Vibration Analyzers - Seminar Report

 vibration analyzers
Many of the earthquakes give the little or no warning before occurring and this is the reason why the earthquake engineering is complex. Experimental testing of the structural system is essential for improving knowledge about component and system performance in earthquake. Shaking table test providing important experimental data about critical issue such as the effect of component damage on the system response, collapse mechanisms, residual, deformation, and post earthquake capacity. Even with this facility, most structural system is too large to test at or near full scale.
In many fields, there is great uncertainty as to whether a new design will actually do what is desired. New designs often have unexpected problems. A prototype is often used as part of the product design process to allow engineers and designers the ability to explore design alternatives, test theories and confirm performance prior to starting production of a new product. Engineers use their experience to tailor the prototype according to the specific unknowns still present in the intended design. For example, some prototypes are used to confirm and verify consumer interest in a proposed design whereas other prototypes will attempt to verify the performance or suitability of a specific design approach.
Even with these recent advantages, structural testing has typically been conducted using customized software that is dependent on the configuration and computation procedure for the test method. Investigation on the dynamic behaviors of the large scale civil engineering structures such as buildings and bridges by performing full scale test is very difficult or often practically impossible to be realize due to the size, weight, and cost etc. therefore the behavior of the whole structure is estimated generally based on the test results obtained by using scaled down model.
Most structures are subjected to vibrations. Vibration means to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.  Vibration is occasionally "desirable". For example, the vibration motions of engines, electric motors, or any mechanical device in operation are typically unwanted. There are two types of the vibrations in the structural dynamics;
a) Free vibration
b) Forced vibration
Free vibration occurs when a mechanical system is set off with an initial input and then allowed to vibrate freely. Examples of this type of vibration are pulling a child back on a swing and then letting go or hitting a tuning fork and letting it ring. The mechanical system will then vibrate at one or more of its "natural frequency" and damp down to zero.
Forced vibration is when an alternating force or motion is applied to a mechanical system. Examples of this type of vibration include a shaking washing machine due to an imbalance, transportation vibration (caused by truck engine, springs, road, etc.), or the vibration of a building during an earthquake. In forced vibration the frequency of the vibration is the frequency of the force or motion applied, with order of magnitude being dependent on the actual mechanical system. Types of the structural vibration of the system .
These vibrations may arise from wind forces, earthquake excitations, machine vibrations, or many other sources. In some cases, especially under strong earthquake excitations, these vibrations can cause structural damage or even structural collapse. The higher the inherent or natural damping in structures, lower will be the likelihood of damage.

Vibration measurement instruments and vibration analyzers are used for measuring, displaying and analyzing vibrations. Typically, these instruments comprise of a transducer, data acquisition and either a local display or some sort of output to a computer or another instrument. Vibration measurement instruments and vibration analyzers can have many features, including incorporating features such as totalizing, local or remote display and data recording. They may be stationary or else portable field-type instruments.
Vibration measurement instruments and vibration analyzers can accept a number of different types of transducers, including acceleration, linear velocity, proximity and displacement, rotary velocity and temperature.  In addition, many vibration instruments can take generic signal inputs, including voltage, current, frequency and serial inputs.  Some of these instruments can even accept wireless data transmissions.
Four main features must be considered when selecting vibration measurement instruments and vibration analyzers: number of channels, accuracy, sampling frequency and ambient conditions. Electrical output options depend on the system being used with the vibration instruments. Common analog options are voltage, current or frequency. Digital output choices are the standard parallel and serial signals. Another option is to use vibration instruments with an output of a change in state of switches or alarms. Two further output options are important to consider. Vibration instruments can often output acceleration, velocity or displacement values as well as standard vibration readings.
Vibration instruments come in different form factors. As mentioned above, they can be stationary or portable. Another slightly different option is a handheld device, meaning that the instrument is actually small enough to operate in one’s hand, as opposed to being a portable device with wheels or a handle. One such device. the above the device shown is single channel FFT analyzer.

The user interface of these devices can be as simple as an analog readout or as complex as an actual computer. Vibration instruments can be operated either manually or via a host computer, can have software support for computer interfacing, and can even have hard drives, removable media or nonvolatile memory options.

