Blast Mitigation Techniques

Dr. Abass Braimah

Canadian Explosives Research Laboratory
Ottawa, ON

Blast Mitigation Design

Structural design for explosive blast has been an

active research topic for the military and security
communities for decades; post WWII through cold
war
A lot of research findings are classified and released
on a “need to know basis” by US government
Recent concerns over worldwide terrorism is leading
to release of some of this research data
Committee on feasibility of applying blast-mitigating
technologies and design methodologies from military
facilities to civilian building
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 Blast Mitigation Design

Objective of every blast-mitigation or blast-retrofit

design is to:
Safeguard personnel
Prevent property damage
Ensure business continuity.

Blast Mitigation Design

At Façade Perimeter Security

Windows Walls
90 m
Walls Bollards
Columns Jersey barriers
Beams Bitsburg barriers
Roof Berms
30 m Ditches
10 m
Stand-off

Within Secure Perimeter

230 m Vegetation (trees)
Ponds
Serpentine approach

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 Windows

Weakest members of façade

Break into “knives and
daggers”
Account for most injuries
About 66% of injuries in the
Oklahoma City bombing was
attributed to flying glass

Window Glass
Glass is produced from soda, lime and silica sand by the float
process. The materials are heated to 1500oC and floated
in a tin bath and cooled
Annealed glass – controlled temperature and uniform
cooling;
low tensile strength,
breaks into sharp pieces
Tempered glass – heat treated or toughened. Annealed
glass is heated to about 680oC and rapidly cooled.
temperature gradient across thickness creates compression
state of stress
high tensile strength
increased impact and thermal resistance
breaks into small cubical fragments 6

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Window Glass
Chemically strengthened glass – annealed glass is
submerged in a chemical solution (molten salts)
increased mechanical resistance,
similar properties to tempered glass
rarely used for window (usually made into thin glass)
Laminated glass – is a composite glass product
manufactured by bonding two or more sheets of glass
with layers of polyvinyl butyral (PVB) or other resin
under high temperature and pressure
PVB or resin holds glass fragments together upon
breakage.
Strength, toughness, impact and heat resistance depends
on type of glass in composite
7

Window Glass

Polycarbonate – high-performance thermoplastic

High shatter resistance
High heat resistance
Increased toughness
Easily scratches
Normal cleaning can lead to deterioration
Glass-clad polycarbonate resolves this problem

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 Window Retrofit

Objective
Protect building occupants from glass
shards
Methods
Anti-shatter film
Blast curtain systems
Catch cable/bar systems

Window Retrofit
Anti-shatter film (Polyester)
ASF consists of polymer films with high tensile strength and
deformation capacity.
When adhered to glass they bind glass shards and mitigates
against glass shard injury.
ASF can be used to increase window performance with
respect to total solar transmittance, reflectance or absorption.
They can also provide protection against UV light
Application Methods
Daylight film application
Wet-Glaze application
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Mechanical anchorage

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 Anti-shatter film

Daylight Application
Frame
Advantages
Mitigates glass shard injury
IGU

Disadvantages
Potential to injure occupants
Does not prevent blast
pressure infiltration into ASF
occupied spaces

11

Anti-shatter film
Wet Glaze
Advantages
Mitigates glass shard injury
Mitigate against trauma injury Structural
Disadvantages Silicone
Transfers load to façade backup
structure
Might require window frame
upgrade
More expensive than daylight
application

12

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 Anti-shatter film
Mechanical anchorage
Advantages
Mitigates against glass shard
injury
Mitigates against blunt trauma Anchor
Disadvantages (Screw)
Might require window frame
and façade backup structure
upgrade
More expensive upgrade than
daylight film and wet-glaze
application

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Blast Curtains (Blinds)

Safety drapes or blinds

added to interior of windows
and anchored to floor and
ceiling
Architectural consideration
Must be drawn during the
explosion event to be
effective

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 Catch Systems
Catch Cables
Catch bars are rigid bars
anchored to the window
frame or façade backup
structure Catch Bars
Catch cable are flexible cords
or bars attached to the
window frame or façade
backup structure
Energy absorbing catch bars
and cables are also being
marketed.
Catch systems are used in conjunction with ASF.
They are designed to catch filmed glazing during
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explosion

Experimental Tests

Filmed Tempered Glass

Anchored single pane 5.6 mm
thick + ASF
Blast Load
Pr = 5 psi (34.5 kPa)
Ir = 35 psi-ms (241 kPa-ms)

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 Experimental Tests
Heavy Laminated Glass
Glass Properties
41.3 mm thick
Blast Load
Pr = 200 psi (1.4 GPa)
Ir = 350 psi-ms (2.41
GPa-ms)

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Experimental Tests
After Test

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 Experimental Tests
Tempered ? Glass
Glass Properties

Blast Load
Pr = 10 psi (69 kPa)
Ir = 50 psi-ms (345 kPa-ms)

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Windows: P–I Diagrams

Every blast load has an associated impulse

The impulse is depended on the decay
coefficient and the duration of the blast.
P-I diagrams presents iso-damage curves that
permit easy assessment of response of an
element to a given load-impulse combination
P-I diagrams are usually drawn for a given failure
or damage criterion and is not universally
applicable

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 Windows: P–I Diagrams

Pressure

Failure
I1 P1 and I1 causes failure

I2 P2 and I2 causes no failure

No Failure
P2 P1 Impulse
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GSA Protection Level for Glazing

