Pradipta Banerji

Tuned liquid dampers for controlling earthquake response of structures

Numerical simulations of a single-degree-of-freedom (SDOF) structure, rigidly supporting a tuned liquid damper (TLD) and subjected to both real and artificially generated earthquake ground motions, show that a properly designed TLD can significantly reduce the structures response to these motions. The TLD is a rigid, rectangular tank with shallow water in it. Its fundamental linear sloshing frequency is tuned to the structures natural frequency. The TLD is more effective in reducing structural response as the ground excitation level increases. This is because it then dissipates more energy due to sloshing and wave breaking. A larger water-depth to tank-length ratio than previous studies suggested, which still falls within the constraint of shallow water theory, is shown to be more suitable for excitation levels expected in strong earthquake motions. A larger water-mass to structure-mass ratio is shown to be required for a TLD to remain equally effective as structural damping increases. Furthermore, the reduction in response is seen to be fairly insensitive to the bandwidth of the ground motion but is dependent on the structures natural frequency relative to the significant ground frequencies. Finally, a practical approach is suggested for the design of a TLD to control earthquake response. Copyright

Commissioning and Evaluation of a Fiber-Optic Sensor System for Bridge Monitoring

This paper describes the design, commissioning, and evaluation of a fiber-optic strain sensor system for the structural health monitoring of a prestressed concrete posttensioned box girder railway bridge in Mumbai, India, which shows a number of well-documented structural problems. Preliminary laboratory trials to design the most appropriate sensor system that could be readily transported and used on site are described, followed by a description of load tests on the actual bridge undertaken in collaboration with Indian Railways and using locomotives of known weight. Results from the load tests using the optical system are compared with similar results obtained using electrical resistance strain gages. Conclusions are summarized concerning the integrity of the structure and for the future use of the sensor system for monitoring bridges of this type. Crack width measurements obtained during the load tests are also described.

Structural vibration control using modified tuned liquid dampers

A tuned liquid damper (TLD) is a passive damper consisting of a solid tank filled with water that uses the water sloshing inside it to dissipate energy. The standard TLD configuration is where a TLD is connected rigidly to the top of the building. It has been popular as a control device for wind excitation. Earlier research has shown that the TLD behaviour is amplitude dependent, i.e. it is more effective when excitation amplitude is increased and more energy is dissipated due to sloshing. A modified TLD configuration is proposed here, where the TLD rests on an elevated platform that is connected to the top of the building through a rigid rod with a flexible rotational spring at its bottom. For particular values of the rotational spring flexibility, the rotational acceleration of the rod is in phase with the structure top acceleration and the TLD base is subjected to a large amplitude acceleration that increases its effectiveness. It should be noted that when the rotational spring is rigid, the modified and standard TLD configurations are identical. It is seen that, for aiven structure with modified TLD configuration, there exists an optimum value of the rotational spring stiffness for which the effectiveness of the modified TLD is maximum. Thus, it is seen that an optimally designed modified TLD configuration may be more effective as a structural control device than a standard TLD configuration, for both harmonic and broad-band earthquake motions.

EARTHQUAKE VIBRATION CONTROL USING SLOSHING LIQUID DAMPERS IN BUILDING STRUCTURES

Sloshing liquid dampers (SLDs), popularly known as tuned liquid dampers, are used as passive control devices for reducing structural vibrations resulting from wind and earthquake excitations in tall buildings and high-rise structures. Available research studies on these dampers mostly deal with single-degree-of-freedom (SDOF) structures although tall buildings and high-rise structures are generally multi-degree-of-freedom (MDOF) structures. In the present investigation, effectiveness of these SLDs has been studied for MDOF building structures. Five-storied shear buildings have been considered as representative of MDOF structures. It is shown that the liquid sloshing is the most important design parameter, rather than tuning of the fundamental sloshing frequency to the structure frequency, for the liquid damper to be effective. Furthermore, the liquid damper design for multistory buildings is different from that for SDOF structures, where not only the optimal tuning ratio of the liquid damper is different,...

Application of fiber-optic strain sensors for monitoring of a pre-stressed concrete box girder bridge

Fiber-optic strain sensors have been used to qualitatively assess structure health. Here, use of fiber-optic strain sensors for quantitative measurement of concrete strain on a railway bridge is investigated. Concrete strains have historically been measured using electrical resistance strain gages. These strain gages, however, have the disadvantages of low signal-noise ratio and degradation of signal over time in the specific application for railway bridges on electrified routes. Fiber-optic strain sensors, apart from the ease of application to a girder concrete surface, have high signal-noise ratios, and are suitable for long term health monitoring as the optical signal and the concrete surface attachment do not degrade with time. It is conclusively shown that appropriately attached fiber-optic sensors coupled with high sampling rates provide a reasonably accurate measurement of strains on concrete surfaces, and can, thus, be used directly for quantitative condition assessment of bridges.

