J.S. Owen

The application of auto–regressive time series modelling for the time–frequency analysis of civil engineering structures

Abstract The traditional approach for producing spectral estimates from experimental data is the Fast Fourier Transform. However, in certain circumstances this approach is inappropriate due to factors such as non-stationarity or non-linearity. In this paper, the authors apply the auto–regressive time series modelling approach to produce spectral estimates of two such problems — non-stationary data obtained from the large amplitude response of a cable stayed bridge to wind excitation and non-linear data obtained from modal testing of cracked reinforced concrete beams. Although the auto–regressive approach is very sensitive to the model parameters chosen and so should always be used with caution, the results of these studies have produced time varying spectral data that give insight into the problems addressed.

The Use of Vibration Data for Damage Detection in Bridges: A Comparison of System Identification and Pattern Recognition Approaches

This paper briefly outlines the rationale for structural health monitoring as an integral component of bridge management systems. Two different approaches, system identification and statistical pattern recognition, are summarised and applied in turn to vibration data collected from three scale modelreinforced concrete bridges. The results show that the system identification paradigm can successfully locate and quantify the damage to the decks when they are loaded to incipient collapse, especially when experience is used to determine the parameters to use in the finite element updating procedure. However, the study also demonstrated that this approach requires a large amount of high quality data, requirements that cannot always be met readily in the field. In contrast, although the statistical pattern recognition approach was not able to quantify or locate the damage, it was able to clearly indicate that damage had occurred from relatively few measurements. A comparison of the strengths and weaknesses of the two approaches suggests that they should be used in a complementary manner. The statistical pattern recognition approach can be employed as a simple, cost efficient way to indicate that damage has occurred. It can then trigger a more detailed investigation using system identification.

The Prototype testing of Kessock Bridge: Response to Vortex Shedding

Abstract Measurements of the dynamic response of Kessock Bridge made between October 1991 and May 1992 indicated that on three occasions the bridge experienced abnormally large response. Data from these three events are presented and discussed. These show that the peak dynamic displacement occurred due to resonance of the first vibration mode with vortex shedding, and was in excess of 110 mm. The duration of the large amplitude displacements was determined by the turbulence of the incident wind and reached 3 min during the more steady Easterly winds. A large torsional response was also noted during one of these events, which indicates a previously unobserved and potentially serious behaviour. Finally, inspection of the tuned mass dampers installed on the bridge to reduce the level of vibration in the first mode showed that they were not functioning correctly after the last event.

The influence of member orientation on the resistance of cross joints in square RHS construction

Abstract The diamond bird beak is a novel joint configuration for square RHS construction and is achieved by rotating the chord and brace of a traditional joint through 45° about their longitudinal axes. This paper describes a finite element examination of the resistance of welded diamond bird beak cross joints under axial brace compression. The effects of the joint parameters — brace/chord width ratio, wall slenderness, chord length to depth ratio, yield stress, and chord-end boundary restraint conditions, are considered. The chord length and the chord end boundary conditions are found to have a significant effect on the joint capacity and are considered in detail to develop an empirical equation for the joint resistance. Comparisons are also made with the resistance of conventional RHS cross-joints.

On the applicability of 2D URANS and SST k - ω turbulence model to the fluid-structure interaction of rectangular cylinders

In this work the practical applicability of a 2D URANS approach adopting a block structured mesh and Menters SST k-ω turbulence model in fluid-structure interaction (FSI) problems is studied using as a test case a ratio B/H = 4 rectangular cylinder. The vortex-induced vibration (VIV) and torsional flutter phenomena are analyzed based on the computation of the out-of-phase and in-phase components of the forced frequency component of lift and moment coefficients when the section is forced to periodically oscillate both in heave and pitch degrees of freedom. Also the flutter derivatives are evaluated numerically from the same forced oscillation simulations. A good general agreement has been found with both experimental and numerical data reported in the literature. This highlights the benefits of this relatively simple and straightforward approach. These methods, once their feasibility has been checked, are ready to use in parametric design of bridge deck sections and, at a later stage, in the shape optimization of deck girders considering aeroelastic constraints.

ROLE OF DYNAMIC TESTING IN ASSESSMENT OF BRIDGES

Six reinforced concrete beams were loaded incrementally up to failure. After each increment the load was removed and measurements of the modal properties of the beams were made by impulse testing. The variation of the natural frequencies, frequency ratios, mode shapes, and the level of damage were investigated. It was found that on completion of the tests the natural frequencies of the beams had been reduced by an average of 25 percent in each mode. However, changes in mode shape were very small, and appreciable differences were only observed when the damage was highly localized. Modeling of the beam by using finite elements predicted trends that compared well with experimental observations. It is concluded that if dynamic testing were used in monitoring reinforced concrete structures, then the changes in frequency due to initial concrete cracking or yield of the reinforcement could be detected. More useful information associated with the spread and type of cracking through a structure may be detectable, although the level of the frequency changes is of the same order as those due to changes in ambient conditions.

