Sherif Beskhyroun

Wavelet packet based damage detection in beam-like structures without baseline modal parameters

In this paper, a new approach for damage detection in beam-like structures is presented. The method can be used without the need for baseline modal parameters of the undamaged structure. Another advantage of the proposed method is that it can be implemented using a small number of sensors. In the proposed technique, the measured dynamic signals are decomposed into the wavelet packet decomposition (WPD) components, the power spectrum density (PSD) of each component is estimated and then a damage localisation indicator is computed to indicate the structural damage. The proposed method is firstly illustrated with a simulated beam and the identified damage is satisfactory with assumed damage. Then, the method is applied to a steel beam. The effect of damage location and the effects of wavelet type and the decomposition level are examined. The results show that the proposed method has great potential in crack detection of beam-like structures.

Measurements of tsunami-borne debris impact on structures using an embedded accelerometer

ABSTRACT This paper presents new experimental techniques utilizing a smart debris device for direct measurement of the impact acceleration forces associated with tsunami-borne debris that impact inland structures. The resulting experimental data will lead to advanced predictive capabilities of such forces for use in design guidelines. The measured debris acceleration data were used to calculate impact forces (mass × acceleration). An image processing technique was used to detect the debris impact angle. The debris impact tests were conducted using a disc-shaped smart debris device with masses of 550, 800 and 1000 g. For calculation of the debris force it was found necessary to include the mass of entrained water. The impact acceleration was found to be a function of debris mass, velocity, and contact duration. An equation is developed to allow estimation of the debris velocity for a known distance between the debris pick-up location by a tsunami and the structure.

Dynamic Field Testing of a Three-Span Precast-Concrete Bridge

AbstractThere has been little full-scale physical experimentation to support the findings of numerous computational studies regarding the contribution of various substructure components to overall bridge-foundation system behavior. In response to this lack of experimental data, a field testing program was undertaken to investigate the in situ dynamic characteristics of a 27-m-long, three-span, precast-concrete bridge. Forced vibration testing and system identification were used to characterize the dominant modal behavior of the bridge-foundation system in each loading direction, leading to identification of the likely force-transfer mechanisms between the structure and the substructure components. It was determined that both the transverse and longitudinal responses of the bridge were dominated by the abutment stiffness, with the passive resistance of the buried settlement slab contributing significantly to the transverse response and the backfill passive resistance dominating the longitudinal response.

Calibration of the finite element model of a twelve-span prestressed concrete bridge using ambient vibration data

The recently constructed Newmarket Viaduct in Auckland is a critical link in the New Zealand state highway network. Newmarket Viaduct is a 12-span, pre-cast, post-tensioned structure built using the balanced cantilever method. A continuous health monitoring system was designed and installed in the bridge. As a critical part in the SHM process, a baseline finite element (FE) model of Newmarket Viaduct was established. This paper describes the implementation of the FE model calibration using ambient vibration data. The initial model of the bridge was developed from the information provided in the design documentation, material testing data and site inspections. Two ambient vibration testing campaigns used some 60 wireless sensors in multiple setups to collect data to map with high density 3D mode shapes of the bridge. The output-only modal identification results obtained from the ambient vibration measurements of the bridge were used to update the FE bridge model. Different parameters of the model were calibrated using an automated procedure to improve the correlation between measured and calculated modal parameters. Careful attention was paid to the selection of the parameters to be modified during updating in order to ensure that the changes to the model were realistic and physically meaningful. The calibrated FE model reflecting the as-built structural condition and dynamic response mechanisms of Newmarket Viaduct will serve as a baseline model for assessment of structural health using continuous monitoring data.

Dynamic testing and long term monitoring of a twelve span viaduct

Because of the critical importance of bridges in land transport networks and broader economy, an increasing interest in permanent observation of their dynamic behavior under traffic, seismic and other live loads has been observed during the past decade. In addition, recent technological advances have made the installation and operation of permanent dynamic monitoring systems much more practical and economical. A multi-channel dynamic monitoring system is being installed in the 12 span pre-cast, post-tensioned Newmarket Viaduct, recently built using the balanced cantilever method and situated in Auckland, New Zealand. This paper first describes the preliminary studies including extensive one-off ambient vibration tests using wireless sensors that provided important information for the design of the monitoring system shortly after construction of the bridge. Then the designed monitoring system is characterized and proposed research that will be undertaken using the monitoring data is outlined.

