Building vulnerability design against terrorist attacks

Some of the concepts that can help structural engineers and building owners mitigate the threat of hazards associated with terrorist attacks on new and existing buildings are discussed. The state-of-the-art methods to enhance protection of the building by incorporating low-cost measures at the early stages of building design are highlighted

Response and Prediction of Dymanic Characteristics of Worn Rail Pads Under Static Preloads

Rail pads are a major component of ballasted railway tracks. It helps attenuate the excessive dynamic stress from wheel/rail impact forces in order to prevent premature breakdown of other track components. Based on numerous analytical and numerical models, rail pad plays a crucial role on the dynamic behaviours of railway track. Over years, the rail pads have been worn by the services to carry either passenger or freight train operations. The characteristics of worn rail pads become imperative to maintenance unit as to plan the renewal schedule. Using methods of modal analysis, this paper adopts an instrumented hammer to excite an innovative rail pad test rig, modelled as an equivalent single degree-of-freedom system (SDOF), incorporating a rail pad as a resilient element. The test rig can allow the static preloads to the worn rail pads through the force sensing bolts up to 400 kN at a railseat. The vibration responses are then recorded using Bruel & Kjar PULSE system. The analytical SDOF dynamic model was applied to best fit the experimental modal measurements that were performed in a frequency range of 0-800 Hz. The curve fitting provides the dynamic parameters as the effective mass, dynamic stiffness, and dynamic damping constant, all of which are required for numerical modelling of a railway track. This would lead to a time-variable dynamic evaluation of dynamic behaviour of railway track

Experimental and Numerical Studies of Railway Prestressed Concrete Sleepers Under Static and Impact Loads

The present paper deals with experimental and numerical investigations of the static and impact behaviours of railway prestressed concrete sleepers under static and dynamic loads. A three-dimensional nonlinear finite element model of a prestressed concrete sleeper was developed using the general purpose finite element package, ANSYS 10. The model was initially validated against static test results. Using SOLID65 solid element, the compressive crushing of concrete is facilitated using plasticity algorithm while the concrete cracking in tension zone is accommodated by the nonlinear material model. Since the cross section of concrete sleeper is fully pre-stressed, the smeared crack analogy is impracticable. Discrete reinforcement modelling with truss elements, LINK8, is then more suitable to utilize. The pre-tensioning was modelled using an initial strain in the tendon elements. Perfect bonding between concrete and pre-stressing wires was assumed. Comparison with experimental static load-deflection response is presented for the prestressed concrete sleeper. Through the linkage between ANSYS and LS-Dyna, the finite element model was subsequently extended to account for ballast support and in-situ conditions for use in the impact analysis. The calibration of the extended model was successfully achieved by using vibration data. Drop-weight impact tests were carried out in the identical setup for verifications of the impact model. Validations were conducted where possible. Very good agreements were found between drop impact experiments and numerical simulations

Non-destructive testing (NDT): A tool for dynamic health monitoring of railway track structures

The applications of NDT are varied and recently, an NDT technique known as ‘modal analysis’ was adopted in railway engineering in order to measure track behaviours under either train services or man-made loadings. OVER THE PAST decade, modal testing has become an effective means for identifying, understanding, and simulating dynamic behaviour and responses of structures. One of the techniques widely used in modal analysis is based on an instrumented hammer impact excitation. By using signal analysis, the vibration response of the structures to the impact excitation is measured and transformed into frequency response functions (FRFs) using the Fast Fourier Transformation (FFT) technique. Subsequently, the series of FRFs are used to extract modal parameters such as frequency, damping, and corresponding mode shape. In a range of practical applications the modal parameters are required to avoid resonance in structures affected by external periodic dynamic loads. Practical applications of modal analysis span over various fields of science, engineering and technology

Designing explosion rated ventilation seals for coal mines using high-fidelity physics-based computer modelling

Questions have been raised about the effectiveness of ventilation control devices (VCDs) to safely resist explosions during their intended life. This functionality depends on the ability of the VCDs and in particular seals to withstand changes in the behaviour of the strata, particularly where longwall abutments influence the stress regime in and around the chain pillars. As a consequence of an explosion impact on a seal, the surrounding strata could experience increased loads possibly resulting in permanent deformation and requiring grout consolidation. These aspects of seal design have been investigated using advanced numerical analysis. Globally since the early 20th century, to protect underground personnel, ventilation seal designs have been required to be tested at an internationally recognized explosion test gallery to achieve pressure ratings required by legislation. The last two decades has seen advances in materials technology and engineering of structures. It has become accepted practice to use numerical methods to provide engineering ratings for mine seals in line with other industries where the elimination of prototype testing provides more rapid product introduction to the market. Before presenting the results of numerical analysis, structural aspects of seal design are simply explained including arching behaviour and the contribution of dynamic magnification due to impact loads. High-fidelity physics-based computer simulations using software LS-DYNA were able to predict the results from physical testing of mine based seals in a most realistic way. Test data from live gas/coal dust deflagration explosions at Lake Lynn, PA, USDA along with pressure-time curves recently developed by the National Institute of Occupational Safety and Health as a result of the study of explosive atmospheres, were used to simulate a realistic loading environment caused by 138 kPa (20- psi) and 345 kPa (50-psi) explosions in physics-based models of seals

Designing explosion rated ventilation seals for coal mines using high-fidelity physics-based computer modelling