1,2 5

4
3a 3b 600 mm
1000 m
3000 m 22

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 GSA Protection Level for Glazing
Performance Protection Hazard Description of Glazing Response
Condition Level Level
1 Safe None Glass does not break. No visible damage
to glazing
2 Very high None Glass cracks but remains in frame.
Dusting of very small fragments near sill
or on floor
3a High Very low Glass cracks. Fragments on floor within 1
m from window
3b High Low Glass cracks. Fragments on floor within 3
m from window
4 Medium Medium Glass cracks. Fragments on floor and
impacts witness panel at 3 m from
window at a height < 600 mm
5 low high Glass cracks. Fragments on floor and
impacts witness panel at 3 m from 23
window at a height > 600 mm

Windows: P–I Diagrams

Effect of Glass Type
1000 Glass Size
1200 x1200 mm
Thickness = 5.563 mm
Pressure (kPa)

100

10

1
1 10 100 1000 10000

Impulse (kPa-ms)
Single pane double glazed double glazed-Tempered
double glazed-Filmed Tempered double glazed - film 24

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 Windows: P–I Diagrams
Effect of Glass Aspect Ratio
100
Glass Area = 1.44 m2
Pressure (kPa)

10

1
1 10 100 1000 10000

Impulse (kPa-ms)
1.2x1.2 0.9x1.6 0.8x1.8 0.6x2.4 25

Window Frame Load

Loads from the glass is

transferred to the window
frame
The window frame must
be designed to resist or
transfer these loads to the
structural backup of the
façade
No design or retrofit is
complete until the load is
transmitted to a structural
member capable of
resisting the load 26

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 Blast Doors

Types of Blast Doors

Hollow metal
Structural steel plate
Built-up structural steel
Reinforced concrete

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Blast Doors
Hollow Metal Blast Doors
Constructed by spot welding a
steel plate (sheet) on to hollow
steel sections (HSS)
Steel sheet thickness < 4.5 mm
to accommodate welds
HSS typically HSS 51x51
Not very effective against
fragment impact
Max. Pr = 130 kPa

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 Blast Doors
Structural Steel Plate Blast Doors
Structural steel plate blast doors
are least expensive because of
lower fabrication cost
Structural steel plate door with
aspect ratio >2 are effective in
reducing deflections.
Steel plates thickness of > 200
mm are readily available
Door size is limited by the weight
of steel
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Blast Doors

Built-up Structural Steel Blast Doors

Constructed by welding a steel plate
(sheet) on to steel shapes (S-Shape)
S-shapes are preferred because of
narrow flanges
Typically used where spans do not
allow use of structural steel plates
Amount of welding required can
dramatically increase fabrication cost
Welding can lead to warping of
members. Also, because welds are
concealed, inspection of welds could
be required 30

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 Blast Doors
Reinforced Concrete Blast Doors
Consists of a doubly reinforced
concrete panel cast into a
structural steel channel frame
Steel reinforcement bars are
often welded to the structural
steel frame
A steel spall plate is provided at
the interior side of the to prevent
injury from concrete scab
Reinforced concrete blast door
are cheaper to construct and
install but can be very heavy and
difficult to operate and move 31

Blast Door: Hardware

Frame
Door Frame
Anchored into structure
Usually a sub-frame is cast into
concrete
Hinges
Hinges Rolling bearing or rolling non-
Latching bearing type
Bearing type is easier to operate
but more susceptible to damage
Latching
Blast Door Multiple latch points
Operated by single or multiple
lever-type handles. Or a wheel
handle
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 Precautions for Blast Door
Door must be operable after an explosion event
Might need gasket seals to increase weather
tightness
Latch bolts must have enough shear strength to
resist blast loads
Door must be easy to operate, door weight vs. ease
of operation must be investigated
Blast door must be looked at as a system consisting
of door, frame, hinges and latching systems. Failure
of anyone component results in failure of the door
Coordination between structural engineer and door
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manufacturer essential

Walls Retrofit
Buildings with masonry infill walls, brick veneer,
or steel stud walls are susceptible to blast
loading
Masonry walls
Are very brittle and fail under low blast loading
Breaks into debris causing injury to occupants and
damage to assets
Steel stud walls
Lack stiffness to resist high pressures
Connections to the structure must be given
special consideration
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 Wall Retrofit

Wall retrofit systems in use

today include:
Elastomeric sprays
FRP sheets
Geotextiles
Steel plates
Concrete skin

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Walls
Adhesive Systems

Elastomeric sprays
Anchor
Polyurea
FRP
CFRP, GFRP, AFRP Elastomeric Spray
sheets FRP Sheet
Epoxy based adhesive Geotextile
Geotextiles
Anchor
Anchorage to structure

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 Walls

Steel plates Anchor

Steel plates anchored to

Steel Plate
backup frame
Steel Column
Steel columns can be
used to support plate
Steel plates mitigate
against debris hazard from Anchor
masonry

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Walls
Concrete skin
Concrete skin mitigates
against debris hazard of Anchor
masonry Steel reinforcement
Resist blast load (depending Concrete wall
on connection)
Difficult to construct
Increase in structural loading

Anchor

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 Steel Stud Wall
Pr = 33 psi
Ir = 200 psi-ms

The wall consisted of steel studs 18 ga.

Steel studs at 400 mm with Exterior
insulation finish system (EIFS). The
exterior flange of studs was sheathed
with 16 ga. steel and the interior with
6 mm gypsum board glued to 20 ga.
steel sheet. 39

Columns

Perimeter columns are often

FRP Wrap
designed for gravity loads
Steel Jacket
Column ties for confinement
(shear reinforcement) Concrete Column
Under blast are loaded in
flexure
Retrofit methods
FRP wraps
Steel jacketting

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Questions ?

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