Monitoring the Degradation of Reinforced Concrete Beams under Cyclic Loading

A particular problem when developing a structural health monitoring s ystem for bridges is that it is not easy to obtain data for the whole design life ( ~125 years) of a test bridge in order to validate the system. Many earlier studies have therefore concent rated on detecting specific artificially induced damage regimes, which in some cases bear little resemblance to the damage observed on prototype structures. To overcome this problem, a pilot study to produce an accelerated whole-life loading regime has been carried out on a s et of four reinforced concrete beams. The beams were each subjected to cyclic load (c. 10 milli on cycles) of varying amplitudes ranging from 25 % to 50% of the ultimate capacity of the beams. At regular intervals the natural frequencies and mode shapes were monitored. The results showed that the re was a weak but discernible trend of a reduction in natural frequency with increasing cycle number. However, the natural frequencies were most sensitive to crack height, which was load, rather than cycle number, dependent. Other monitoring parameters, such as cosh spectral distance and reciprocity were less sensitive, although repeated FE model updating was able to detect a g radu l reduction in Young’s modulus of the concrete. Introduction There has recently been a growing interest in developing damage det ection and structural health monitoring (SHM) methodologies to help in managing an ageing Civil Eng ineering infrastructure. Much of this work has focused on improving non-destructive testing techniques and on applying monitoring schemes such as acoustic emission; all of which have shown pr mise. One area where research has yet to make a real breakthrough is in the use of modal test data. Vibration data have always been a popular choice for prototype SHM systems because they are easy to collect, provide a global picture of structural behaviour from relatively few measure ments and are dependent on both the level of stiffness and its distribution through a structure. Howe ver, although the detection of damage in reinforced and pre-stressed concrete structures using vibration s ignatures has been widely investigated over the past two decades on both full [e.g. 1] and lab scal e structures [e.g. 2], the resulting techniques have yet to find widespread practical acceptanc . Reasons for this include the difficulty in getting adequate modal data from large civil engine ering structures and the inherent weak non-linearity of damaged RC elements. In the light of this r ecent research has considered alternatives to the traditional system identification methodology [ 3] and has sought to exploit the inherent system non-linearities for monitoring damage [4,5]. A further reason why many studies have yet to convince a sceptical profession is that the damage considered is artificial; either the structures are built “dam aged” or damage is induced using unrealistic methods such as removing material, cutting the concrete/ reinforcement or by overloading the structure with an arrangement that doesn’t reflect the real whole-life loading on the structure. In reality, the degradation of RC structures occurs over a long period of time and may arise from the cumulative effect of repeated loads, corrosion of the reinforcement a nd exposure of cracked elements to repeated freeze thaw cycles in addition to the one off overloading e vent. Although there have been some attempts to address these other damage mechanisms w ithin the SHM systems [6] there is still a lot of work to be done. Key Engineering Materials Online: 2003-07-15 ISSN: 1662-9795, Vols. 245-246, pp 307-314 doi:10.4028/www.scientific.net/KEM.245-246.307

Influence of Preload and Nonlinearity of Railpads on Vibration of Railway Tracks under Stationary and Moving Harmonic Loads

In railway track dynamics, the stiffness and damping properties of railpads have a significant effect on track vibration, decay rates as well forces transmitted to the track supporting structure. Many studies have shown that railpads exhibit pronounced nonlinear behaviour, with preload and frequency dependent properties. This paper presents a three parameter railpad model, together with its differential equation of motion and the required model parameters obtained from experimental data. A time domain model of a rail discretely supported on these railpads is then formulated using the finite element method. The model is subjected to static and dynamic loading in order to study the effects of preload and frequency on its dynamic behaviour. Results are shown as time histories and frequency spectra for the track displacements and reaction forces for various preload levels. They emphasise the necessity of accounting for nonlinear behaviour based on the large disparities (up to 20 dB) observed between the linear and nonlinear cases for the parameters used in this study.

Torsional behaviour of rectangular hollow sections.

The results of a series of full-scale torsion tests on rectangular hollow sections (RHS) are presented and discussed. The observed torque–twist behaviour is compared to that predicted by an extended version of Marshall’s simplified thick wall torsion theory and finite element (FE) analysis, and significant differences are highlighted and examined. The behaviour predicted by the FE models is shown to be identical to that predicted by Marshall’s thick wall theory, which forms the basis of the British and European design procedures. However, even though the experimental measurements agree with the FE and theoretical predictions in the elastic range, the measurements of torsional capacity are significantly lower than those calculated, and this has important implications for design that may be wider than just torsion of RHS. A number of potential causes for this behaviour are examined, but it is yet to be fully explained. Evidence of similar behaviour in previous large-scale testing is highlighted and discussed.

Design and Implementation of a New System for Large Bridge Monitoring—GeoSHM

Structural Health Monitoring (SHM) is a relatively new branch of civil engineering that focuses on assessing the health status of infrastructure, such as long-span bridges. Using a broad range of in-situ monitoring instruments, the purpose of the SHM is to help engineers understand the behaviour of structures, ensuring their structural integrity and the safety of the public. Under the Integrated Applications Promotion (IAP) scheme of the European Space Agency (ESA), a feasibility study (FS) project that used the Global Navigation Satellite Systems (GNSS) and Earth Observation (EO) for Structural Health Monitoring of Long-span Bridges (GeoSHM) was initiated in 2013. The GeoSHM FS Project was led by University of Nottingham and the Forth Road Bridge (Scotland, UK), which is a 2.5 km long suspension bridge across the Firth of Forth connecting Edinburgh and the Northern part of Scotland, was selected as the test structure for the GeoSHM FS project. Initial results have shown the significant potential of the GNSS and EO technologies. With these successes, the FS project was further extended to the demonstration stage, which is called the GeoSHM Demo project where two other long-span bridges in China were included as test structures. Led by UbiPOS UK Ltd. (Nottingham, UK), a Nottingham Hi-tech company, this stage focuses on addressing limitations identified during the feasibility study and developing an innovative data strategy to process, store, and interpret monitoring data. This paper will present an overview of the motivation and challenges of the GeoSHM Demo Project, a description of the software and hardware architecture and a discussion of some primary results that were obtained in the last three years.