Questions have been raised about the effectiveness of ventilation control devices (VCDs) to safely resist explosions during their intended life. This functionality depends on the ability of the VCDs and in particular seals to withstand changes in the behaviour of the strata, particularly where longwall abutments influence the stress regime in and around the chain pillars. As a consequence of an explosion impact on a seal, the surrounding strata could experience increased loads possibly resulting in permanent deformation and requiring grout consolidation. These aspects of seal design have been investigated using advanced numerical analysis. Globally since the early 20th century, to protect underground personnel, ventilation seal designs have been required to be tested at an internationally recognized explosion test gallery to achieve pressure ratings required by legislation. The last two decades has seen advances in materials technology and engineering of structures. It has become accepted practice to use numerical methods to provide engineering ratings for mine seals in line with other industries where the elimination of prototype testing provides more rapid product introduction to the market. Before presenting the results of numerical analysis, structural aspects of seal design are simply explained including arching behaviour and the contribution of dynamic magnification due to impact loads. High-fidelity physics-based computer simulations using software LS-DYNA were able to predict the results from physical testing of mine based seals in a most realistic way. Test data from live gas/coal dust deflagration explosions at Lake Lynn, PA, USDA along with pressure-time curves recently developed by the National Institute of Occupational Safety and Health as a result of the study of explosive atmospheres, were used to simulate a realistic loading environment caused by 138 kPa (20- psi) and 345 kPa (50-psi) explosions in physics-based models of seals

Simulation of impulsive loading on column using inflatable airbag technique

The purpose of this study was to simulate impulsive loading on columns by an innovative lab-based experimental technique that utilises inflatable airbags. Mild and stainless steel hollow sectioin columns with effective lengths of 955mm and under simply supported condition were used in this study

The state of the art of explosive loads characterisation

This paper presents the state of the art of characterisation of explosive loads of engineering structures. In recent years, high explosive devices have become the weapon of choice for the majority of terrorist attacks. Such factors as the accessibility of information on the construction of bomb devices, relative ease of manufacturing, mobility and portability, coupled with significant property damage and injuries, are responsible for significant increase in bomb attacks all over the world. In most of cases, structural damage and the glass hazard have been major contributors to death and injury for the targeted buildings. Following the events of September 11, 2001, the so-called “icon buildings” are perceived to be attractive targets for possible terrorist attacks. Research into methods for protecting civilian buildings against such bomb attacks has been initiated. Several analysis methods available to predict the loads from a high explosive blast on buildings in complex city geometries are examined. Analytical and numerical techniques are presented and the results obtained by different methods are compared. Results of the numerical simulations presented in this paper for multiple buildings in an urban environment have demonstrated the importance of accounting for adjacent structures when determining the blast loads on buildings

A Review of Methods for Predicting Bomb Blast Effects on Buildings

In recent years, the explosive devices have become the weapon of choice for the majority of terrorist attacks. Such factors as the accessibility of information on the construction of bomb devices, relative ease of manufacturing, mobility and portability, coupled with significant property damage and injuries, are responsible for significant increase in bomb attacks all over the world. In most of cases, structural damage and the glass hazard have been major contributors to death and injury for the targeted buildings. Following the events of September 11, 2001, the so-called “icon buildings” are perceived to be attractive targets for possible terrorist attacks. Research into methods for protecting buildings against such bomb attacks is required. Several analysis methods available to predict the loads from a high explosive blast on buildings are examined. Analytical and numerical techniques are presented and the results obtained by different methods are compared. A number of examples are given

Application of vibration measurements and finite element model updating for structural health monitoring of ballasted railtrack sleepers with voids and pockets

Although vibration measurements are very useful and common in practice nowadays in many applications related to mechanical, civil, and aerospace engineering, the utilisation in railtrack infrastructure and facilities is not widely established. This chapter deals with the application of vibration measurements and finite element model updating to the assessment of ballasted railtrack sleepers, in particular with a void and pocket condition. It describes the concept of vibration measurements and the understanding into the dynamic behaviour of ballasted railtrack sleepers. It discusses briefly on the development of finite element model of in-situ sleeper and its updating. Then, the application to structural health monitoring of the rail-track sleepers is demonstrated. Dynamic load effects on the in-situ concrete sleepers in a railway track system are very significant and highly regarded in the viewpoint of structural engineers since the resonant excitation frequencies amplify the vibration magnitudes and cause cracking of railway concrete sleepers. New concept for the design criteria of railway concrete sleepers considers such resonant effects. As one of the main components of railway tracks, ballast interacts with the sleepers and influences the dynamic characteristics of railway sleepers, as well as the dynamic modulus of railtracks. The ballast support configurations are often changed by the effects of wheel load, breakage of gravel, or loss of confinement, which therefore creates voids and pockets underneath railway sleeper. This chapter presents a comparison between the experimental investigation on free vibration behaviour of an in-situ railway concrete sleeper with voids and pockets underneath, and the numerical prediction incorporating sleeper/ballast interaction. It is aimed at demonstrating the development and application of the vibration measurement and model updating to railway engineering practice. Using finite elements, Timoshenko-beam and spring elements were used in the in-situ railway concrete sleeper modelling, whereas the voids and pockets could be superiorly treated. This model had been proven its effectiveness for predicting the free vibration characteristics of insitu sleepers under different circumstances. In addition, the modal testing results have clearly exhibited that the simplified approach is ample to predict the natural vibrations of voided railway concrete sleepers. This study has led to the prediction and understanding of dynamic characteristics of railway concrete sleepers under various configurations of voids and pockets