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Historically, the most common cause of bridge failure has been “scour” – the gradual erosion of soil around bridge foundations due to rapid water flow. A reliable technique to monitor scour could potentially guide timely repair and, in turn, mitigate the risk of future scour-induced bridge failure. Currently, there are various, mostly underwater, techniques employed by bridge managers to monitor scour, ranging from diving inspections to autonomous underwater vehicles; however, none have gained wide acceptance. A particular disadvantage of underwater monitoring techniques is that the equipment underwater is relatively difficult to install and prone to damage from fast-flowing water and debris. One possible solution might be to use a vibration-based method to monitor scour indirectly, using changes in dynamic modal parameters (e.g. the natural frequency of vibration) captured by sensors mounted on the bridge deck or piers above the water level. There has been extensive research into the use of vibration-based monitoring methods to identify other causes of failure, such as cracking and deterioration in bridge superstructures; however, this has proven to be ineffective in practice, as the expected sensitivities in modal parameters were only single-digit percentages and therefore insufficient to overcome environmental/operational sensitivity. In contrast to superstructure damage, scour is a special damage case, which changes a boundary condition of the bridge in the form of an increase in effective pier height as a result of the lowering of the ground level and therefore, significant changes in modal parameters can be expected. Recently, this concept has been studied primarily using numerical modelling simulations of a hypothetical integral bridge with piled foundations. Only one modal parameter – natural frequency – was investigated in most of these studies and it was predicted to change by up to double-digit percentages due to scour. Although such a high change could potentially overcome environmental and operational sensitivities, a critical problem is that this has been difficult to observe in practice with experiments on either real field bridges or small-scale soil-structure models. Another problem is that there is little knowledge of the applicability of this technique to different types of bridges and forms of scour. This research proposes a vibration-based technique based on a combination of three vibration parameters (spectral density, mode shape and natural frequency), which were studied using first-of-a-kind experiments and numerical modelling simulations on various types of bridges and forms of scour. A field trial was carried out on a bridge with pre-existing scour, which was monitored for ambient vibrations throughout a repair process involving controlled scour backfilling, i.e., “scour in reverse”. The effect of this scour backfilling was captured by measuring changes in two of these parameters, mode shape and spectral density, derived from the ambient vibrations. The mode shapes, in particular, showed the potential to localise the presence of scour to a specific pier. The most commonly measured vibration parameter of natural frequency was also observed from ambient vibrations, but this did not capture the effects of backfilling due to high measurement uncertainties. In order to study all three of these vibration parameters in a controlled environment, a centrifuge model testing programme was developed. These tests considered small-scale models representing three full-scale bridges with different bridge deck and foundation configurations (i.e. integral/ simply supported decks and shallow/deep foundations) and two forms of scour (i.e. local/global). The observed results of these small-scale centrifuge models were used to calibrate numerical models of full-scale bridges representative of these centrifuge models. Numerical simulation techniques were also developed to simulate the experimentally observed effects of local and global scour. These centrifuge experiments and the associated numerical modelling found that vibration-based methods have broad applicability for bridges, although only some parameters showed sufficient sensitivity to be viable as a monitoring technique in certain types of bridges. For example, the centrifuge bridge models with a shallow foundation did not show a significant change in natural frequency or mode shapes, but they did show a significant change in modal spectral density. This research therefore concludes that a vibration-based scour monitoring technique, examining the combined effect of natural frequency, mode shape and spectral density parameters, has significant potential to measure and even localise the change of scour depths at bridge foundations.

