Long duration blast loading and debris distribution of complex masonry panel structures

Blast loading and its interaction with structures is a complex phenomenon even in the simplest of urban settings. Modelling the effect of air blast and coupled structural response is a non-trivial task. The difficulty is increased when considering long duration blast due to the considerable drag loads imparted by the dynamic pressure phase. Long duration blast loading is defined here as an explosive event in which the positive phase duration, clearly exceeds 100msec; conventional explosives have a positive phase duration of less than 50msec. These types of load cases are most commonly associated with chemical vapour cloud detonation, e.g. 2005 Buncefield Disaster (between 150-250 tonnes TNT equivalence). Academic literature presents both researcher and practitioner with little understanding pertaining to the long duration blast response of commonplace masonry or segmental structures. Knowledge gaps exist in the methods available, principally: (a) we cannot currently calculate the amount of rubble or debris blockage that will prevent emergency services providing life saving assistance, (b) we do not have accurate tools to predict resulting casualties or net damage and, (c) current calculation methods are flawed and cannot model real structures beyond crude approximations. To solve this key gap in knowledge, the latest techniques in advanced computational modelling are required coupled with instrumentation intensive national test facility sponsored experiments.

By their nature, long duration blast loads transmit large magnitude impulse and the non-negligible effects of drag loads make interactions with structures complex to model; intrinsically more so than a conventional explosive source. When modelling structural collapse, the reliability of readily available numerical methods (e.g. Finite Element Analysis) fail in the discrete phase, particularly for brittle systems susceptible to particulate fragmentation. Newer adaptive techniques in blast effects research such as the Applied Element Method, overcome these limitations through the use of continuum decoupling techniques and collision detection algorithms. It is now possible to model complex segmental, jointed arrangements and determine a reliable debris field distribution following breakage. Preliminary research has shown that pressure equalisation on the rear structural face in the long duration case can reduce net loading by 25-30%. These effects are further complicated by dynamic pressures entraining broken fragments. Importantly for long duration blast, incident and reflected impulses are at least one order of magnitude greater leading to rapid over-matching of comparatively smaller structures.

This research proposal will use advanced computational techniques in conjunction with comprehensive experimental trials conducted in the UK, Ministry of Defence Air Blast Tunnel to derive breakage algorithms and debris fragmentation profiles. Precise mapping using mass distribution grids, 3D laser scanning and high speed video will allow the comparison of analytical and trial results. The effects of blast clearing and net pressure effects across individual panels will be examined carefully. This will form the reference benchmark for the analysis of complex interlinked structural geometries. Linking breakage algorithms to the current limited guidance for conventional small explosive exclusion zones will be a key objective.

Infrastructure resilience and the ability to resist severe extreme loading is an important topical issue facing all Civil Engineers without exception. These loads can be separated into unplanned and planned events, accidental or deliberate. Each scenario contains a high potential to cause irrecoverable structural damage, considerable economic loss and life threatening consequence. The effect of explosive blast on structures and components, particularly building infrastructure has become a key design consideration, bought to the forefront by recent events: (i) accidental large detonations at Buncefield, UK and West, Texas USA, (ii) terrorist actors intent on maximum destruction and disruption and, (iii) construction of important national infrastructure requiring protection e.g. large explosions causing fragmentation damage at power stations.

Traditional masonry, monolithic masonry and segmental structures constructed from blocks occur frequently in the built environment. These structures have a low resistance to long duration blast effects and cause problems with respect to rubble pile generation and fragment scatter. The ability of emergency services to respond can be hampered by the lack of safe passage across any debris field. Successful emergency planning is entirely dependent upon the ability to reasonably foresee the worst case scenario. Over pressure sensitive upon initial breakage, broken structural sections can travel considerable distances at very high velocities, particularly if fragments become entrained in the flow field; a problem observed during large fuel vapour detonations and nuclear source explosions. Research in this proposal will provide a detailed understanding and nationally important body of knowledge that will be directly used by both the support partners and allied parties working in this discipline. New algorithms for debris fields will be used to make a significant step change in understanding: Firstly, support partner AWE plan to incorporate the research findings within the UK Blast Damage Assessment capability models. These are used in emergency situations to define worst case scenarios. Secondly, infrastructure engineers advising on layout (both industrial and domestic) will have recourse to accurate information for safe exclusion zones.

Major asset owners will be direct beneficiaries through a direct ability to plan and appraise for the onset of an accidental or unplanned explosive event. Proposal support partner Stone Security Engineering have agreed to actively work with the PI and PDRA to disseminate research findings towards early adoption by both UK and US expert practitioners to achieve maximum industry impact.