysis of the models was done by modeling in OpenSees software..

The investigated parameters were final load (maximum base section), maximum displacement, absorbed energy and ductility, which were evaluated by changes in structure characteristics, including geometry such as the length of openings and the height of the structure.

The bending frame structure includes columns and beams that are connected to each other by means of rigid connections, where the lateral stiffness of the frame depends on the stiffness of the columns and beams and its connections. In terms of behavior, this system is relatively malleable and shows a high ability to dissipate energy, but the stiffness of this system is relatively low and it suffers from weakness against lateral loads, for this reason, members resistant to lateral loads such as braces are used in this type of system. Below are the definitions of the types of bending frame system described in the regulation. 1-2-1-1 Normal bending frame Normal bending frames are the frames with low ductility that are discussed in the Iranian concrete regulation (ABA) which cannot be used in areas with high and high seismicity and should be more careful in choosing this type of system for different areas [1].

1-2-1-2 Medium bending frame

Medium bending frames are the same frames with medium malleability as discussed in the Iranian Concrete Regulations (ABA) which are allowed to be used for buildings of medium importance and different seismic areas provided that the conditions of height and regularity of the structure are controlled [1].

1-2-1-3 Special bending frame

Special bending frames are those frames with high plasticity that are discussed in Iran's Concrete Regulations (ABA) which are used in areas with high and very high seismicity and different constructions and special structures [1]. 1-3 types of bracing. May­ Braces are often considered as an obstacle for the architectural design of the structure, so they are usually placed in openings that create the least obstacle and at the same time satisfy the structural conditions of the brace in the action of the shearing and twisting forces of the building. In many cases, the type of bracing is determined based on the available space and opening. In general, braces are divided into two categories [2]: concentric braces (convergent) eccentric braces (divergent) In recent years, in addition to the above two groups, new bracing systems (gate braces, knee braces, and swing buckling braces) have been developed, each of which is We will briefly explain.

1-3-1- Concentric braces[1]

These systems were invented in the way of completing the steel structure systems in order to deal with the wind forces. In this type of bracing, it is assumed that the neutral axes in different members, such as columns, beams and bracing members, meet at a common point in each connection. In frames with concentric bracing, the lateral resistance of the structure is provided by the diagonal members that form a truss system with the beams of the frame. Types of concentric braces include cross, diagonal, V-braced, inverted-v, and k-braced as shown in Figure 1-1. Due to the truss-type configuration, the lateral rigidity of these systems is very high, so that a steel frame system with cross-type concentric braces can be up to 10 times harder compared to a similar bending frame system [3].

One of the major problems of these systems is low formability and energy absorption, mainly due to the local or overall buckling of the compression member of the brace and to some extent the weakness and improper performance of its connections. May­ Below, we discuss the main problems of each of the above types of wind braces according to AISC [4]. The bracing that bears the compressive force is buckled and removed from the lateral load system. In the next cycle, this happens to the other brace, and after a few cycles, both braces are removed from the lateral load system. As can be seen in Figure 1-2, the hysteresis rings of the steel frame with cross braces are very unstable and irregular.