Nicholas Haritos

Mechanical stability of trees under dynamic loads

Tree stability in windstorms and tree failure are important issues in urban areas where there can be risks of damage to people and property and in forests where wind damage causes economic loss. Current methods of managing trees, including pruning and assessment of mechanical strength, are mainly based on visual assessment or the experience of people such as trained arborists. Only limited data are available to assess tree strength and stability in winds, and most recent methods have used a static approach to estimate loads. Recent research on the measurement of dynamic wind loads and the effect on tree stability is giving a better understanding of how different trees cope with winds. Dynamic loads have been measured on trees with different canopy shapes and branch structures including a palm (Washingtonia robusta), a slender Italian cypress (Cupressus sempervirens) and trees with many branches and broad canopies including hoop pine (Araucaria cunninghamii) and two species of eucalypt (Eucalyptus grandis, E. teretecornus). Results indicate that sway is not a harmonic, but is very complex due to the dynamic interaction of branches. A new dynamic model of a tree is described, incorporating the dynamic structural properties of the trunk and branches. The branch mass contributes a dynamic damping, termed mass damping, which acts to reduce dangerous harmonic sway motion of the trunk and so minimizes loads and increases the mechanical stability of the tree. The results from 12 months of monitoring sway motion and wind loading forces are presented and discussed.

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.

Damage identification in plate-like structures using bending moment response power spectral density

In this article, a new damage-sensitive parameter based on bending moment response power spectral density (MSD) is presented for damage identification in two-dimensional plate-like structures. The total energy or the average output power under the bending MSD graph quantified by the zero order moment of the response spectral density, known as mean square value (MSV), is implemented as a principal response parameter. Damage indices (DIs) derived from MSV, namely relative changes in MSV, mean square value curvature (MSVC), normalized damage index, and relative root mean square error (RRMSE) are then used to detect and localize structural damage. The effectiveness of this approach is illustrated by comparing the results with those obtained from existing and well-established techniques, namely relative changes in natural frequencies, modal flexibilities, uniform load surfaces, and changes in curvatures, such as mode shape curvatures, modal flexibility curvatures, and uniform load surface curvatures. The significant advantage of the proposed technique is that both input–output and output-only damage identification problems can be treated. For the latter condition, the only assumption made is that the forcing function is stationary, ergodic white noise. The methods are illustrated on a simply supported RC rectangular plate subjected to simulated damage cases. Artificial damage simulating local stiffness degradation is introduced to the plate in terms of the material modulus at selected locations in the finite element (FE) model. The modal properties obtained from FE-based modal analyses of this plate for different damage condition states are used to generate the bending moment frequency response functions and MSD at simulated measurement grid points. Subsequently, MSV is computed for undamaged and damaged states from which the appropriate damage indices are obtained. The DIs obtained using different algorithms are used to identify and localize both single and multiple damage conditions.

Performance of Distributed Multiple Viscoelastic Tuned Mass Dampers for Floor Vibration Applications

Floor vibration due to human activities is becoming a significant concern to designers and developers of long span lightweight floor systems. Modern office floors are now constructed with longer spans and lighter structural members. Actual office dead loads and floor damping are significantly lower than what they were in the past, increasing the potential for annoying floor vibration. Traditional techniques to reduce vibrations through structural modifications have some serious shortcomings, especially with existing structures. This paper discusses the development of an innovative configuration for a tuned mass damper (TMD) using viscoelastic material in rectifying problematic floors. General analytical formulae to predict the response of a floor fitted with multiple identical TMDs are developed. Experimental and numerical studies on the performance of a custom-made distributed multiple viscoelastic TMD system in suppressing the vibration level on an actual office floor subjected to various types of excitations are discussed. The effects of the damper location, the variation in the floor and/or dampers dynamic characteristics on the effectiveness of this control approach have been taken into account. The installed damper system has successfully suppressed the floor response level to an acceptable limit for human comfort, in the case study presented.

Load testing to collapse limit state of Barr Creek Bridge

VicRoads, the road authority for the state of Victoria, Australia, has been undertaking extensive research into the load capacity and performance of cast-in-place reinforced concrete flat slab bridges. One of the key objectives of this research is the development of analytical tools that can be used to better determine the performance of these bridges under loadings to the elastic limit and subsequently to failure. The 59-year-old Barr Creek Bridge, a flat slab bridge of four short continuous spans over column piers, was made available to VicRoads in aid of this research. The static testing program executed on this bridge was therefore aimed at providing a comprehensive set of measurements of its response to serviceability level loadings and beyond. This test program was preceded by the performance of a dynamic test (a simplified experimental modal analysis using vehicular excitation) to establish basic structural properties of the bridge (effective flexural rigidity, EI) and the influence of the abutment supports from identification of its dynamic modal characteristics. The dynamic test results enabled a reliably tuned finite element model of the bridge in its in-service condition to be produced for use in conjunction with the static testing program. The results of the static testing program compared well with finite element modeling predictions in both the elastic range (serviceability loadings) and the nonlinear range (load levels taken to incipient collapse). Observed collapse failure modes and corresponding collapse load levels were also found to be predicted well using yield line theory.