The literatures on shake table tests are reviewed for past studies on dynamic analysis.
Agababian et al. (1990) in this paper, procedures are discussed for the evolution of some of the earthquake damage mitigation methods in use or under development at the museum. Generic models for various categories of subjects have been formulated and devised that allow the assessment of susceptibility of objects to rocking, sliding and overturning and stress failure when subjected to earthquake induced forces. The possibility of damaged to dedicated objects on display museum during an earthquake is significant even if the building structure itself remains intact. Although resistance to damage can be improved by modifications either to the object or its supporting altering the support is preferred method.
 Augusti and Ciampoli (1996) summarizes and completes previous researches on the seismic response of damage of art museum and presents simple low coast rules for reducing the risk damage in case of medium intensity earthquake.
Atkiston and Beresnev (1998) presented, ground motion time history which is capable with uniform hazard spectra provided by the new national seismic hazard maps of Geotechnical Survey of Canada are simulated. It has compatible time histories for these spectra, in order that dynamic analysis requiring the use of time histories can be employed. The simulated records provide a realistic representation of ground motion for earthquake magnitudes and distances that contribute most strongly to hazard at the selected cities and probability level.
Calio and Marletta (2003) presented the passive control of vibration art object excitations. The art objects modeled as a rigid block simply supported on pedestal which is connected to a visco-elastic device in order to obtained passive controlled system. If subject to seismic excitation the object may tilt over the movable supporting mass and eventually overturn, while sliding is presented by means of seismic restrains.
Shakib and Fuladgar (2003) presented that evolution of effect of three component of earthquake excitation on the response of pure friction base isolated structure. The structure is idealized as a three dimensional single storey resting on the sliding support. The transmission of ground motion of structure can be effectively controlled through isolation of the structure at its base. This involves an especially foundation system that limits the intensity of the ground motion transmitted the superstructure.
Takahashi and Fenves (2005) presented, structural testing involves imposing displacement or forcing boundary conditions on a specimen according to a test method and loading protocol. There are many types’ experimental setups for applying boundary conditions. The actuators are controlled by the control system but the most software for computing the control signals is customized for an experimental site and test method, as consequence of the method. As a consequence of the specialized software, it is difficult to developed and implement new test methods, such as hybrid testing or geographically distributed testing, or to exchange software from one laboratory to another. Prior to developing software from one address this problem, the requirements for the test methods are summarized.
Neurohr (2005) presented, building elements be divided into two groups; structural component and operational and functional components (OFCs). Experience from past earthquakes has shown that damage from these components can be substantial and can pose significant life safety hazards, especially in areas of low seismic risk. Hence, like structural systems, OFCs have to be designed to resist seismic forces. Observation of damage due to earthquakes in past demonstrated that proper design for non structural components cannot be neglected. As a result, research on OFCs has increased over the past decade, and the need to research specific topics still exists.
Rai and Sinha (2009) presented, a relatively simple system has been assembled with care to ensure an adequate replication of input motion by the shake table system. Subjective comparisons of input signal vs. shake table response, in both time and frequency domain have been utilized to provide a measure of the capabilities of the simulators  to reproduce earthquake motion scaled according to similitude laws. The ground motion is multidirectional in reality and its simulation in the simulation laboratory with multi-axis shake table system complex and costly. A single axis shake table is the simplest form of earthquake simulator which is not only useful for many investigations when it is only desirable to excite the specimen in one axis, but also simplifies sub-sequent interpretation of the results.

Analytical methods are now available to analyze and evaluate structures installed with these devices. The study of structural dynamics in civil engineering curriculum is commonly perceived to be a difficult exercise, more so in India. One of the most   effective ways to achieve this would be to develop a suite of simple experimental setups   which would enable the study of basic issues related to vibration behavior, such as, damping, dynamic response magnification, and resonance, structural vibration under support motions, normal modes, vibration isolation, and vibration absorption. These   setups would provide valuable physical insights into the basic vibration behavior of structures in general and, structural dynamic responses under base motions, in particular.
This is the only experimental technique for direct simulation of inertia forces, which can be used to simulate different types of motion such as recorded earthquake ground motions, sine sweeps, etc. Shake table test results enhance further the understanding of the behavior of various numerical tools used for analysis. This facility can be utilize for verification of earthquake resistant design of buildings, other structures, mechanical components devices , etc.

The main objective of this project is to study the behavior of various structural models on shake tables available in the department of applied mechanics, by making shake table systems operational.
The study is reported in different scaled models of buildings of various plans and materials like aluminum and steel. The acceleration of different responses of the structural models is measured with sinusoidal base excitation of different amplitudes and frequencies.
It is observed that the study also involves comparison of shake table (experimental) responses with the corresponding area time history analysis results obtained using the structural analysis package SAP2000.
The specific objectives of the present work are:
1. To study in detail the properties and parameters of the different types of shake table placed in NIT Nagpur.
2. To study and understand the results obtained from the FFT analyzer SA 77.
3. To study and understand input and output of the 50 kg-f shake table.
4. To develop scaled model of buildings of aluminum and steel material of various plans and dimensions and study using nonlinear time history analysis under sinusoidal base excitation.
5. To measure the experimental results of the above models
6. To analyze the mathematical models of the scaled setups in SAP2000 under sinusoidal base excitation.
7. Comparison between analytical and the experimental results.

No comments:

Post a Comment

leave your opinion