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Structural Dynamics and Earthquake Engineering

by Biswajit JenaBiswajit Jena
Type: NoteInstitute: Dhaneswar Rath Institute of Engineering and Management Studies (DRIEMS) Specialization: Civil EngineeringOffline Downloads: 565Views: 8919Uploaded: 10 months ago

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Biswajit Jena
Biswajit Jena
STRUCTURAL DYNAMICS AND EARTHQUKE ENGINEERING UNIT I THEORY OF VIBRATIONS UNIT-I 1. 1
CONTENTS No TITLE TECHNICAL TERMS 1.1 INTRODUCTION TO STRUCTURAL DYNAMICS 1.2 IMPORTANCE OF STRUCTURAL DYNAMICS 1.3 TYPES AND SOURCES OF DYNAMIC LOADS 1.4 DISTINGUISHING FEATURES OF A DYNAMIC PROBLEMS 1.5 METHODOLOGY FOR DYNAMIC ANALYSIS 1.6 TYPES OF STRUCTURAL VIBRATION QUESTION BANK UNIT-I 1. 2
TECHNICAL TERMS 1. Mass: Mass is obtained by dividing the weight on body by the acceleration of gravity. Unit of mass is given in kilograms (kgs) 2. Stiffness: It is defined as the force required producing unit deformation. It is an elastic property that describes the level of resisting force that results when a body undergoes change in length. 3. Natural Period: It is defined as the time required to complete one cycle of free vibration 4. Natural frequency: When no external force acts on the system after giving it an initial displacement, the body vibrates. These vibrations are called free vibration and their frequency is natural frequency 5. Simple Harmonic Motion: The motion of a body to and fro about fixed point is called simple harmonic motion (SHM) 6. Amplitude: The maximum displacement or deformation of a vibrating system from its mean position is called as amplitude. 7. Mode of Vibration: The structure possesses as many natural frequencies as it has the degree of freedom. For each frequency the structure has its own way of vibration is known as mode of vibration 8. Fundamental mode of vibration: The fundamental mode of vibration of a structure is the mode having the lowest natural frequency. 9. Resonance: When the frequency of external force is equal to or matches with one of the natural frequencies of the vibrating system, the amplitude of vibration becomes excessively large. This phenomenon is called resonance. 10. Free Vibration: Free vibration is a vibration in which energy is neither added to nor removed from the vibrating system. It will just keep vibrating forever at the same amplitude. Except from some superconducting electronic oscillators, or possibly the motion of an electron in its orbit about an atomic nucleus, there are no free vibrations in nature. They are all damped to some extent. 11. Forced Vibration: Forced vibration is one in which energy is added to the vibrating system, as for example in a clockwork mechanism where the energy stored in a spring is UNIT-I transferred a bit at a time to the vibrating element. 1. 3
I BASICS OF STRUCTURAL DYNAMICS 1.1 Introduction to Structural Dynamics Structural analysis is mainly concerned with finding out the behaviour of a structure when subjected to some action. This action can be in the form of load due to the weight of things such as people, furniture, wind, snow, etc. or some other kind of excitation such as an earthquake, shaking of the ground due to a blast nearby, etc. In essence all these loads are dynamic, including the self-weight of the structure because at some point in time these loads were not there. The distinction is made between the dynamic and the static analysis on the basis of whether the applied action has enough acceleration in comparison to the structure's natural frequency. If a load is applied sufficiently slowly, the inertia forces (Newton's second law of motion) can be ignored and the analysis can be simplified as static analysis. Structural dynamics, therefore, is a type of structural analysis which covers the behaviour of structures subjected to dynamic (actions having high acceleration) loading. Dynamic loads include people, wind, waves, traffic, earthquakes, and blasts. Any structure can be subject to dynamic loading. Dynamic analysis can be used to find dynamic displacements, time history, and modal analysis. A dynamic analysis is also related to the inertia forces developed by a structure when it is excited by means of dynamic loads applied suddenly (e.g., wind blasts, explosion, earthquake). A static load is one which varies very slowly. A dynamic load is one which changes with time fairly quickly in comparison to the structure's natural frequency. If it changes slowly, the structure's response may be determined with static analysis, but if it varies quickly (relative to the structure's ability to respond), the response must be determined with a dynamic analysis. Dynamic analysis for simple structures can be carried out manually, but for complex structures finite element analysis can be used to calculate the mode shapes and frequencies. UNIT-I 1. 4

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