Investigating the effect of span length on bridge behavior under the simultaneous effect of horizontal and vertical components

Master's Thesis in Civil Engineering, Structural Orientation

Abstract

Investigating the effect of span length on bridge behavior under the simultaneous effect of horizontal and vertical components

Among the types of structures, bridges are one of the pillars of vital arteries that must be used after an earthquake to access hospitals, fire stations and other services; Therefore, it can be said that bridges have a special place in maintaining the required level of safety and operability. Although the investigation of recent earthquakes clearly shows that the vertical component of an earthquake may even exceed its horizontal component, currently, in order to design bridge vibrations, in the code of design and calculation of reinforced concrete bridges in Iran, there is no mention about the effect of the vertical component of the earthquake and also about the combination of all three seismic components (2 perpendicular horizontal components and 1 vertical component). The length of its decks has been analyzed. Expansion joint modeling, which includes modeling the impact of the decks on each other and modeling the cushions, modeling the soil interaction and its effect on the longitudinal displacement of the bridge according to the "F-H-W-E" relationships, assuming an elastic half-space medium in order to calculate the translational and torsional stiffness under the piles and columns and considering the effect of the bridge deck deflection, are among the most important parameters studied in this research. Bridges have been studied under the effect of 7 earthquakes far from the fault and 7 earthquakes close to the fault, once without considering the vertical component and again with the consideration of the vertical component. The results showed drastic changes (more than 50%) in the axial forces on the columns, these changes caused drastic changes in the forces in the column heads and the anchor in the middle of the deck. Finally, coefficients are presented in order to consider the vertical component of the earthquake.

Key words: vertical movements of earthquakes, response of earthquakes, load combinations, span length, structure and soil interaction.

Introduction

Among the types of structures, bridges have a more complex system structure than other normal structures. Also, they are one of the pillars of the vital arteries that need to be used after the earthquake for access to hospitals, fire stations and other services. According to the mentioned reasons, it can be said that bridges have a special place in maintaining the required level of safety and operability. Most of the bridge design regulations in the discussion of seismic analysis of bridges, either do not consider the effect of the vertical component or do not provide a specific method to consider the vertical component of the earthquake. However, the investigation of recent decades' earthquakes shows that the effect of the vertical component of the earthquake can be one of the main factors in the destruction of bridges in some cases.

In cases where the effect of the vertical component is included in the design, the spectrum function is usually 0.66 of the response spectrum of the horizontal component. However, new studies show that this ratio in the low periods and in the areas near the fault is an estimate against the direction of certainty.

In this research, in addition to investigating the simultaneous effect of two horizontal and vertical components of the earthquake, the simultaneous effect of all three components of the earthquake on the response of bridges has been investigated. In the first chapter, the researches and studies conducted on the effect of the vertical component of the earthquake on the response spectrum of the earthquake and the forces acting on the bridge have been investigated. Considering the importance of considering the interaction of the soil in the piles and under the columns with the bridge structure, in the second chapter, the forces acting on the soil and also the existing relationships have been investigated in order to consider the interaction of the soil. In the third chapter, the members of the superstructure and the substructure have been introduced. Types of column modeling methods, deck modeling, the effect of bridge deck deflection on the rigid performance of the deck, expansion joints and cushions as force transmitters from the deck to the columns have been introduced and investigated. In the fourth chapter, the most prominent features of OpenSys [1] software, which is the reason for choosing it for this research, are mentioned. Then, Paul Clements[2] is introduced for modeling purposes. At the end, by comparing the values ??of force responses and spatial change and the results in the article, the accuracy of the model made in Opensys software is controlled. In the fifth chapter, the results of the analysis of the bridges modeled in the fourth chapter are presented.In this regard, the effect of the vertical component of the earthquake on the seismic behavior of bridges has been investigated. Finally, in the sixth chapter, summaries of the results and suggestions of this research are stated. Iran, like other parts of the world such as Japan and America, is increasingly used due to the density of cars and the need to expand roads. However, the destruction of such huge bridges on highways and in cities as a result of various earthquakes in countries such as the United States, Japan and New Zealand shows the weaknesses in the current regulations of these countries. In this chapter, a review of past earthquakes that have a vertical component with high maximum acceleration, the effect of the vertical component on the deck and column of bridges, and the purpose of this research have been discussed. Prove the vulnerability of reinforced concrete structures against severe earthquakes, due to economic reasons, damage to structures is allowed to a certain extent, and the recognition of this damage is based on linear theory and engineering judgment. Knowing the ultimate capacity of reinforced concrete members under the effect of inelastic alternating loading.

In the case of seismic design of bridges, the San Fernando earthquake is considered a turning point. During this earthquake, 62 bridges were damaged in the central area of ??the earthquake and more than 15 million dollars were damaged. The different performance of this earthquake compared to previous earthquakes and characteristics that were not considered in the seismic design of bridges have been reported as the cause of these failures. During the past earthquakes, most of the damage was related to damage under the structure and the soil around it, while the main cause of damage or damage to bridges in the San Fernando earthquake was structural vibration. The most important causes of damage in this earthquake were: [[i]]:

Lack of ductility

Shortness of seat width in expansion joints and supports and finally deck damage

Shear failure in bridge columns and foundations, before flexural yielding occurs

Outing of reinforcements in vertical columns that are in the deck or The foundations were restrained

Failure of the foundations and embankments and its retaining walls and wing-shaped walls

After the San Fernando earthquake, extensive planning was prepared, many bridges were equipped with accelerometers, analytical modeling and various types of linear and non-linear analysis were developed to better understand the behavior of bridges, and a program to strengthen the existing bridges was implemented, which continues until now, but during subsequent earthquakes such as Kobe[5] and Lamaprita[6] again collapsed many bridges. Below is a brief description of the above three earthquakes.

1-2-1-         Report of the Lamaprita earthquake

The Lamaprita earthquake with a magnitude of 1.7 showed the excellent performance of the bridges designed according to recent regulations (Ashto[7] and ATC[8]). This earthquake also proved the effectiveness of restraining devices that were added to existing bridges in the bridge strengthening program. However, this earthquake questioned many of the design principles of old bridges. Thirteen bridges were severely damaged and closed, and seventy-eight bridges suffered a lot of damage [[ii] and [iii]].

Damages to bridges during this earthquake include:

- Complete destruction of parts of the Nimitz Bridge [9] due to the weakness of the structural system and inappropriate details (Figure 1-1).