Investigation of progressive collapse of truss bridges

Dissertation

Master's Course

Field: Civil Engineering

Abstract

Bridges are inseparable members of communication facilities and are very influential in economic and social conditions. The passage of time and random loads are among the things that threaten the health of bridges. Abnormal loads caused by natural accidents, execution errors and some other issues can cause progressive collapse in structures, so early detection of bridge damage can prevent disasters. Therefore, it is felt necessary to carefully investigate the behavior of these structures against progressive collapse. In this thesis, a method based on the investigation of the collapse of a multi-span truss bridge against the phenomenon of progressive collapse with scenarios of detection and removal of critical members and a comparison between different situations of removal and also the effect of scenarios of removal of critical members on the stability of the remaining structure is presented. After simulating 3D finite element models, it was subjected to different scenarios in order to evaluate the potential of progressive collapse. By comparing the results of linear static analysis, non-linear static analysis and non-linear dynamic analysis during the times of 0.5 seconds and 1 second, the dynamic load increase factor of the critical members in the average state of force changes for the upper and lower edge members was discussed. By comparing the dynamic load increase coefficient for two groups of key members of the upper and lower ridge and the average value of the key members in each group, it was concluded that the shorter the time of the applied impact due to removal, the greater the load increase coefficient, and it was also concluded that the faster the member removal is done, the load increase coefficient increases. Persian keywords: progressive collapse, critical area, truss bridge, load increase coefficient.

Chapter 1:  

Introduction and Generalities

1-1- Introduction

After the Second World War, the construction industry developed rapidly all over the world. However, the passage of time and the existence of various internal and external factors cause the structural components to be damaged and the structure to suffer serious problems and even destruction under operational loads. Considering all aspects in the design and early detection and appropriate action to fix the damage can prevent the catastrophic collapse of the structure. Therefore, in recent decades, a lot of research has been done in the field of identifying damage in structures.

Communication lines and infrastructure structures play an essential role in all countries, and a lot of money is spent on their construction and maintenance every year. Meanwhile, bridges play a key role in the economic, social and political conditions of a country. Therefore, in recent years, the amount of attention paid to monitoring the health of bridges has greatly increased, because there is a fundamental need to assess the condition of many bridges in the world. Studies show that more than 40% of existing bridges in Canada need to be repaired and retrofitted [1]. Among the 57,000 highway bridges in the United States in 1997, 187,000 of them were reported to be defective and it is stated that 5,000 more bridges are added to this number every year [2]. In 2001, it was stated that Japan has 140,000 bridges, most of which were built before 1980. Therefore, many of them are in dire need of maintenance[3].

Due to the location of Iran in an earthquake-prone region, the occurrence of multiple earthquakes can cause severe damage to various types of structures. In addition, daily traffic and its increase can be a factor for the damage of bridges. Also, the lifespan of many existing bridges in the country reaches more than 30 years. On the other hand, the amount of construction of different types of bridges in Iran is increasing. As a result, the maintenance and control of bridges can provide an effective role in the country [4].

A rare but very harmful phenomenon in structures is the phenomenon of progressive collapse [1]. This phenomenon is a dynamic effect resulting from the expansion and successive development of the primary rupture in a structure, which indicates a severe mismatch between the causative factor and severe collapse. The factor that causes progressive collapse is a local and concentrated factor such as lack of local resistance that causes a catastrophic phenomenon [5].

Due to the occurrence of unpredictable events such as earthquakes, explosions, collisions and accidents, as well as the possible reduction of the structure's capacity due to the passage of time and the effect of these factors on the occurrence of progressive collapse in bridges, the investigation of the effects of strengthening structural components on the resistance against progressive collapse has been considered. Past studies show that earthquake strengthening of the structure can make the structure resistant to progressive collapse in the face of unusual factors. On the other hand, increasing ductility can accelerate the mechanism of progressive collapse.

One of the famous incidents in the field of collapse is the collapse of the I-35W steel truss bridge over the Mississippi River, located in the state of Minnesota[2], in the United States. As shown in Figure (1-1), this bridge suddenly collapsed on August 1, 2007, killing 13 and injuring more than 100. The investigation report on the causes of the collapse of the I-35W bridge shows that the dead load of the deck was increased several times due to the repair and strengthening of the slab, and the thickness of the gusset plates used in the bridge was only half of the designed thickness. In addition, on the day of the collapse, there were construction materials and heavy machinery on the bridge for maintenance. These factors have caused the collapse of the I-35W bridge[6]. According to Figure (1-2), in 2007, in China, a cargo ship collided with the foundation of the Jiangdong Jingjiang Bridge[3], causing the collapse of four spans adjacent to the foundation. Investigations show that the collision force of the ship was more than the permissible design force and as a result caused damage to the bridge foundation. As a result of this damage, the internal forces changed and the redistributed forces exceeded the capacity of the side foundations, and as a result, progressive collapse occurred[7]. Other examples of progressive collapse occurred in Baihua Bridge[4] in Figure (1-3) as a result of the Wenchuan earthquake[5]. In this bridge, due to the damage of the supports, the bridge deck has collapsed along with twisting deformations [7, 8].

1-2-

(Images are available in the main file)

Ensuring the safety of human lives and vehicles passing over the bridges, as well as maintaining the health of the bridge structure, as well as preventing possible accidents such as settlement due to the weight of the structure, cracks in the structures, or partial collapse of the structure. The cause of decay, wear and tear, corrosion, construction error, collision of passing vehicles on or under the bridge or collision with a bridge foundation, explosion or terrorist attacks and similar cases, has become one of the important issues in the field of bridge construction and maintenance, which includes truss or cable-stayed bridges. Various solutions to solve these problems have been provided by researchers. But one of the important issues in the field of truss bridges is the sudden removal of one or more members of the bridge due to an external factor, which causes the collapse of a part of the bridge, or sometimes appears as a progressive collapse phenomenon and leads to the collapse of the entire bridge. Today, the regulations and especially the regulations for the design of bridges and suspended structures against progressive collapse around the world have covered these weaknesses to some extent by providing special design solutions to reduce the possibility of progressive collapse. However, designing with high reliability coefficients is not always cost-effective. Therefore, in this thesis, an attempt has been made to present a practical method to identify critical members and strengthen them in order to reduce the possibility of progressive collapse of the bridge.

1-3-

In this research, after performing a static analysis on the truss bridge in a healthy structural state, by removing the candidate members in four groups, during 48 scenarios with the help of a factor called the change index, a comparison of the force ratio of each member in the state of healthy and damaged structure was made and finally Each group of two members was identified as a critical member, and a total of eight critical members (with a key member) were identified, of which 4 members were removed due to the instability of the structure due to the failure of the key member. Then, by removing the key member in each scenario with the help of linear static analysis and nonlinear static analysis and nonlinear dynamic analysis, the change index was obtained for each key member, and finally, with the help of dynamic load increase coefficient, for the remaining four key members, the dynamic load increase coefficient was compared in three modes of linear static analysis, nonlinear static analysis, nonlinear dynamic analysis in 0.5 second mode and nonlinear dynamic analysis in 1 second mode for each member. was paid Finally, appropriate solutions will be presented to deal with the phenomenon of progressive collapse.

Therefore, the purpose of this research is:

<>Introducing the phenomenon of progressive collapse and explaining the cause of its occurrence as well as the process of this phenomenon in structures, especially bridges, and the methods of dealing with this dangerous phenomenon.