This work is on the free vibration of swept (skewed, trapezoidal and tapered) cantilevered beam-plates and an investigation on the use of modal patterns for the design of viscoelastic damping treatments of the beam-plates. The generic name beam-plate is used to define beam-like and plate-like structures. The natural vibrations of swept cantilevered beam-plates play an important role in various structural applications such as aircraft wings and rocket fins. The natural frequencies and mode shapes of aluminium beam-plates of aspect ratios ranging from 1 to 10 with several leading and trailing edge sweep angles were predicted using the ABAQUS FE programme. To classify complex mode shapes, a new nodal pattern matching technique was established in which mode shapes with similar nodal pattern features were put into the same modal families that were identify by the number of nodal lines ('i') of the mode shapes along the lengthwise direction of the uniform beam-plate design. Then modal families were classified into modal groups which were identified by the number of nodal lines ('j') of the mode shapes along the breadthwise direction of the uniform beam-plate design, namely: predominantly bending (j =0), predominantly torsional (j=1), composite (j=2, 3...) and in-plane bending (n =1) modal groups. The modal assurance criterion (MAC) was used to validate the visual nodal pattern matching technique. The modal data was used to produce frequency parameter graphs which showed the variations of the dimensionless natural frequencies against aspect ratios and swept angles of the beam-plates. Then the data from these graphs were combined together to produce graphs which are referred to as frequency parameter charts. Experimental modal testing and analysis were performed on swept cantilevered beam-plates with aspect ratios 2, 4, 4.85 and 6 to validate the natural frequencies and mode shapes. The use of the frequency charts to identify mode types from measured FRF was illustrated. Thus, the charts facilitate the identification of mode types and prevent the need to perform a full experimental modal testing and analysis. Another major application of the mode shapes was to design constrained viscoelastic layer damping (CLD) treatments of beam-plates. CLD patches were cut according to the nodal patterns and used to suppress the vibrations of the beam-plates. It is shown that to control the first bending mode, CLD patch should be placed at the fixed end of the beam-plate which covers a passive nodal line. To control the second bending mode the CLD patch should be placed in the middle of the beam-plate and the later bending modes are best controlled if the patch is placed towards the free end of the beam-plate. It was concluded that to obtain higher damping then the size of the damping patch should be just big enough to cover all the active nodal lines in a deformed beam-plate.

Bearings today need to be able to run at very high speed, providing high positional accuracy for the structure that it supports, and requiring very little or no maintenance. For this to happen, bearings must have tight tolerances and very low or zero friction during operation. This pushes many traditional contact-type bearings to their limits as they often fail due to friction, generating heat and causing wear. By comparison, existing non-contact bearings fare better because of their very low or zero friction. But some have their own problem too. For example, the fact that aerostatic bearings require an air supply means having to use a separate air compressor and connecting hoses. This makes the installation bulky. Aerodynamic and hydrodynamic bearings cannot support loads at zero speed. Both hydrodynamic and hydrostatic bearings may cause contamination to the work-pieces and the work environment because of the use of lubricating fluid. A potential solution to the above-mentioned problems is the new squeeze film air bearing. It works on the rapid squeeze action of an air film to produce separation between two metal surfaces. This has the benefit of being compact with a very simple configuration because it does not require an external pressurized air supply, can support loads at zero speed and is free of contamination. For this research, two squeeze film air journal bearings, made from material of Al 2024 - T3 and Cu - C101 with the same geometry, were designed. The bearing is in the shape of a round tube with three fins on the outer surface and the journal, a round rod. When excited at a certain normal mode, the bearing shell flexes with a desirable modal shape for the squeeze film action. The various modes of vibration of Al bearing were obtained from a finite-element model implemented in ANSYS. Two Modes, the 13th and 23rd, at the respective frequencies of 16.320 kHz and 25.322 kHz, were identified for further investigation by experiments with respect to the squeeze film thickness and its load-carrying capacity. For Cu bearing, the two Modes are also 13th and 23rd at the respective frequencies of 12.184 kHz and 18.459 kHz. In order to produce dynamic deformation of the bearings at their modes, a single layer piezoelectric actuator was used as a driver. The maximum stroke length and the maximum blocking force of the single layer piezoelectric actuator were determined using manual calculation and ANSYS simulation. In the coupled-field analysis, the single layer piezoelectric actuator was mounted on the outside surface of the bearing shell and loaded with an AC and a DC voltage in order to produce the static and dynamic deformation. For the static analysis, the maximum deformation of Al bearing shell is 0.124 μm when the actuators are driven at the DC of 75 V. For the dynamic analysis, the actuators are