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RE: FALLING ICE

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Michael,

This is not a particularly easy problem, but it is one that the tower
industry has mitigated based on past history.

In a general sense the problem is a missile barrier analysis.  The
deferability of the falling ice shard and the barrier characteristics
(material type, thickness, mass, etc.) play major roles in trying to
calculate an equivalent static force.  There are separate formulas for
concrete and steel barriers that have been around for years from the nuclear
industry.  You can find further help by looking at the Petry formula if it
is a concrete barrier; or the research from the Ballistic Research Lab and
Stanford Research Institute for steel barriers.

There are plastic impacts and elastic impacts.  You have to access a local
response (probably a plastic impact) and an overall barrier response
(probably an elastic impact).  Another consideration is missile rebound.

One reference you might want to consider is "Barrier Design Procedures" by
the NRC.

The specific problem in the terrestrial microwave and broadcast tower
industry is generally referred to ice accretion and ice shedding.  The tower
industry response is more mundane.  They try something and see if it works.

Membrane roof structures adjacent to these towers are generally protected
with 2 inch thick pavers.  Smaller equipment shelters constructed of
composite FRP generally perform well in resisting the impact of the ice
shards.  One potential problem area is the wave guide and coax bridges,
which are usually protected by a 10 gage galvanized steel plate.

Regards,
Harold Sprague
The Neenan Company


-----Original Message-----
From: Michael O'Neil [mailto:moneil(--nospam--at)f-w.com]
Sent: Wednesday, May 26, 1999 1:14 PM
To: SEAINT (E-mail)
Subject: FALLING ICE


I AM LOOKING FOR INFORMATION ON THE IMPACT FORCE FROM ICE FALLING FROM A
TOWER OR TALL BUILDING AND HOW THIS FORCE IS DISSIPATED.

THANKS

MICK O'NEIL