BLAST LOADING ASSESSMENT AND MITIGATION IN THE CONTEXT OF THE PROTECTION OF CONSTRUCTIONS IN AN URBAN ENVIRONMENT (Sub-chapter IV.

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1 BLAST LOADING ASSESSMENT AND MITIGATION IN THE CONTEXT OF THE PROTECTION OF CONSTRUCTIONS IN AN URBAN ENVIRONMENT (Sub-chapter IV.3) presented by Dr Peter D Smith Reader in Protective Structures Cranfield University, Defence Academy, Shrivenham, UK

2 INTRODUCTION Civil engineers today need guidance on how to design structural systems to withstand various acts of terrorism. Remennikov [2002] Sub-chapter IV.3 offers this guidance by: Assessing blast loads on buildings in an urban environment using simple (though sometimes limited) techniques Introducing empirical approaches that account for shielding and channeling effects and discussing the use of numerical simulation. Discussing the desirability of creating both real standoff (by means of barriers ) and virtual stand-off (using blast walls ). Developing robust buildings by providing a building façade incorporating a glazing system that prevents blast entering the building.

3 Blast load assessment based on scaled distance in simple geometries FOR SINGLE BUILDINGS WITH SIMPLE GEOMETRY, TOOLS TO CALCULATE BLAST RESULTANTS INCLUDE: Manuals: Software: Books: TM (now UFC ) TM (now UFC ) ConWep BECv4 Explosion hazards and evaluation Blast and ballistic loading of structures Blast effects on buildings See References for details

4 A number of images used both in Sub-chapter IV.3 and in this presentation are taken from: BLAST EFFECTS ON BUILDINGS (2 nd Edn) edited by David Cormie, Geoff Mays and Peter Smith [see References ]

5 Pressure, MPa Blast loading assessment in simple geometries CONWEP OUTPUT: 1000kg TNT AS SURFACE BURST (REPRESENTING VBIED) Pressure vs. Range Hemispherical Surface Burst Incident Pressure, MPa Reflected Pressure, MPa Charge weight 1000 kilograms TNT Range, meters

6 Blast load assessment in more complex geometries VBIED in a complex urban geometry: many buildings near the point of detonation assessment of the loading experienced by a particular building becomes more difficult. more complicated if building façades partly or completely fail and the blast enters.

7 The effect of buildings along a street Vehicle bomb detonated at some location along the centre of the street: how are blast resultants at building A affected? Buildings VBIED Buildings Pressure measured here

8 Presence of buildings enhances blast pressure and the impulse delivered to Building A THIS ENHANCEMENT COULD NOT HAVE BEEN ACCURATELY PREDICTED WITH SIMPLE TOOLS

9 Influence of street configurations Sub-chapter IV.3 summarises: Confining effects of bends, X-roads, T-junctions etc Effect of street width in producing multiple reflections Effect of building height influence on blast resultants at street level Effect of detonation at some distance from a bend or junction etc Effect of façade failure on loading of adjacent buildings Effects of arrays of buildings in providing shielding or creating channelling effects in the urban environment

10 POROSITY : How does façade failure affect blast propagation along a street? Model wall with 48% porosity showing location of pressure transducers

11 Impulse (kpa-msec) Impulse vs porosity at a scaled distance along porous façade of 3.0 m/kg 1/ Porosity (%) INCREASING POROSITY - BLAST LESS INTENSE FURTHER ALONG STREET

12 SHIELDING AND CHANNELLING: A SCHEMATIC VIEW OF THE PROCESSES Target Channeling effect Explosive charge Shielding effect

13 Real array with 2t VBIED VBIED

14 Measurements at Location A PLAN OF REAL ARRAY A VBIED

15 Pressure (KPa) Pressure-time histories captured in small-scale experiments at Location A in real array of buildings 100 Pressure-Time History Time (msec) RECORDS ARE REPEATABLE BUT EXHIBIT CONSIDERABLE COMPLEXITY

16 Simulations of real array using LS-Dyna by Kiliç

17 Areas of interest in real array VBIED RED areas shielding? YELLOW areas channelling?

18 SIMULATED PRESSURE CONTOURS ON LEFT AND RIGHT FAÇADES (from Air3d by Rose) RED = high; PURPLE = low LEFT RIGHT 55m HIGH PRESSURE REGIONS OCCUR WHERE LOW PRESSURE MIGHT BE EXPECTED!

19 Real and Virtual standoff Real standoff is created by obstacles that maintain distance between threat and target (e.g. bollards, planters, etc.) Virtual standoff is created by barriers that absorb and/or deflect blast energy away from the target (e.g. blast walls)

20 Real standoff - passive barriers Images from Cormie et al [2009]

21 Real standoff active measures Images from Cormie et al [2009]

22 New US Embassy, London real and virtual standoff [ KieranTimberlake/studio amd] (from USEmbassy.org.uk [2010])

23 Examples of blast walls

24 Blast wall performance Peak pressure behind plane, canopied and mounded blast walls [from Protective Structures Automated Design System v1.0 Sept 1998]

25 RESPONSE: PRE- and POST- TYING REQUIREMENTS Chamber of Shipping, London built prior to post-ronan Point Building Regs amendments: VBIED attack 1992 Progressive collapse at Ronan Point, London: 18th floor gas explosion 1968 Images from Cormie et al [2009] Kansallis House, Bishopsgate, London, design incorporated the post-ronan point tying requirements: VBIED attack 1993

26 Progressive collapse on removal of key element Murrah Building, Oklahoma City, USA designed to the American Concrete Institute code ACI A transfer beam destruction promoted a progressive collapse: VBIED attack 1995 Image from Cormie et al [2009]

27 Robust building design Three design methods for structural robustness generally common to the different international codes and standards are identified in Ch IV Tie-force based design methods Alternate load-path methods Key element design Detailed discussion of these approaches is provided in Chapter 10 of Cormie et al [see References ].

28 Façade failure allows blast to enter a building SELECTION OF AN APPROPRIATE GLAZING ELEMENT IN A ROBUST FRAME WILL AVOID....shards from failed annealed glass being projected into the building...and even dice-like fragments from failed tempered glass are undesirable Images from Cormie et al [2009]

29 Laminated glass in a robust framing system Severely cracked laminated panes with the polyvinylbutyral [pvb] interlayer stretched but not torn has been completely retained in frames and the blast has been excluded from the building s interior

30 Conclusions A summary of current knowledge and understanding of the factors that are important in the development of blast loading assessment and mitigation in the context of the protection of constructions in an urban environment has been provided. Understanding of the threat and the loading that it can generate is of primary importance and the paper reviews the methods available for such load prediction. Methods for mitigation of the effect of blast loading by the provision of both real and virtual stand-off have been presented The effects of blast can be further reduced by strengthening the building s fabric to ensure that: it is of robust construction to prevent disproportionate collapse it has a façade that will not readily be breached, keeping blast from entering the building. By application of these approaches the level of building damage will be reduced and EVEN MORE IMPORTANTLY the safety of the building s occupants will be increased. THANK YOU FOR YOUR ATTENTION!