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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
INTRODUCTION
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.
“Civil engineers today need guidance on how to design
structural systems to withstand various acts of
terrorism.” Remennikov [2002]
Blast load assessment based on scaled distance
in simple geometries
FOR SINGLE BUILDINGS WITH SIMPLE GEOMETRY, TOOLS
TO CALCULATE BLAST RESULTANTS INCLUDE:
Manuals: TM5-1300 (now UFC3-340-02)
TM5-855-1 (now UFC3-340-01)
Software: ConWep
BECv4
Books: Explosion hazards and evaluation
Blast and ballistic loading of structures
Blast effects on buildings
See „References‟ for details
A number of images used
both in Sub-chapter IV.3
and in this presentation are
taken from:
BLAST EFFECTS ON
BUILDINGS (2nd Edn)
edited by
David Cormie, Geoff Mays
and Peter Smith
[see „References‟]
Range, meters
Pre
ssu
re,
MP
a
Pressure vs. RangeHemispherical Surface Burst
0.5 0.7 1 2 3 4 5 6 7 8 910 20 30 40 50 70 100 200 300 5000.002
0.003
0.005
0.01
0.02
0.03
0.05
0.1
0.2
0.3
0.5
1
2
3
5
10
20
30
50
100
200
300
500
1000
Charge weight 1000 kilograms TNT
Incident Pressure, MPaReflected Pressure, MPa
Blast loading assessment in simple geometries
CONWEP OUTPUT: 1000kg TNT AS SURFACE BURST (REPRESENTING VBIED)
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.
The effect of buildings along a street
Buildings
Buildings
Pressure measured here
Vehicle bomb detonated at some location along the centre of
the street: how are blast resultants at building A affected?
VBIED
Presence of buildings enhances blast pressure and the
impulse delivered to Building A
THIS ENHANCEMENT COULD NOT HAVE BEEN
ACCURATELY PREDICTED WITH SIMPLE TOOLS
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
„POROSITY‟: How does façade failure affect
blast propagation along a street?
Model wall with 48% „porosity‟ showing location of pressure transducers
Porosity (%)
Imp
uls
e (
kP
a-m
sec)
0 10 20 30 40 50 60 70 80 90 10015
20
25
30
35
40
45
50
55
Impulse vs porosity at a scaled distance along porous
façade of 3.0 m/kg1/3
INCREASING ‘POROSITY’ - BLAST LESS INTENSE FURTHER ALONG STREET
SHIELDING AND CHANNELLING:
A SCHEMATIC VIEW OF THE PROCESSES
Shielding effect
Target
Channeling effect
Explosive charge
„Real‟ array with 2t VBIED
VBIED
PLAN OF ‘REAL’ ARRAY
VBIED Me
asu
rem
en
ts a
t
Lo
ca
tio
n A
A
Time (msec)
Pre
ss
ure
(K
Pa
)
Pressure-Time History
0.6 1.2 1.8 2.4 3 3.6 4.2 4.8 5.4 6 6.6-75
-50
-25
0
25
50
75
100
Pressure-time histories captured in small-scale experiments at
Location A in ‘real’ array of buildings
RECORDS ARE REPEATABLE BUT
EXHIBIT CONSIDERABLE COMPLEXITY
Simulations of „real array‟ using LS-Dyna by Kiliç
Areas of interest in „real‟ array
RED areas – shielding? YELLOW areas – channelling?
VBIED
55m
HIGH PRESSURE REGIONS OCCUR WHERE LOW
PRESSURE MIGHT BE EXPECTED!
LEFT
RIGHT
SIMULATED PRESSURE CONTOURS ON LEFT AND RIGHT FAÇADES
(from Air3d by Rose) RED = high; PURPLE = low
„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)
„Real‟ standoff - passive barriers
Images from Cormie et al [2009]
„Real‟ standoff – active measures
Images from Cormie et al [2009]
New US Embassy, London „real‟ and „virtual‟
standoff
[© KieranTimberlake/studio amd] (from USEmbassy.org.uk [2010])
Examples of blast walls
Peak pressure
behind plane,
canopied and
mounded blast
walls
[from Protective Structures
Automated Design System
v1.0 Sept 1998]
Blast wall
performance
Progressive collapse at
Ronan Point, London: 18th
floor gas explosion 1968
Chamber of Shipping, London
built prior to post-Ronan Point
Building Regs amendments:
VBIED attack 1992
Kansallis House, Bishopsgate, London, design
incorporated the post-Ronan point tying
requirements: VBIED attack 1993
RESPONSE: PRE- and POST- TYING
REQUIREMENTS
Images from Cormie et al [2009]
Progressive collapse on removal of key element
Murrah Building, Oklahoma
City, USA designed to the
American Concrete Institute
code ACI 318-7. A transfer
beam destruction promoted a
progressive collapse: VBIED
attack 1995
Image from Cormie et al [2009]
Robust building design
Three “design methods for structural
robustness” generally common to the
different international codes and
standards are identified in Ch IV.3.5.3
– 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”].
Façade failure allows blast to enter a building
…..shards from failed annealed glass
being projected into the building….
SELECTION OF AN APPROPRIATE
GLAZING ELEMENT IN A ROBUST
FRAME WILL AVOID……………..
…..and even dice-like fragments from
failed tempered glass are undesirable Images from Cormie et al [2009]
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
Image from Cormie et al [2009]
ConclusionsA 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!