A study of seismic improvement of highway bridges, a case study of Shahid Haqqani bridge in Tehran

Thesis‌ to receive a master's degree

in the field of civil engineering with structural orientation

February month  2013 

Abstract:

In recent years, significant progress has been made in the field of improvement and retrofitting of buildings in our country, however, in the field of infrastructure structures such as bridges, a completely codified set has not yet been presented. Many of the existing bridges in our country are designed based on the old by-laws, and with the change of the by-laws, these bridges do not have the necessary strength to withstand moderate to strong earthquakes. In this regard, the seismic vulnerability of the Shahid Haqqani Bridge in Tehran has been studied in detail, the detailed investigation is carried out in both qualitative and quantitative ways, so the forces of the main members of the bridge, including columns and other components, are calculated using spectral dynamic analysis, and based on FHWA regulations 95, using the capacity-to-demand ratio method, the possible weaknesses of the bridge are expressed. Ratios smaller than one will be evaluated for improvement, and finally, according to economic and operational issues, improvement is considered and the necessary plans for improvement are presented. The studied bridge is concrete and has an arch in the plan, the modeling of the bridge is done by SAP 2000 V15 software. The results of the analysis show the weakness of the bridge in some parts. The ratio of the capacity to the bending and axial demand of some bridge piers, especially the middle piers, is inappropriate and needs to be strengthened. The bridge bags and the bases under it are considered suitable. The cell deck of the bridge was analytically evaluated from the bending and shear point of view and its adequacy was confirmed. The length of the patch and the patch location of the reinforcements are evaluated as suitable. Regarding the required seat width, no problem was observed according to the regulations, but since the bridge deck is diagonal to the piers, the longitudinal component of the earthquake force in the seat has both a longitudinal and a transverse reaction, so it is suggested that the deck be connected to the seat by means of a restraining rod. Necessary measures should be taken to prevent soil movement during an earthquake from between the pile foundations. Finally, based on the principles of value engineering and the investigations carried out, it is suggested that the reinforcement of foundations be evaluated and carried out by the external posttension method by CFRP fibers.

Key words: vulnerability, capacity-to-demand ratio, seismic design, reinforced concrete bridges

Chapter 1: Speaking

1-1- Introduction

The importance of considering earthquake effects in the design of engineering structures was recognized for the first time in 1923 after the devastating damage of the Kanto[1] (Honshu Island[2] in Japan) earthquake. In those days, bridges were built based on technology from the United States of America, England, Germany, and France, regardless of the effects of seismic disturbances. The common damage caused by earthquakes at that time was in the form of lateral displacement, which caused the collapse of the entire bridge system. This was a common damage that did not consider seismic effects, or when seismic design was not sufficient, especially for the foundation. This type of damage was more visible in bridges from 1920 to 1950. Therefore, counter-seismic measures were started after the 1923 Kanto earthquake. The equivalent static lateral force method using the seismic coefficient of 0.3-0.1 based on the allowable stress design method, which is often called the seismic coefficient method, was determined as a method to deal with earthquake force and was first registered in the design of bridges in 1927. Since then, seismic design practice has improved based on seismic damage and research progress. However, the knowledge of design was still in the initial stage and mostly to ensure the seismic performance, that is why many other important factors such as the real movement field of the earth, plasticity and dynamic response and other points were not included in the designs, but this was a start to deal with the effect of the earthquake on the structure. 2-1- The purpose of the research and its objectives. Important structures are affected by seismic excitations. In any country, roads and railways are very important as vital arteries of a country. In times of war or other crises such as earthquakes, these roads are considered as the main ways to provide aid, and if they are damaged, it is very difficult to replace them.The requirement for continuous use of these types of structures is to reduce the damage caused to them to the minimum possible level, so that they can be rebuilt without traffic restrictions. Surveys show that the bridges built in the last few decades have major weaknesses in design and implementation. These bridges are usually designed by gravity and special seismic criteria are not included in them. On the other hand, unlike residential structures, in order to function uninterruptedly, bridges must have operational conditions after an earthquake. In the design of many bridges in the past, due to the lack of necessary design knowledge at the time of their construction, the destructive effects of earthquakes were not considered. The occurrence of numerous failures and failures in past earthquakes in different countries such as 1995 Kobe, Japan[3], 1994 Northridge  California[4] in the United States, showed the vulnerability of these structures against earthquakes. Since the earthquakes that occurred in Iran in the last century were mostly not within the limits of big cities, and also because bridges are planned and implemented by government institutions and under better supervision than normal buildings, fortunately we have not seen severe damage of this type of structures in Iran, but the examination of bridges in Tehran has determined that in the event of relatively severe earthquakes, damage to these bridges is not far from expected. Existing bridges can be considered among the most effective measures in reducing costs and losses caused by earthquakes. Seismic retrofitting includes two basic steps, checking the seismic behavior of existing structures and then checking the methods of improving the seismic behavior. In addition to their use for the seismic strengthening of existing structures, these surveys provide an effective means to increase the knowledge of seismic design and improve the quality of design and implementation of new structures by identifying the weak points of the structure. Before using seismic strengthening methods for a structure, it is necessary to know which part of the structure is vulnerable and with what quality. In order to detect this vulnerability, there are various methods that are generally divided into two categories: qualitative methods and quantitative methods. In fact, the purpose of seismic vulnerability assessment is to determine the level of classified seismic risk that threatens the bridge using quantitative results. The purpose of this research is to collect information regarding the seismic vulnerability assessment of one of Tehran's bridges, as we know the need to conduct vulnerability assessment studies and also to improve bridges that are considered as national capital, especially for bridges that were designed and implemented with old regulations, is a vital matter. But since it is not possible to perform these steps for all types of bridges, during this research, some important and practical methods are explained for the purpose of evaluating the seismic vulnerability of bridges. The evaluation process and its steps are similar in many bridges, so by using some similarities in the process of analytical evaluation of bridges, the results of this research can be generalized to concrete bridges with two-element deck system and frame-shaped columns in some cases that will be mentioned. This subject has much room for research and it is appropriate for the researchers to continue the open research horizons in a detailed and detailed manner so that by recognizing the weakness of bridges and implementing  seismic rehabilitation plans, we will not witness severe damage to these sensitive structures when an earthquake occurs.

The second chapter has looked at the types of damage caused to bridges caused by earthquakes, on the basis of this, the reasons for this damage and the weak points of bridges have been examined, as well as the research conducted in the field of bridge design and vulnerability.

In the third chapter, the methods of assessing the vulnerability of bridges, design situations, modeling methods and determining the seismic vulnerability of bridges have been discussed in qualitative and quantitative methods.