Dissertation for Master's Degree
Civil-Structural Field
Winter 1392
Abstract
In recent years, the number of buildings that have suffered damage due to the loss of their gravity-bearing members has increased. Therefore, it has become important to design structures so that they are resistant to progressive damage in addition to the effects of earthquakes. One of the factors that are very important in the design of structures against progressive deterioration is the type of structural system and building configuration, as well as the retrofitting of these structures and the behavior of the retrofitted structure against progressive deterioration. In this research, using alternative path method and non-linear dynamic analysis based on UFC regulations, the potential of progressive failure has been evaluated in two models with a combined structural system of flexural bracing frame with two different bracing arrangements designed using Iranian regulations. After that, these two models with the presence of truss belt in the roof floor were analyzed against the phenomenon of progressive failure and the effective role of this type of retrofitting in reducing the probability of structural failure against progressive failure was evaluated. In the following, the same structure with a bending frame system and also a braced frame with articulated connections were examined in two-dimensional and three-dimensional states. After the modeling and analysis, it was observed that the structures that were designed according to Iran's regulations against earthquakes do not have the ability to resist progressive damage and need to be strengthened. After retrofitting the mentioned structures, it was observed that the structure with the combination of brace and truss belt has the ability to deal with progressive damage. Considering that in this method, the wind braces of the structure, while being lateral load-bearing members, also increase the resistance of the structure against progressive failure, the amount of increase in the mass of the skeleton of the structure resisted against progressive failure is not much compared to the structure designed according to the Iranian earthquake code.
Key words: progressive failure, nonlinear dynamic analysis, alternative paths method, combined load-bearing system, truss belt.
Chapter 1
Generalities
1-1 Introduction
According to ASCE7 [1] progressive failure is defined as the spread of failure in a structure from one element to another so that it eventually leads to the failure of the entire structure or a major part of it. The factors that can lead to this type of failure are: car impact, gas explosion, plane crash, construction error, fire, Accidental excessive loading on members, explosion, etc. Most of these events have a short duration of impact, which result in dynamic responses.
In traditional structural design regulations, the discussion of progressive failure was considered indirectly and by defining the degree of importance for structures, but recently, regulations have been compiled for the discussion of progressive failure in structures. Among the most valid and newest of these regulations, UFC 4-023-03 [2] and GSA2003 [3] regulations can be mentioned. The UFC regulations were first compiled in 2005 and then revised in 2010.
The presence of two factors is required for progressive failure to occur in a structure. The first factor is an abnormal loading that can cause initial failure in structural members, and the second factor is the lack of continuity, ductility, and sufficient degree of uncertainty in the structure that causes initial failure in structural members. In order to control the phenomenon of progressive failure in structures, one of the above two factors must be controlled. That is, either structural members should be designed in such a way that they will not be damaged by abnormal loads, or the structure should be designed in such a way that in case of failure of one of its gravity-bearing elements, the damage will not spread, which requires continuity, ductility and sufficient degree of uncertainty in the structure.
was analyzed and its results were compared. The structure must be designed in such a way that after the removal of each of its gravity-bearing elements, there are alternative paths to bear the incoming loads and the structures do not suffer damage. In this method, after removing the desired element, the changes in the members are calculated and according to the type of analysis, they are compared with the permitted values ??of the regulations, and all values ??must be within the permitted limits of the regulations. In the UFC regulations, three methods of linear static analysis, nonlinear static, and nonlinear dynamic analysis are proposed to investigate this phenomenon, and in this research, the nonlinear dynamic analysis method was used to analyze the models. The end of the structures was examined with a truss belt on the roof floor so that finally a structure can be achieved with a combination of bracing system and truss belt, which has the most resistance to the progressive failure phenomenon. and then how to investigate the phenomenon of progressive failure using nonlinear static and dynamic nonlinear analysis according to the criteria of the UFC regulations, and also an overview of the work done in the field of progressive failure phenomenon. In the third chapter of this thesis, the analysis and design, geometry and loading of the structure and the sections used for the models are given.
In the fourth chapter of this thesis, the results of the nonlinear dynamic analysis for all the models in the retrofitted state with the truss belt have been examined. The fifth chapter of this thesis also includes conclusions and suggestions for the continuation of this topic. Chapter Two: Basics of Progressive Failure Introduction 2-1 Introduction Progressive failure is defined as follows in a book compiled by the American National Institute of Standards and Technology [4]: ??Progressive failure refers to a chain of failures in a structure that are caused by initial failure in the structure. So that this damage occurred in the structure is disproportionate to the initial damage. UFC regulations determine the design requirements for existing buildings and new buildings in such a way that the potential for progressive damage in them is minimized and these structures suffer minor and limited structural damage against the usual unpredictable events. 2-2 Importance of discussing progressive damage In examining the progressive damage phenomenon, the factor Abnormal loading and minor damage caused by it should be investigated and then, according to the damage that has occurred, some of the structure's abilities, including continuity, ductility, and the uncertain degree of the structure, should be investigated to prevent the spread of damage.
The phenomenon of progressive damage is a rare phenomenon in most parts of the world, however, if this phenomenon occurs, it can leave very unfortunate results. Considering the increase in terrorist attacks on important buildings in recent years, it seems necessary to study and examine this discussion more closely.
For example, on April 15, 1995, in the bombing of the Alfredmora building [1] in Oklahoma City, 168 people died due to the damage of part of the structure and not due to the direct effects of the explosion, and more than 800 people were injured. The north side of this 9-story reinforced concrete building, due to the explosion of a bomb equivalent to 4000 pounds of TNT, could not withstand the vertical distribution of the load and finally caused this collapse (Figure 1-2). The said building, a 22-story building made of building panels, occurred on the 18th floor. The explosion blew out the front load-bearing wall and the relief wall, thus removing the support of the upper floors. The lack of continuity between the structural components and the lack of a secondary road for carrying cargo led to the rupture of all the upper and lower floors. This is an example of progressive failure where the loss of a load-bearing member leads to the failure of the entire structure (Figure 2-2).