driven at three levels of AC, namely 55, 65 and 75V with a constant DC offset of 75V and the driving frequency coincided with the modal frequency of the bearing. The maximum dynamic deformation of Al bearing shell is 3.22μm at Mode 13 and 2.08μm at Mode 23 when the actuators were driven at the AC of 75 V and the DC of 75 V. Similarly, the FEA simulation was used for analyzing Cu bearing. Furthermore, the dynamic deformation of both Al and Cu bearing at Mode 13 and 23 are validated by experiments. This research developed two theoretical models that explain the existence of a net pressure in a squeeze film for the levitation. The first model uses the ideal gas law as first approximation whilst the second uses the CFX simulation to provide a more exact explanation. In terms of the load-carrying capacity, Mode 13 was identified to be better than Mode 23 for both bearings. However, at Mode 13, Al bearing has a higher load-carrying capacity than Cu bearing. This is due to Al bearing having a higher modal frequency and amplitude. Finally, the coupled-field analysis for fluid solid interaction (FSI) was studied at both Mode 13 and 23 for Al bearing. The findings are that: a) the fluid force in the squeeze film can affect the dynamic deformation of the bearing shell, especially at high oscillation frequency, more at Mode 13 than at Mode 23 due to the relatively high pressure end-leakage in the latter; b) the dynamic deformation of the bearing shell increases with the gap clearance in a logarithmic manner at Mode 13; and c) the micron levels of gap clearance provide a damping effect on the dynamic deformation of the bearing shell at Mode 13 and at Mode 23, though much less dominant.

Vibration-based damage detection methods have been extensively used in structural health monitoring as these are response-based techniques and can be applied to experimental/operational data. The conventional methods of obtaining full-field vibration measurements are limited due to the location and number of sensors. Advancements in imaging technology have enabled the use of the digital image correlation (DIC) technique to measure the full-field deformation of a vibrating structure. In this study, the DIC technique was used to obtain vibration measurements from an impact test of a cantilever beam for damage identification.The application of curvature mode shapes (CMSs) developed from the mode shapes (MSs) of the beam is studied to detect and locate the damage. The CMSs of the undamaged state of the beam are determined only from the damaged state of the beam, without prior information about the associated undamaged beam, provided the beam is geometrically smooth. It is shown that the polynomial fit of the appropriate order of measured MS is equivalent to the associated MS of the undamaged beam. The objective of this study was to investigate the use of DIC technique and CMSs to locate and detect damage in the form of a machined area with reduced thickness in a cantilever beam. The modal parameter estimation (MPE) was done using X-Modal software, based on the unified matrix polynomial approach (UMPA), to obtain mode shapes and natural frequencies from the vibration measurements. It is shown that the proposed method can successfully detect and locate damage in the beam, for the data obtained from a single-input impact test. The work focuses on understanding how the parameters used in the DIC technique and MPE influence the damage detection. The influences of parameters such as subset size and step size used in the DIC technique are studied. The influences of parameters such as type of MPE algorithm, frequency band selection and model order during MPE are studied. The damage identification is represented by squared difference between the CMSs of the damaged and undamaged beam defined as curvature damage index (CDI) and averaged CDI for multiple modes defined as damage index (DI).

In many areas of engineering such as telecommunications industry, lenses and optical cooling, waveguiding and the ability to control wave propagation is necessary. Phononic Crystals (PnCs) influence the propagation of mechanical waves by their periodic variation in elastic impedances. Depending on Lattice type and inclusion's spacing, one can manipulate the frequencies that are blocked or allowed to transmit through the crystal in a particular direction. In this work, two dimensional PnCs plates are design and also numerically and experimentally demonstrated. Finite Element Analysis (FEA) was performed to study the dispersion behavior and eigenmodes of a PnC. The simulations show the existence of a complete bandgap, thus acoustic PnC devices such as cavity, resonator and waveguide are created accordingly by introducing a line defect to the crystal. The experimental response of the devices was then measured by fabricating and then testing them. These results show excellent agreement with the FEA. For the first time waveguide and 90áµ’ bend is designed that have several isolated modes in the bandgap with transmission efficiencies above 90% for a specific frequency range. In this seminar also a few novel patents will be introduced. We consider a two-dimensional periodic square array of the air holes in a finite aluminum medium. The defect introduced to the crystals are acting as local acoustic resonances/guiding with the substrate and lead to the possibility of finding a low-frequency confined acoustic waves or guided modes. Studying the mode shapes will help to shed light on the origin of BG formation and as the results BG optimization. The numerical and experimental results presented here are related to the case of Aluminum-air devices, but the conclusions remain valid for other materials and compositions as well.

