Dissertation for obtaining a master's degree
in the field of civil engineering – Earthquake
Winter 2013
Abstract
Earthquakes close to the fault have important differences with earthquakes far from the fault due to having a pulse-type movement with a long period at the beginning of the record, impact force on existing structures, high ratio of maximum speed to maximum acceleration, and the presence of maximum acceleration, speed and displacement compared to earthquakes far from the fault.
In this study, the performance of steel bending frames in Tall structures are evaluated in the area near the fault. Due to the lack of data related to the earthquake near the fault and to solve this problem, a new method is used to simulate the earthquake near the fault by considering the frequency content. To perform analysis of three, nine and twenty story structures with steel bending frame system The SAC project, designed in Los Angeles, is used. Dynamic analysis of nonlinear time history is performed on the designed models in order to determine the performance levels and the amount of deformation of the members of the studied instruments, under the effect of earthquakes in the nearby area. Various damage criteria such as the maximum displacement of the floors and the displacement of the roof are evaluated as the response of the systems. The results of the analysis show that the special characteristics of the earthquake near the fault, such as the directional effect and the permanent displacement effect, can create a significant seismic demand in engineering structures compared to an earthquake without a velocity pulse.
Keywords: Earthquake near the fault, orientation, Mangar displacement, time history analysis
1 First chapter: Overview
1-1 Introduction
Inspection and explanation of strong earth vibrations in two branches of structural engineering (structure behavior analysis) and earthquake engineering (earth behavior analysis) are very important. It is worth noting that in order to minimize the damage caused by large earthquakes, the need to formulate, generalize and expand analytical perspectives on powerful earth vibrations has become more apparent.
The earthquake near the fault has special characteristics that distinguish it from the earthquake in the area far from the fault. The experience of past earthquakes has shown that these specifications have significant effects on the seismic demand of structures. Especially in the records of the speed of earthquakes near the fault, it means the presence of large development speeds of the earth, which are created in long-term acceleration pulses. A manifestation of these effects is in the form of creating large oscillatory displacements, which can also be seen in the Earth displacement record. The presence of these large values ??in the parameters of the ground motions near the fault is a characteristic feature of the records of earthquakes such as the Northridge earthquake, the Kobe earthquake, the Chichi earthquake in Taiwan, and for earthquakes near the fault, or in other words, earthquakes with a short distance from the seismogenic fault. Considering that many big cities of the world, including the metropolis of Tehran, are exposed to near-fault earthquakes, the need to know the characteristics of near-fault earthquakes and its effect on engineering systems seems more and more necessary. Statement of the problem
The increase in the population of cities close to active faults (such as Tehran, Tabriz, Los Angeles and Tokyo) doubles the probability of an earthquake with huge casualties in the near future. This problem comes from the fact that an earthquake near the fault [1] compared to an earthquake far from the fault, can impose a greater seismic demand on the structure. and as a result caused a lot of damage in past earthquakes[1]. This issue shows that investigating the seismic behavior of structures under an earthquake near the fault is very important.
Some of the characteristics that distinguish an earthquake near the fault from an earthquake far from the fault are the directionality effect [2], the permanent displacement effect [3] When the direction of the fault rupture is towards a particular site and the speed of the fault rupture is close to the speed of propagation of shear waves, the directionality effect is observed at that site. In this case, a significant share of energy is transferred to the site in a short period of time [2].A large amount of incoming energy in a short period of time causes a distinct pulse in the time history of the earthquake speed. This effect can often be seen in the component perpendicular to the fault [3]. The presence of this pulse makes the scale of the response spectrum (Sa) larger in periods close to the pulse period. Therefore, the response of structures under a pulsed earthquake will be different compared to an earthquake away from the fault. In addition to the above feature, the maps recorded during recent earthquakes near the fault, such as the Kokaeli earthquake[4] in Turkey (1999) and the Chi Chi[5] earthquake in Taiwan (1999), contain large amounts of permanent earth displacement, which is called permanent displacement. This deformation occurred during the sliding time in the direction of the fault slip and therefore it is generally visible in the parallel component of the fault. For this reason, in most cases, it is not combined with the effects caused by orientation [4]. Unlike the response of the structure under a normal earthquake, the distribution of requirements in the height of the structure for an earthquake near the fault is completely non-uniform [5]. These special characteristics for the response of structures under near-fault earthquakes make the study of the behavior of structures under these earthquakes worthy of further scrutiny. The development of design guidelines for structures close to seismically active springs requires a correct and proper understanding and perspective of the response and performance of structures under such earthquakes. One of the problems in examining the seismic response of structures under near-fault earthquakes is that the number of earthquake records that have characteristics of near-fault earthquakes has been recorded so far. In areas where information about past earthquakes is not available, this problem is double. One of the solutions for these areas is the use of artificial earthquakes. There are many ways to generate artificial earthquakes for a particular region. In general, these methods can be divided into probabilistic methods and deterministic methods. Probability methods can be divided into two general methods: point source [6] and wide source [7]. Since the earthquake away from the fault has a probabilistic nature, probabilistic methods are used for such earthquakes. Earthquakes near the fault have a different nature from earthquakes far from the fault, and deterministic methods are used to simulate these earthquakes. Hisada and Bilak [7] presented an efficient method based on the kinematic spring model, which is well able to simulate the effects of an earthquake near a fault, including the effects of directionality and permanent displacement. In this method, the earthquake source is modeled as a surface fault and the characteristics of the fault as well as the characteristics of different soil layers are also modeled. Finally, by solving the Green's functions, the earthquake map can be calculated at the desired site. For this purpose, we use the earthquake generated for the site, especially near the fault, and examine the seismic response of the structure under this earthquake. Finally, we will compare the seismic response of the structure under the earthquake near the fault with velocity pulse and also the earthquake without pulse.
1-3 Hypotheses and research questions
Hypotheses:
Simulation methods based on the spring kinematic model are well able to produce earthquakes near the fault.
• An earthquake near the fault has significant effects on the seismic performance of structures.
Questions:
• Is the need to change the location of records close to the fault compared to the need to change the location of records far from the fault?
• Are the effects of earthquakes near the fault on structures and the design of structures resistant to these types of movements possible without fully understanding the seismic characteristics of this phenomenon and its difference with the movement of the earth in the far area?
1-4 Research objectives
In this study, we are trying to investigate the response of multi-degree-of-freedom structures under near-fault earthquakes. The general goal is to know and improve one's understanding of the earthquake characteristics near the fault and its effect on the seismic response of structures. Therefore, we will be able to evaluate the current status of the design regulations to provide a safe plan against earthquakes near the fault. In this study, the earthquake produced with the help of seismological methods is used to produce an earthquake near the fault.