Behavior of FRP-RC Slabs under Multiple Independent Air Blasts

In some terrorist attacks, it is possible that RC structures might be subjected to more than a single explosion. RC structures designed without the consideration of blast effects tend to lose their capacity after the first explosion. The use of a fiber reinforced polymer (FRP) sheet has been proven to enhance the performance and resistance of an RC member under a single explosion test. However, there appears to have been no experimental programs conducted to assess the performance of FRP-strengthened RC members subjected to multiple explosions reported in the literature. This paper, therefore, presents experimental results for the behavior of RC slabs strengthened by an FRP sheet after undergoing single, double, and triple independent explosion testing. Results from these blast tests indicate that the FRP sandwich RC slab tested was able to sustain the subsequent second explosion of greater impact. A brittle shear failure with FRP debonding was observed following the third explosion on this FRP-strengthen...

Repair and Strengthening of Reinforced Concrete Structures Using CFRPs

CFRPs offer a high strength lightweight alternative strengthening strategy to traditional methods using concrete overlays and/or steel plates in bridge engineering applications. In addition, the use of CFRPs may offer a viable retrofit/repair strategy in the case of damaged structures, where this damage may be significant. This paper reports on the performance of CFRP-based strategies for the repair and strengthening of two 40% scale continuous flat slab bridge models with significant damage arising from prior static testing to incipient collapse conditions. In addition, the performance after repair using a CFRP scheme of a RC beam-slab-column subassembly, following severe high-load cyclic testing, was also investigated. Results indicate that CFRPs offer a viable repair strategy in structural applications involving severe damage from the influence of static overload or extreme earthquakes but care needs be exercised to ensure secure adhesion where the surface under repair has locally adverse geometric features or has suffered large geometric deformation from the damage concerned.

Finite Element Simulation of FRP Strengthened Reinforced Concrete Slabs under Two Independent Air Blasts

Multiple detonations might occur in both accidental explosions and terrorism attacks. Generally, normal reinforced concrete (RC) structures which are not designed to withstand high intensity blast loads are not capable of withstanding explosions from a single blast let alone a sequence of more than one blast. Since concrete is often highly cracked and damaged from the first blast, the remaining deteriorated concrete and steel reinforcement in a RC member becomes very vulnerable to collapse. This paper reports on the feasibility of using fibre reinforced polymer (FRP) to strengthen a normal RC slab capable of sustaining two independent air blasts. Apart from the experimental investigation, numerical studies have been conducted to verify the concrete and FRP material models when they are utilized to predict the behaviour of FRP-RC structures under multiple blasts. This article provides guidance on how to choose appropriately between the two existing concrete models available in the LS-DYNA code.

Non-linear curve fitting for modal analysis

Abstract This paper discusses deficiencies of techniques widely adopted for curve fitting in modal analysis applications. A new Direct Simultaneous Modal Approximation (DSMA) method is introduced that can significantly improve the accuracy of eigenvalues and eigenvector reconstructed from experimental measurements on real structures. The method employs a Newton iteration technique to approximate simultaneously all eigenvalues and eigenvectors of the structure concerned. A major advantage of the method is that it minimises the non-linear function of weighted global least square error for all available data, enabling accurate reconstruction of modal parameters even when this data contains significant levels of noise and/or when modes are very closely spaced. The concept of the DSMA algorithm can be transferred to other disciplines with non-linear curve fitting requirements where estimation of a large number of parameters is required.

Accuracy of Satellite-Measured Wave Heights in the Australian Region for Wave Power Applications

This article focuses on the accuracy of satellite data, which may then be used in wave power applications. The satellite data are compared to data from wave buoys, which are currently considered to be the most accurate of the devices available for measuring wave characteristics. This article presents an analysis of satellite- (Topex/Poseidon) and buoy-measured significant wave heights for a 1-year period at Cape Sorell and Rottnest Island, off the Australian coast. The analysis found that the satellite-measured wave heights showed a slight positive bias. This is contrary to the findings of most other authors, who have analyzed data from the Northern Hemisphere and generally found a negative bias in the satellite-measured wave heights. The implication is that calibration functions to improve the correlation between the buoy and satellite data may vary for different hemispheres or even regions within these.