Daniel Ambrosini

Effects of Large and Spread Explosives Loads

This paper presents experimental data and numerical analysis of the effects of detonating explosive charges ranging from 1000 to 26288 kg laid out on the ground. The charges consist of different ordnances widespread in a carpet-like form. Numerical analysis using ANSYS/Autodyn are carried out with a view to gain a better understanding of the blast loading resulting from the detonation of large masses of ordnance. Good correlation between the numerical results and experiments in terms of crater dimensions and blast wave parameters is obtained. The influence of the charge configurations on the blast wave parameters and crater shape are also investigated. While the cube root scaled distance works well for the evaluation of pressure and impulse values produced by a relatively compact charge layout, the scaled distance parameter has to be modified for cases where charges are spread in a carpet-like fashion.

Numerical Study of Confined Explosions in Urban Environments

In recent years, events such as the attack on the World Trade Center as well as many other attacks around the world have shown that terrorism-related activity is dramatically increasing. Such a situation makes necessary to study the effects of blast loading on complex urban environments. In this paper, the phenomenon of channelling in congested urban environments is studied. Channelling is a phenomenon that can significantly increase the destructive potential of a shock wave of an explosion. Hence, channelling must be taken into account in the design of structures subjected to extreme loads in congested urban environments. This paper presents a novel approach to understanding channelling effects using maps of overpressure amplification and impulse amplification. These maps are developed for different street widths and amounts and locations of explosive loads and could be used as a first approximation for design purposes. The characteristic channelling zones are defined and characterised for the first time in the open technical literature. Finally, quantitative conclusions about the influence of the different parameters are highlighted.

A heuristic approach to output-only system identification under transient excitation

Output-only system identification under non-white-noise excitation is addressed.The classical approach consists in using long records.The white-noise excitation assumption is trivially validated during free-vibration.An algorithm is developed to automatically find free-vibration record segments.The proposed approach requires short records, leading to hardware simplifications. Output-only system identification is a very attractive technique for its implementation simplicity. However, it requires long records to validate the white-noise assumption of the excitation, mainly under transient forced vibration. Alternatively, free-vibration record segments can be selected before the identification process. This improves the accuracy, even using less data, but it requires human intervention or input recording. In the present paper, an approach is proposed for accurate system identification from short output-only records of vibration induced by transient excitation, without human intervention. The approach is based on a novel heuristic search algorithm to find free-vibration record segments, which is fully automatic and it handles the possibility of free-vibration absence. Tests with real-life data from Structural Health Monitoring (SHM) of a bridge showed that the free-vibration finding improves the accuracy of the modal parameter estimates up to ten times, as compared to using record segments starting at the response peak. The proposed approach drastically reduces the need to transmit large amounts of data, which impacts on hardware requirements of SHM implementations.

Evaluation of classical reinforcement and passive control systems on a reinforced concrete bridge subjected to seismic loading

In this paper, different strategies are studied with the main objective to reduce structural demand and damage of an existent bridge in Mendoza, Argentina, with post-tensioning reinforced concrete girders that are a common structural typology in the region. A numerical study was carried out with this purpose. The numerical model was calibrated against experimental measurements of vibration natural frequencies. The alternatives studied are: a) classical reinforcement of the structural overall stiffness; b) tuned mass dampers; c) viscous dampers; d) metallic dampers. The model is subjected to real near-fault seismic records, thereby obtaining response parameters to evaluate the efficiency of each protective system. The response is evaluated not only in terms of reduction of displacements, but also in increasing of shear force in key elements, which is a negative characteristic of some of the systems studied. Advantages and disadvantages of each studied system are highlighted.

New sacrificial cladding system for the reduction of blast damage in reinforced concrete structures

Over the past few decades, explosive devices have become the weapon of choice for many terrorist attacks. In some of these attacks, large amounts of explosives are detonated within a short distance of building structures. It has been shown that current design methods are inadequate for the prevention of progressive collapse. The protection of key elements, such as columns, is necessary to prevent this catastrophic response sufficiently. This article discusses the performance of a new sacrificial cladding that protects reinforced concrete members under blast loading. It consists of reinforced resin panels with an insulation layer. For comparison purposes, two reinforced concrete members were previously experimentally studied, a reinforced concrete column without protection and reinforced concrete column with classical steel jacketing. It can be observed from the experimental results that the new sacrificial cladding achieves an important reduction in damage and displacements.

