Master's Thesis
Field: Civil - Structures
Page 1-1-Problem definition:
Always in the science of structural engineering, there has been an attempt to predict a set of events that will have a significant effect on the efficiency and endurance of the structure during its useful life. These factors should be taken into consideration by the design engineer during the design of the structure and should be able to ensure the stability of the structure in the face of it.
Progressive failure is one of those factors whose reason for its occurrence is the failure to predict its occurrence at the time of the structure design and unfortunately causes serious injuries and disasters. The structure or a large part of it ends [8].”
Generally, buildings are not designed for loading conditions including gas explosions, bomb explosions, vehicle collisions, airplane collisions, storms, tornadoes and such loads. Hence, when buildings are exposed to such unusual loads, they may suffer great and irreparable damage.
The history of the progressive collapse problem as an engineering problem can be said to be rooted in the famous destruction of the Ronan Point 2 apartment building (Figure 1-1) in 1968. This is an example of progressive failure, which led to the total failure of the structure with the loss of the load-bearing member. The repetition of these types of progressive collapses made the design engineers look at such collapses as one of the design concerns and try to include them in the structure design process.
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In this regard, extensive research has been conducted by structural researchers in the world and its results have been presented in the form of guidelines and regulations in industrialized countries.
Progressive failure phenomenon can be studied by various analysis methods that range from very simple analyzes to very complex analyses, generally these analyzes can be done by using common finite element software such as SAP2000, which has full capability to consider dynamic and nonlinear properties.
It is clear that the phenomenon of progressive failure, due to its occurrence in a A very short period of time and the imposition of non-linear deformations on the elements before rupture is a dynamic and non-linear phenomenon. It has become an important research axis. Also, the structures designed according to the current regulations, due to the presence of any weakness in the design or implementation of the structural elements, may cause the phenomenon of progressive failure in the structures, while facing unusual loads or earthquakes, which may cause catastrophic human and financial losses. For this reason, if any of these parameters are likely, the structural designer should consider a plan to control the possible progressive collapse of the structure.
Perhaps it can be said that the structural designer should, like a director, plan the continuation of this scenario and have a solution in mind to stop it. done over the past few decades is well understood. However, the behavior of these systems in the face of progressive rupture when critical members are lost has not been fully investigated. Structures designed according to earthquake codes will not necessarily be resistant to progressive failure unless this is addressed. Considering that in Iran as well as in other parts of the world, the use of this type of system has a special place, therefore, it seems necessary to study and investigate the effect of this phenomenon on structures.
It is worth mentioning that in this thesis, the different methods of current valid regulations in the field of progressive deterioration are examined and instructions related to reducing deterioration are taken into consideration.Also, using nonlinear dynamic analysis, the resistance against progressive rupture in off-axis braced steel bending frames for a 9-story building sample has been investigated by designing models with different placement of braces in the openings and height of this building. In most of the researches conducted in the field of progressive failure, one plan with different heights has been used, but in this research, by using several different plans with the same height, the potential of progressive failure in the structure has been investigated.
The aim of this research is that in the first stage, the degree of vulnerability of braced bending frame steel structures designed by Iranian regulations and also the effect of factors such as changing the placement of braces in openings and height The structure, the place where the member is removed (side or middle) and the number of the floor where the member is removed, should be checked against the phenomenon of progressive failure, and after determining the vulnerable elements, in the second step, structures that are resistant to progressive failure should be designed. At the end, the resistance of these structures against progressive failure will be compared.
1-3- Research method:
In order to investigate the progressive rupture in the braced steel building, according to the national building regulations of Iran, including the sixth and tenth topics related to the loads on the building [1] and the design and implementation of steel buildings [2], a 9-story building with a system The double bending steel frame and off-axis bracing were designed with models with different placement of bracing in the openings and also in the height of the building. In the design of the models, the seismic considerations of the regulations for the design of buildings against earthquakes (Standard 2800)]3 were considered. The design of the mentioned building was done using ETABS software. Then, according to the steel sections related to the beams, columns and bracing, the desired building frame has been modeled in order to check the forces created in the frame members and the change of the location of the nodes using the nonlinear time history analysis method in SAP2000 software [4 and 10]. According to the incoming loads, the dynamic response of the system has been determined under the combination of the special progressive failure load from the UFC 2009 regulations [11] and their application model to the structure, as well as according to the analysis of the alternative path method and the sudden removal of the brace and column. It has been prepared and arranged, and its chapters are as follows:
The first chapter includes the definition of the problem, the goals pursued in this thesis, as well as the method and process of the research.
In the second chapter, first, a history of the occurrence of progressive damage in structures and its effect on the correction, authoring and development of regulations related to this phenomenon are examined. In the following, the basics of progressive deterioration and a comprehensive comparison of related regulations are discussed in valid international construction regulations. At the end of the chapter, a summary of the studies conducted in this field is presented.
The third chapter describes the concept of performance level and plastic joints, structural modeling and various analysis methods used in the UFC 2009 regulations.
The fourth chapter deals with how to apply loading in nonlinear dynamic analysis and examine the results of this analysis.
The fifth chapter deals with the conclusion and discussion of the results and finally offers suggestions for future research.
References and appendices are also provided at the end of this collection.
Chapter two: Basics of progressive failure and review of related regulations
L 2-1- Introduction:
Progressive failure can be defined as a chain reaction or propagation of failure in which, under certain causes, local damage occurs in a relatively small area of ??the structure, and in some circumstances, this local damage spreads to other parts of the structure and ultimately leads to the overall failure of the structure; In other words, sometimes the local failure of the member does not remain localized and spreads throughout the entire structure [12].
Possible hazards and unusual loads that can cause progressive failure include: design or construction error, fire, gas explosion, accidental overload, vehicle accident, bomb explosion, etc. Because the probability of occurrence of these risks is low, they are not considered in the structural design or they are addressed by indirect measurements. Most of them have the characteristic of action during a relatively short period of time and lead to dynamic responses [13].