Dynamic properties of an eleven-span motorway bridge at different levels of excitation

Bridge dynamic properties measured under a given vibration intensity condition would give a true picture of the behaviour for that particular condition. However, the use of the model derived from such data may not be reliable when applied for the prediction of response under a different vibration intensity condition. Therefore, it is necessary to investigate the structural dynamic behaviour at different levels of excitation in detail. This paper focuses on the experimental investigation of modal property variability at different levels of excitation. Both weak ambient vibration tests (induced by nearby traffic, wind and possibly microtremors) and forced vibration tests with different applied input force induced by eccentric mass shakers were performed on the Nelson St off-ramp bridge (an 11-span post-tensioned concrete, box girder structure forming a part of the motorway network in Auckland’s CBD). Three separate system identification methods, namely peak-picking (PP), the frequency domain decomposition (FDD) and the data-driven stochastic subspace identification (SSI) method, were applied for accurate structural modal parameter identification. It was found that the three output only identification techniques are able to extract natural frequencies of the structure reliably, while the time domain SSI method yields the best mode shape estimates and PP may not be able to give accurate mode shape estimates for some modes. The variability of the dynamic properties for the 1 vertical and lateral bending modes was examined. A general trend of decreasing natural frequencies and increasing damping ratios was observed with increased level of vibration intensity.

Assessment of a segmental post-tensioned box girder bridge using ambient vibration testing

The condition and load carrying capacity of a bridge needs to be properly assessed to operate the structure safely and efficiently. The conventionally utilized static and quasi-static loading tests with weight-controlled heavy trucks have shown distinct limitations in practical applications for the assessment of existing bridges. New and efficient approaches are thus required. In this paper, the assessment of condition and load carrying capacity of the 12-span Newmarket Viaduct, situated in Auckland, New Zealand was conducted using finite element modelling and vibration testing. A finite element model was built to predict the response of the bridge. The ambient vibration testing provided the real properties of the full-scale structure and improved the accuracy of the finite element model of the bridge via model updating process. The significant parameters in calibrating the model were the stiffness of the deck and the boundary conditions at the bearings. The load capacity of the structure was then studied under dead and live loading through the analysis using the calibrated finite element model. The availability of the calibrated, reliable finite element model enhanced the effective structural assessment.

EVALUATION OF THE DYNAMIC AMPLIFICATION FACTOR FOR RAILWAY BRIDGES SUBJECTED TO A SERIES OF MOVING MASS

Abstract. This paper investigates the vertical dynamic behavior of steel rail viaducts subjected to train passage. The analyses in this study take into account the dynamics of the vehicle suspension systems, the mass of bogies and car bodies, and the dynamic properties of the viaduct as emulated by idealized mass and stiffness values. The rail vehicles are simulated as a series of moving mass with various wheel distances traversing over simply supported girders. Using finite element software, LUSAS, the dynamic amplification factors are evaluated and the results are compared with standard design practice for a wide range of speeds and a range of viaduct spans. The results show that the impact factors proposed by design codes are unconservative in many cases especially for high speed trains traversing the short span viaducts.

Ambient vibration based evaluation of a curved post- tensioned concrete box-girder bridge

This paper describes ambient vibration based evaluation of a curved, post- tensioned, concrete, box-girder bridge, the Newmarket Viaduct. The procedure includes ambient vibration testing, system identification, finite element modelling and finite element model updating. Since the dynamic excitations were not measured in the ambient testing, two operational modal analysis methods, namely enhanced frequency domain decomposition and stochastic subspace identification, were applied to identify the experimental dynamic modal characteristics. A three dimensional finite element model of the bridge was created to determine the dynamic characteristics analytically. Analytical and experimental dynamic modal characteristic were compared with each other and the finite element model of the bridge was updated by changing the material properties and boundary conditions to reduce the differences between the experimental and analytical results. It is demonstrated that the proposed procedure can successfully identify the most significant modes of the bridge and the in-situ material properties and boundary conditions. Finite element (FE) analysis of important engineering structures such as highway bridges is now commonly performed in the design or reassessment process. With the advances in numerical modelling, the FE models are first built in the design phase based on technical design data, as built drawings, on-site geometry surveys and professional engineering knowledge to predict structural behaviour, both static and dynamic. However, material properties, boundary conditions and section properties accepted in the analysis are subject to inherent stochastic variability, can change as a result of construction errors and will deteriorate with time due to aging and damage. Therefore, the performance and structural behaviour of bridges in service have to be determined with the help of field testing or experimental measurements. The experimental results can later be used to check the

Experimental and analytical modal analysis of a multiple-span motorway bridge

The paper presents the experimental and analytical modal analysis for an eleven-span continuous concrete deck bridge in New Zealand. Sine-sweep forced-vibration testing by rotating weight shakers was conducted to excite the bridge within the frequency band of interest. One of the most advanced time domain system identification techniques, i.e. the data-driven stochastic subspace identification, was used to extract the modal parameters of the bridge. Correlation analysis among the developed beam-element girder, shell-element girder models and experimental measurements was carried out in terms of natural frequencies and mode shapes, and a good agreement was achieved. This suggests that both developed numerical models have adequate accuracy from the point of view of engineering practise for precisely calculating dynamic response in different circumstances, such as earthquakes and traffic loading, as long as linear response range is considered.