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Blast Resistant Design

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Ref: Dileep Kulkarni's post

Dileep,
Sorry for this late response.
I was out of station for a few days.

I had occasion to do Blast Resistant Design 16 years
ago when I worked as a Structural Design Engineer for
a Govt Owned Consultancy organisation.

The memories are vague.
But here are a few tips that may help.

It's not often that one gets an opportunity to do
designs to resist blast loads.
Harold Sprague is correct.
Blast resistant design is something special.
It needs to be taken more seriously.

When we had this assigment (from the Defence), it took
us hardly any time to realize that we were up against
something special and did not hesitate to seek
professional help. Luckily, we found ourselves
deficient only in theoretical knowledge, not in the
detailed structural design and detailing. Our client
also gave us enough guiding material from previous
designs (all classified and I have no access to them
now)


We tied up with Two academics in the Indian Institute
of Science at Bangalore for the theoretical back up.
The professors gave us "Private tuitions" on
calculation of loads and computation of natural
frequencies for three months while  we did the design,
 detailing and construction supervision without their
help. They also vetted our final design.

Luckily our problem was not too complex and the
learned academics decided we could treat the load as
an Impulse load and use an equivalent static load
approach for design.

If my memory serves me right, I think the duration of
the load must be less than one tenth of the natural
time period of the structure subjected to blast
loading for this approach to be valid.

The equivalent static load could then be calculated by
the formula

esl = Impulse x Omega

where esl = equivalent static load.
Impulse = area under the curve when Force is plotted
against time. 
Omega is the circular natural frequency of the
structure (= 2 x Pi x f, where f = natural frequency
in cycles per second)

The design was interesting. 
If the structure was made stronger, the natural
frequency increased and so did the equivalent static
load.
We had to find an optimum and find ways to increase
the mass of the structure without increasing the
moment of inertia.
We opted for a double walled structure with sand
filled in the space in-between to accomplish this.

We referred to the book "Structural Design for Dynamic
Loads" by Biggs.

For computing natural frequencies we referred to a
book called "Forumulas for Natural fequency and mode
shapes" by Blevins.

Luckily, the charge in equivalent weight of TNT was
specified by our client (the Defence Scientists).
The professors helped us in working out the peak
overpressures and the duration of the blast on a
structure at a specified distance from the centre of
the blast.

You could also try the book "Explosive shocks in air"
by Kinney for guidance in arriving at peak
overpressures and the duration.

Sorry, it is now several years since I did hardcore
design and those were the days when personal computers
and sophisticated software were not so freely
available. (Helpful lists like this were also not in
existence!)

If you are located in India, (your name is Indian),
try contacting Dr A K Kar at Calcutta who has
published papers in the ASCE journal on blast
resistant design . I remember his articles on
Projectile penetration into Earth, Steel and Concrete
which we found quite relevant.


Dr R Narayana Iyengar(Professor of Structural Dynamics
was the academician we consulted, way back in 1984) He
later took over as the Director of Central Building
Research Institute at Roorkee. I wonder if he is still
active now.

In Bangalore, where I live, the scientists of INBRI
may also be of help. They were very cooperative and
freely parted with technical information when we
approached them and also made their library available
to us for reference.

(INBRI = Indian Nobel Basic Research Institute. They
deal with explosives routinely. I think it is
privately funded and is not a Government organisation)

Hope this has been of some help to you.
With Best Wishes

G Vishwanath
General Manager
FI Sofex Ltd.
Bangalore

Formerly
Deputy General Manager
MECON Limited (A Govt of India Undertaking)






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