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!