The presented research examined three areas: best practices in high quality dynamic testing of in-situ floor systems, extensive dynamic testing of three bare (non-fit out) in-situ multi-bay steel composite floors to estimate their dynamic parameters/response and to identify trends in dynamic behavior, and development of a set of fundamental finite element (FE) modeling techniques to adequately represent the dynamic response of steel composite floors for the purpose of evaluating vibration serviceability. The measurement, analysis, and computation of a floor's accelerance frequency response function (FRF) is the core premise linking all areas of the presented research. The burst chirp signal using an electrodynamic shaker is recommended as the most accurate and consistent source of excitation for acquiring high quality measurements suitable for use in parameter estimation, operating deflection shape animation, and calibration/validation of FE models. A reduced mid-bay testing scheme is recommended as a time-saving alternative to modal testing over a full coverage area, provided the only desired estimated parameters are frequencies, damping, and mid-bay acceleration response. Accelerance FRFs were measured with an electrodynamic shaker located within 23 unique bays on the three tested floors. Dominant frequencies ranged from 4.85 Hz to 9 Hz and measured estimates of damping varied considerably, ranging from 0.44% to 2.4% of critical (0.5%-1.15% was typical). Testing showed several mode shapes were localized to just a few bays and not all modes were adequately excited by forcing at a single location. The quality of the estimated mode shapes was significantly improved using multi-reference modal testing. FE models for the tested floors were developed based on high quality measured data and were shown to provide adequate representations of measured floor behavior. Fundamental techniques are presented for modeling mass, stiffness, boundary conditions, and performing dynamic analysis. A method of evaluating vibration serviceability was proposed using the FE model's computed accelerance FRF for comparison with a design accelerance curve that represents an acceleration response threshold in the frequency domain. An example design accelerance curve is presented based on current serviceability guidelines for acceleration tolerance and effective harmonic forces due to human activities such as walking.

The purpose of this study was to examine the effect of bottom chord extensions on deflections and vibration characteristics of joist supported floor systems when joist bottom chord extensions are installed. To understand the effect of bottom chord extensions on deflections, natural frequency, damping, mode shape and effective mass, extensive analytical and experimental studies were conducted on single span and three span joist supported laboratory footbridges with different bottom chord extension configurations. Finite element computer models were created to simulate and compare the results of stiffness and vibration tests. Testing was done with a) the bottom chord extensions in-place before the concrete was placed, b) with all or part of the bottom chord extensions removed, and c) after the bottom chord extensions had been reinstalled with jacking for the single span footbridge and without jacking for the three-span footbridge. Results from the stiffness tests indicate that re-installing the bottom chord extensions to the joists of the single span footbridge with cured concrete with the center of the span raised helps to reduce the uniform load deflections to some extent, but not as much as placing the bottom chord extensions before the concrete placement. Likewise, for the three span footbridge, placing the bottom chord extensions before the concrete placement is observed to be a better solution. Results from the dynamic tests indicate that the effect of bottom chord extensions on the single span footbridge is consistent for natural frequency, 20 psf live load deflections, sinusoidal excitations with high amplitudes, quarter point heel drop excitations, walking excitations, and effective mass values. The effect of bottom chord extensions on the three span footbridge is consistent for the natural frequency and 20 psf deflections. However, the FRF (Frequency Response Function) peaks of chirp, heel drop, sinusoidal excitations, accelerations from walking data, and the MEScope and Finite Element model effective mass results do not follow a common trend. It can be concluded that even though the footbridge was stiffened by the bottom chord extensions, that does not necessarily mean that the acceleration levels, and hence the frequency response function peaks, decrease. However, bottom chord extensions do increase the natural frequencies for all the three governing bending modes.