Dynamic Parameter Characterization for Railway Bridges Using System Identification

The problem of structural health monitoring of railway bridges is an ongoing research area, and numerous researchers have worked in this domain. The fundamental problem of condition assessment can be addressed by identifying the dynamic parameters like fundamental frequencies and mode shapes of tested structures and validating numerical models using these identified parameters. The entire problem can hence be broken down into two major areas, one of identifying the dynamic parameters in a robust manner from site data and the other of creating updated numerical models which exhibit convergence with these. In the present paper, the first of these areas is addressed. Various system identification techniques are presented along with their results to illustrate the robustness of the various techniques. A novel method is also developed to identify the mode shapes using sparse sensor applications. In this paper the model updation techniques developed elsewhere by the authors is used in conjunction with system identification to as a synthesized approach to identification of mode shapes. All the techniques are illustrated by application on an existing in-service railway bridge to verify these techniques for real structures. In this paper, the concept of global and local modes is introduced where the structural modes involving the movement of the entire structure are termed as global modes, whereas the modes involving only certain elements are termed as local modes. It is shown that a system identification technique is possible which identifies and differentiates these.

Assessment of fibre optic strain gauges for field use in India

The commissioning and evaluation of a fibre optic strain sensor system for the structural health monitoring of a pre-stressed concrete post-tensioned box girder railway bridge in operational use in Mumbai, India, is described. Preliminary laboratory trials to identify the most appropriate sensor system are detailed followed by a description of the load tests on the actual bridges which were undertaken in collaboration with Indian railways. Results from the load tests using the optical system are compared with similar results obtained using electrical resistance strain gauges. Conclusions are drawn concerning the integrity of the structure and for the future use of the sensor system for monitoring bridges of this type. Crack width measurements obtained during the load tests are also summarised.

Neural networks for delamination flaw detection in FRP laminated composite plates

In this study, the detection of delamination flaws in laminated composite plates is carried out using artificial neural networks (ANN) in a two-level cascading manner. The three damage parameters detected using ANN are the size of the delamination, its vertical location (across the plate thickness) and horizontal location (along the plate surface). The numerical data in the form of frequency domain Greens function for the displacement response on the surface of the plate containing the delamination flaw is generated first using an available numerical method. Pseudo-experimental data is generated adding artificial random noise into the numerical data. At the first level, a counterpropagation neural network (CPN) is trained for qualitatively classifying the damage parameters using the numerical data generated above. Next, a second level back-propagation network (BPN) is used for each subclass to quantify the damage parameters. An overlapping data set is used for the training of each class of the second level network. As a result, any pattern misclassified by the CPN due to its closeness to the boundary of any two classes is still quantified correctly. By feeding pseudo-experimental data to the trained networks, it is seen that the classification success rate and noise tolerance level of CPN is excellent. The quantification of damage by the second level BPN is also good. It is possible to stop after the first level if only a qualitative assessment of the damage and its approximate location is required. These cascaded networks show promise in providing a successful delamination damage detection tool.

Three-dimensional guided waves in laminated composite plates excited from point source

Three dimensional wave propagation characteristics in laminated plates are studied considering the anisotropic and viscoelastic properties of fiber reinforced composite material. A Rayleigh-Ritz based stiffness method is used to discretize the plate in the vertical direction to determine propagation characteristics (wave number, phase velocity, group velocity) and mode shapes for a plane wave front. For 3-dimesional cases, wave propagation problem is decomposed into a series of two-dimensional plane wave problems with three displacements coupled. Double Fourier transform integral transformations are used to get the governing equation in a transformed wave number domain. Steady state elastodynamic Greens functions for the laminated composite plates are constructed through summing the contribution of all two-dimensional problems and the application of modal summation technique. Numerical integration of double infinite integrals is performed by summations over a finite range. The wave propagation characteristics for a 16-layer unidirectional fibre reinforced laminated composite plate show the orthotropic nature of the plate reflected in its 3-D wave propagation characteristics. It is also seen that the Greens functions for 3D waves are very different from those for plane strain 2D waves. Furthermore, the direction of propagation has a significant effect on the Greens function for surface displacements.

Elastic wave propagation in laminated FRP plates

In this paper, guided elastic Lamb wave propagation characteristics in laminated plates are studied using a proper phenomological model for the fiber reinforced composite material. It is well established that ultrasonic waves attenuate in FRP composite material. This is caused by the visco-elastic behavior of the resin ad scattering due to the fiber. The material here is, therefore, numerically modeled using complex material properties. Both the material and frequency-dependent damping for each layer of the laminated plate is incorporated in the formulation. A Rayleigh-Ritz based stiffness method is used to discretize the plate in the vertical direction to determine wave numbers. Furthermore, elasto-dynamic Green functions for both displacement and stress fields in the laminated composite plate are also derived. In this manner, the effect of guided wave attenuation on wave numbers and displacements and stress Green functions are studied. All wave numbers, including those for propagating modes, are complex. However, the relative value of the imaginary part of the wave number of the propagating modes are small, and it is seen that incorporation of damping has an insignificant effect on dispersion characteristics, irrespective of the excitation frequency. However, it significantly affects both displacement and stress fields, especially as the distance from the source increases. This effect is dependent on the excitation frequency. At relative low frequencies, the attenuation of the displacement and stress fields is small. The attenuation increases with excitation frequency, although seemingly the effects on the stress field are relatively less significant than those on the displacement field.