Investigating nonlinear analysis methods in seismic evaluation of steel frame with combined bending system and cross bracing

To obtain a master's degree

Civil engineering field - structure orientation

Abstract:

The subject of the current research is to investigate the level of performance and accuracy of methods based on non-linear static analysis and the need to retrofit steel buildings with a double system of bending frame with cross brace and integrated 7-shape brace designed with Iran earthquake code 2800 (3rd edition) by means of improvement instructions. Seismicity of the existing buildings is based on the purpose of improvement required for these buildings.

For this purpose, plans with the number of floors 4, 8, and 16 are selected and designed according to the 2800 code of Iran earthquake in a statically equivalent way. The critical frame of these buildings is under foundation improvement and is analyzed with two methods of nonlinear static and nonlinear dynamic time history under acceleration of different mappings with the help of ETABS and SAP2000ver12 software and controlled according to the seismic improvement guidelines. Finally, in the case of buildings designed with the 2800 regulation, it is concluded that nonlinear static analysis is of sufficient accuracy compared to dynamic analysis. It is not nonlinear. Chapter 1: Generalities Considering that most common structures enter the non-linear region during an earthquake and show inelastic behavior, therefore, by using the traditional methods of the regulations, which are based on linear analysis, it is not possible to control the behavior of the structure after it enters the inelastic region. On the other hand, nonlinear dynamic analysis, which is often referred to as the most accurate method for investigating the behavior of structures during an earthquake, cannot be suitable for practical and engineering problems due to its high cost and time-consuming nature. Meanwhile, the idea of ??non-linear static analysis has been repeatedly proposed, which, while not having the problems and complexities of the non-linear dynamic method, can evaluate the behavior of the structure in the inelastic region with an acceptable approximation. Nonlinear static analysis is the basis of performance-based design method. Design based on performance actually refers to a method in which the structure design criterion is described as achieving a target behavior and performance. This method is opposite to the design criteria of conventional structures, in which the design criteria of the structure is defined only by limiting the forces of the members that result from the application of specific amounts of design loads. In this method, by leveling the earthquake risk, the employer is given the authority to choose the level of risk for the structural designer. On the other hand, by predicting the behavior of the structure with a certain risk, it is possible to decide on the use and vulnerability of the structure after the earthquake. 1-2- Thesis topic: In the course of a renovation project, it is first necessary to collect information from the existing structure, then it is necessary to model and analyze the desired structure in a way, so that the behavior and performance of the structure during an earthquake can be determined. Now, in order to control the obtained results, special criteria are needed to determine the need or not for the improvement of the existing buildings, and after carrying out all these measures and taking into account the results and if improvement is needed, the implementation method of the improvement will be determined. According to the contents stated and the beginning of the necessary measures for the seismic improvement of the existing buildings in the country, a guideline has been prepared by the country's management and planning organization, which is the basis of the building improvement studies. be placed This instruction is based on the regulation that was recently developed and recommended in the United States for the improvement of buildings. This regulation has been prepared by the American Society of Civil Engineers (ASCE) and the Federal Emergency Management Agency (FEMA) and has been published under the title of FEMA440 & FEMA356 & FEME273.

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The main purpose of this thesis is to evaluate the effectiveness of the methods of determining the performance level of the structure, proposed in the earthquake improvement guidelines in estimating the performance level of the steel building with a dual system of cross bracing and bending frame. In fact, the accuracy of static analysis is compared with nonlinear dynamic analysis.. It also answers the question of what level of performance the building designed with the third edition of Code 2800 is based on the guidelines for seismic improvement and whether the building in question meets the purpose of Code 2800. The working method in the thesis is that three symmetric and regular buildings of 4, 8, 16 stories steel double bending frame system - door brace and 7-shaped brace are designed with ETABS software in three dimensions (3D) based on regulation 2800, third edition, and on type 2 soil against earthquakes. Then, these three buildings were subjected to 3D non-linear static pushover analysis in the ETABSver9 software and the results were compared with the non-linear dynamic analysis of the time history of the models in the SAP2000ver12 software.

                       Chapter Two

Overview of the concepts and basics of improving building vibrations

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2-1- Introduction

In recent years, according to the economic value of the existing buildings, it has been tried to examine the stability of the buildings from the point of view of the necessary minimums, and in this regard, retrofitting guidelines have been formulated. According to the philosophy of these guidelines, the safety coefficients considered in the design regulations must be larger than the safety coefficients of these instructions. According to the above discussion, it is expected that the structures designed according to the usual design regulations will be approved by the retrofitting guidelines. In fact, the methods of retrofitting structures that are detailed in guidelines such as FEMA 356 and the retrofitting guidelines of our country can be considered as more accurate methods and in accordance with the design of existing structures. 2-2- Overview of seismic retrofitting preparations In this section, some basic definitions and preliminary seismic retrofitting, building performance levels, and earthquake risk levels from the point of view of retrofitting instructions are reviewed. Due to summarizing the definitions and methods, some of these definitions and methods are not included in this thesis, which will be referred to the improvement instructions if needed. Foundation improvement is expected to ensure the safety of the residents under a "risk level-1" earthquake.

2-2-1-2 - Optimal improvement

In optimal improvement, it is expected that the goal of improving the base is achieved and, in addition, the building will not collapse under a "risk level-2" earthquake.

2-2-1-3- Special improvement

In special improvement, a higher performance is achieved for the building in question than the desired improvement. is placed For this purpose, a higher performance level is considered for the building under the same level of earthquake risk used in the desired improvement, or by maintaining the same performance level as the desired improvement, a higher earthquake risk level is considered. "Danger Level-1", ensure the safety of residents.

2- Under an earthquake milder than "Danger Level-1", the building does not collapse or the limited life safety of residents is ensured. (1-2-1-4) The improvement instruction is carried out, but for some reasons, only a part of it is implemented in the existing conditions. In this case, the renovation should be planned and implemented in such a way that the goal of the renovation of other parts is met in the next stages [1].

Local renovation should be done according to the following:

1- The renovation of a part of the building should not lead to a decrease in the performance level of the entire building. caused by an earthquake in members that are in a critical state under an earthquake. 2-2-2- Building performance levels The building performance levels are defined based on the performance of structural and non-structural components and are briefly indicated by a number for the performance of structural components and a letter for the performance of non-structural components. 2-2-2-1- Performance levels of structural components The performance levels of structural components include four performance levels. Main and two intermediate performance levels.