Michael D. Netherton

Security risks and probabilistic risk assessment of glazing subject to explosive blast loading

A probabilistic risk assessment (PRA) procedure is developed which can predict risks of explosive blast damage to built infrastructure. The present paper focuses on window glazing since this is a load-capacity system which, when subject to blast loading, has caused significant damage and injury to building occupants. Structural reliability techniques are used to derive fragility and blast reliability curves (BRCs) for annealed and toughened glazing subjected to explosive blast, for a variety of threat scenarios. The probabilistic analyses include the uncertainties associated with blast modelling, glazing response and glazing failure criteria. Damage risks are calculated for an individual window and for windows in the facade of a multi-storey commercial building. If threat probabilities can be estimated then the paper shows illustrative examples of how this information, when combined with risk-based decision-making criteria, can be used to optimise risk mitigation strategies.

Blast Load Variability and Accuracy of Blast Load Prediction Models

A statistical analysis of explosive blast loading field (test) data has revealed a high level of variability of peak reflected pressure, impulse and time of positive phase duration for repeatable tests where variability would be expected to be a minimum. The model error (accuracy) of a widely used predictive blast load model is also assessed. A probabilistic model of blast loading is then developed that considers variability and/or uncertainty of explosive mass, net equivalent quantity of an explosive in terms of TNT mass, stand-off distance, air temperature, air pressure, inherent variability and model error. Two widely used explosives are considered: Tritonal (military) and ANFO (terrorism). This type of statistical and probabilistic analysis is essential for structural reliability analysis of structures subject to explosive blast loading where load variability is an important contributor to damage and safety risks. It was found that the TM5–1300 design values for peak reflected pressure and time of positive phase duration adequately represent median values of the probability distribution of blast loads. The TM5–1300 design values for peak reflected impulse were 40% higher than median values with probabilities of exceedance of only 4% to 23%. This over-estimation of actual blast loads on a structure may lead to conservative design outcomes.

Reliability-Based Design Load Factors for Explosive Blast Loading

AbstractReliability-based design allows the decision maker to select the level of reliability for a specific blast loading scenario and key to this is an understanding of airblast uncertainty. Hence, explosive field trials have been conducted in Australia that measured the variability of free-field blast loading caused by military standard plastic explosives. The results have revealed a high level of variability of peak incident pressure, impulse, and time of positive phase duration for repeatable tests where variability would be expected to be a minimum. The accuracy of predictive blast load models (model error) was also assessed. A probabilistic blast load computer model is revised to capture these observed variabilities. The effect of a 20% mass-increase safety factor typically applied to explosive mass on the probability of exceeding a design blast load is assessed. Reliability-based load factors are calculated where the nominal load is multiplied by the load factor to ensure that the actual load is e...

Risk-based blast-load modelling: techniques, models and benefits

There are many deterministic blast-load methods currently in use, such as (1) those for the ready calculation of explosive pressure, impulse and duration; (2) the derivation of explosive safety distances; or (3) the determination of safety hazards (and other consequences) following an explosive’s detonation. In this article, we argue that deterministic blast-loading methods do not fully account for society’s usual acceptance (or rejection) of the risks associated with damage, safety and/or injury as a result of an explosive blast-load. This article details the state of the art of probabilistic blast-load modelling that supports a quantitative calculation of risk, with respect to damage, safety and injury. The probabilistic models draw data from the literature and from our own field trials. The article details the benefits that flow from this form of blast-load characterisation and concludes with a discussion on how probabilistic methods be used to derive cost–benefit advice with respect to any proposed risk mitigation solution.

Probabilistic modelling of safety and damage blast risks for window glazing

There are many computational techniques to model the consequences to built infrastructure when subject to explosive blast loads; however, the majority of these do not account for the uncertainties associated with system response or blast loading. This paper describes a new computational model, called “Blast-RF” (Blast Risks for Facades), that incorporates existing (deterministic) blast-response models within an environment that considers threat and (or) vulnerability uncertainties and variability using probability and structural reliability theory. The structural reliability analysis uses stress limit states and the UK Glazing Hazard Guides rating criteria to calculate probabilities of glazing damage and occupant safety hazards conditional on a given blast scenario. This allows the prediction of likelihood and extent of damage and (or) casualties, useful information for risk mitigation considerations, emergency services contingency and response planning, collateral damage estimation, weaponeering, and p...

Field testing and probabilistic assessment of ballistic penetration of steel plates for small calibre military ammunition

The penetration of projectiles into semi-infinite targets helps in the understanding and modelling of terminal ballistics. The article describes field test results of 5.56×45 mm F1 Ball and 7.62×51 mm M80 Ball ammunition. The targets were 25-mm-thick mild and high strength steel plates of Grade 250 MPa and 350 MPa, respectively. The tests recorded penetration depth, muzzle and impact velocities, and bullet mass. Despite its smaller calibre, the 5.56 mm × 45 mm F1 Ball ammunition recorded deeper penetrations than the larger calibre 7.62 mm × 51 mm M80 Ball ammunition. This is due to the 5.56 mm ammunition comprising a hardened steel penetrator and lead core, whereas the 7.62 mm ammunition comprised only a lead core. Multiple shots were fired for each type of munition. The coefficient of variation of steel penetration is approximately 0.10 and 0.03 for 5.56 mm and 7.62 mm rounds, respectively. The article also presents predictive models of steel penetration depth and compares these to the field test results.

Performance, Reliability and Security Risks of Glazing Subject to Explosive Blast Loading

Abstract Risk assessment techniques need to be developed to quantify the safety hazards and economic losses posed by explosive blast loading, and the assessment of risk mitigation measures. The paper applies a probabilistic risk assessment to explosive blast damage caused by terrorist attacks to built infrastructure. Window glazing is adopted as a typical load-capacity system for detailed analysis. Fragility and blast reliability curves are developed for annealed and toughened glazing, for various threat scenarios comprising combinations of explosive charge weights and stand-off distances. Knowledge of the relative likelihood of these threat scenarios are then used to calculate probabilities of glass failure for a typical building facade. A life-cycle cost analysis is used to assess the sensitivity of the most cost-effective design solution for glazing to the attack probability.