Influence of the nonlocal parameter on the transverse vibration of double-walled carbon nanotubes

Abstract A high-order nonlocal continuum beam model is proposed, which can be applied to study the transverse vibrations of double-walled carbon nanotubes (DWCNTs), including those that could have initial deformations due to defects or external actions. A beam element is developed adopting Hermite cubic polynomials as shape functions, and mass and elastic stiffness matrix are presented. The influence of the nonlocal parameter on the vibrational properties of DWCNTs is studied. Using the proposed model, it was found that the nonlocal parameter has a strong influence on the natural frequencies.

Computational Modelling in Protective Engineering Research - Recent Advances and Good Practice

PREFACE Protective engineering has attracted significant renewed research interest in recent years. In the development to tackle the challenges arising from the needs of in-depth and detailed information about the complex dynamic structural and material responses, the use of sophisticated computational modelling has become a seemingly indispensable means. Apart from being driven by the traditional difficulties when it comes to performing physical experiments for the types of extreme loads concerned, the trend has been fuelled by the rapid advancement in the computing power and software capabilities. It is not difficult nowadays for an analyst to model even quite complex loading and nonlinear responses using just a desk-top computer. On the balancing side, however, we should always be reminded that the ability to exploit the modelling capabilities and make good sense of the results is still and will remain to be dependent upon sound comprehension of the underlying mechanics and physical processes, as well as calibration of the models by rigorous comparison with experimental results. This special issue of International Journal of Protective Structures (IJPS) has brought together a state-of-the-art representation of the latest advances in the computational modelling of protective structures against a variety of high impulsive loads. By sharing the experiences on the successes, challenges, and pros and cons in the development and implementation of various modelling techniques, it is hoped that the special issue will also serve to promote the best practice in performing competent numerical modelling analysis to meet the challenges on both the research and practical design fronts. Contributions to this special issue have been solicited by invitation, and all the papers submitted have been reviewed by at least two experts in the relevant field of studies. Topics covered in this special issue include fundamental modelling considerations such as the effects of shear dilatancy and failure limits in the concrete models on the simulated blast behaviour of reinforced concrete structures; specific modelling considerations such as coupling in a multi-layer medium environment and the erosion limits; FE analysis involving high strength materials, fibre-reinforced concrete, and shell structures; as well as modelling of explosive hazard in a broader spatial domain. We wish to take this opportunity to thank all contributors for sharing their research experiences and findings in this special issue of IJPS. We also wish to extend our thanks to all the reviewers for providing timely review of the papers and valuable comments. International Journal of Protective Structures – Volume 4 · Number 3 · 2013 i

Determinación de cargas generadas por explosiones en ambientes urbanos

El objetivo principal de este trabajo es la determinacion de la masa de explosivo y la ubicacion del foco de la explosion generada por un ataque terrorista en un ambiente urbano congestionado. La metodologia presentada es especialmente util cuando las dimensiones del crater generado por la explosion son inciertas o desconocidas. Se realizo un analisis computacional para la determinacion de presiones e impulsos en las fachadas de los edificios de una cuadra en un ambiente urbano real. Se realizaron simulaciones correspondientes a diferentes combinaciones de masa de explosivo - ubicacion del foco. Por otra parte, se realizo la evaluacion del dano producido en los edificios a traves de la utilizacion de las curvas de isodano que relacionan, en forma aproximada, las presiones e impulsos con los danos producidos. Posteriormente se definieron contornos de igual dano y se trazaron los mapas de isodano en toda la cuadra, para las diferentes alternativas analizadas. Por ultimo, a traves de la comparacion de los danos reales y los simulados computacionalmente, pudieron descartarse las alternativas no coincidentes.