Investigating the performance of concrete bridges with retracted box section under the effect of blast loads

Master's Thesis in Civil Engineering

Earthquake Tendency

Abstract:

Bridge structures are affected by various loadings and are designed according to the level of risk that is considered for them. Explosive loading is one of the loads that threaten the health of bridges. It is necessary to evaluate the amount of damage and performance of bridges under the effect of this loading due to their strategic importance. In this thesis, during four chapters, it has been tried to investigate the performance of concrete bridges with prestressed box section under the effect of explosive load. In the first chapter of the statement of the problem, the necessity of conducting this research and general information about the problem is written. In the second chapter, the phenomenon of explosion, explosive loading and its parameters are introduced, then the behavior of materials at high strain rates (impact loads) is briefly stated. The history of numerical and laboratory research conducted in the field of explosion effect on bridges is also explained in this chapter.

In the third chapter, the researched bridge and its modeling techniques in finite element software are described. The research method and explosive scenarios are defined in this chapter. The characteristics and modeling of materials are presented. In the fourth chapter, the results of explosive scenarios are presented and discussed. The results show that the bridge is very vulnerable under the effect of different explosive scenarios and the possibility of its collapse is high. After breaking the deck slab, the blast wave penetrates into the box and intensifies. The reinforcements are broken and the prestressing cables flow in the end restraining areas, then due to the increase in the strain in the cables, the effective prestressing force on the section decreases. This is while the tension in the cables is more than the static flow, but due to the effect of the strain rate in the points outside the end restraining area, they do not flow or break. Chapter 1: Generalities 1-1-Introduction Undoubtedly, the event of September 11, 2001 was one of the biggest terrorist attacks of mankind. This incident shows that structures are always exposed to explosion risks. During the past few decades, many terrorist attacks have occurred all over the world, which have left many financial and human damages. The result of this event has been the need for more engineers to design explosion-resistant structures. For example, the lessons learned from the bombing in Oklahoma[1] in 1995 and the attack on the American embassy in Tanzania[2] and Nairobi[3] in 1998 initiated the development of regulations for the design of structures against explosions. With the expansion of terrorist operations, the occurrence of various wars, the occurrence of accidents that lead to explosions (explosion of a vehicle carrying fuel), the need to study and determine appropriate solutions to prevent serious damage to strategic and vital structures is felt more than ever. After the birth of explosion technology, many experiments and researches have been conducted by engineers and scientists on explosive materials and charges. Steel and concrete as the most common construction materials due to the importance and extent of use in construction projects have allocated the majority of these researches and explosive researches. Few significant researches have been done in the field of explosion effects on bridges, so for the design of bridges under the effect of explosions, experimental, numerical and analytical researches are needed in order to provide sufficient knowledge for the development of bridge design regulations against explosions.

After September 11, many efforts were made to focus on the security of the transportation system. So that several specialized groups came together to provide suggestions and solutions to prevent terrorist attacks against bridges. One of the first research projects in this field was initiated in the Texas Department of Transportation. The focus of this research was to develop solutions to improve the performance of all types of bridges against terrorist attacks. This research was done with the help of methods based on parametric studies with simple analytical models [1]. Recently, US Army engineers investigated the performance of metal and concrete towers of suspension and cable bridges that have been subjected to near-explosive loads. The rest of the research focused more on bridge members. For example, Fujikura[4] investigated the performance of frame foundations of bridges during laboratory research. Also, US Army engineers conducted similar research on pre-stressed beams.Although a lot of research has been done in this field, this field is still new[2]. 1-2- The necessity of investigating the behavior of bridges under the effect of explosive load. This tragic incident is a warning to officials and engineers to pay more attention to structures that may be damaged by explosive charges. If we think more about this issue, we can realize the vulnerability of strategic structures under explosive loading. Transportation structures are among these strategic structures. As mentioned, in the field of transportation, transportation systems are at risk of damage due to explosions. Information collected by Minta Transportation Institute[5] indicates that at least 53 terrorist attacks have occurred between 1998 and 2006 to destroy bridges in different parts of the world[3].  Out of this number, about 60% were bombed. Past observations have shown that terrorists are interested in attacking highway bridges that exist in industrial cities, because by damaging these bridges they can effectively hit their target. For example, suicide attacks and car explosions on two highway bridges in Iraq, which led to their collapse.

Besides terrorist attacks, accidents that lead to explosions on bridges are also among the explosion risks that threaten bridges. The car accident and their explosion that led to the collapse of the Weber Fall Bridge on I-40 [6] is an example of this type of risk. Also, the i-35 bridge in Minnesota [7] also collapsed in a similar way [4].  In the 8 years of Iraq's imposed war against Iran, PMP bridges that were used for the passage of manpower and military equipment over the rivers show the importance and bold role of bridges in the hard days of the war. According to the mentioned cases, it is necessary to pay more attention to bridges and to design resistant bridges or to improve existing bridges against explosive loads. 3- Research structure and objectives Designing and construction of concrete bridges with retracted box section has developed a lot. The parts of such bridges can be prefabricated and then sewn together with the help of cables. In addition to providing the possibility of producing concrete with very high resistance, the precast method reduces shrinkage and creep strains to a great extent. This type of implementation is very suitable for the construction of valley bridges and city bridges where it is not possible to stop the traffic under the bridge due to the removal of any timber under the bridge. The range of economic span for such bridges is 30 to 120 meters, while the span length of most bridges is also in this range. Therefore, due to the large implementation of these types of bridges, the necessity of investigating the performance of concrete bridges with retracted box section under the effect of the loads caused by the explosion appears.

The present research includes 6 chapters. In this thesis, first, the retracted box bridge with spans of 37+68+37 meters is analyzed and designed in Sap2000 software according to valid regulations. Then 8 finite element models of this bridge are modeled in ANSYS-AUTODYN software. The difference between these 8 models is the location of the explosive and the distribution of the live charge on the bridge. The objectives of this research are as follows.

Evaluation of the amount of damage caused to the bridge, under the blast wave that has intensified due to penetration into the deck box.

Observation and follow-up of the stress distribution in the bridge components

The effect of the traffic load distribution on the vulnerability

1 Chapter 2: Review of the subject literature

2-1-Introduction

Explosion prediction techniques are often divided into explosive charge determination and response determination methods. Each of these categories can be divided into two main and experimental groups. The main group begins to predict the explosion using the laws of physics, while the experimental group pursues this goal with the help of experiments.

In order to propagate the explosion waves realistically, it is necessary to consider atmospheric conditions, boundary effects, explosive materials and many other parameters, which is a difficult task. Also, explosion pressure changes due to large deformations of the structure and local damages should be included in the calculations.