Ice Missles 1

[msquared48]

I have an ice shield I have to design that is to be placed over some elex=ctrical equipment next to a 340 foot lattice tower. Does anyone know the mass of the chunk of ice that is required to be used to compute the impact force to the shield?

I am checking in TIA 222-G. IBC and ASCE7 do not cover that from what I can see.

Mike McCann
MMC Engineering

 

[msquared48]

Nothing in TIA either. I'll just have to assume something logical I guess.

Mike McCann
MMC Engineering

 

[Robbiee]

Mike,
According to this article, chunks as long as 4 feet fell from the cables of this bridge.

http://www.theglobeandmail.com/news...ce-chunks-on-port-mann-bridge/article6594158/

 

[ztengguy]

Just shape like a dog house to deflect off

 

[SteelPE]

My boss did one of these years ago (20+). I believe he said he used a 10lb piece of ice and used a heavy grating to smash the ice into little pieces so when it hit the structure below it didn't go through the roof. He show it to me one time and as far as I know the structure is still in place and working fine.

 

[msquared48]

Yea. I calculated a 10# chunk, but used 20#. Impact force is 4K. No worries. Thanks.

Mike McCann
MMC Engineering

 

[msquared48]

You know though, in fact, the calculated velocity (air neglected) is about 148 fps. With terminal velocity at 88 fps, and the associated air pressure, the ice will more than likely break into smaller pieces by the time it hits the ice shield. I assumed a chunk of 1/2" ice with an interior radius of 6", 5 feet long for my culprit. I think I will be more than OK.

Mike McCann
MMC Engineering

 

[msquared48]

Sorry - terminal velocity at 88 mph or 129 fps.

Mike McCann
MMC Engineering

 

[azcats]

Just for my curiosity, how did you determine the terminal velocity?

 

[msquared48]

Kinda what I have heard from watching parachuting videos... Real precise I know.

Mike McCann
MMC Engineering

 

[SkiisAndBikes]

Do not know answer to your question regarding the mass of an ice chunk, however, I did work on a similar project where falling concrete was an issue. No one could accurately determine the maximum mass of a potential falling object, therefore we adopted a two layer system.

The upper layer (roof/canopy structure) was designed to be more flexible than the lower layer. There was a reasonable gap between the layers (approx. 12") We found via testing that the upper layer would dissipate much of the energy and in many cases would retain much of the smaller and/or medium sized test chunks. In the event of a larger sized chunk, the upper layer would break, however, much of the energy was dissipated and the lower layer would easily retain the larger sized chunk. More expensive construction but if you have a small area to protect, it might be work considering.

I have also seen a large wire mesh net (supported via steel poles)installed above a large surface parking lot that is located directly beneath a tall tower to protect the vehicles from ice damage.

 

[JedClampett]

I'd design it using the common sense recommendations above and put a sign there, "Beware of Falling Ice." Some artwork showing ice hitting someone's head would be a nice touch.

 

[DRWeig]

No structural experience here nor much knowledge, but I thought I would throw in an outlier I experienced. I lost a car to one ice chunk, and there were over 20 other cars in the parking lot either totaled or nearly destroyed as well.

10-story apartment building with a low-sloped mansard roof of standing-seam metal construction. The seams were about six feet apart, and each section was about 10 feet long. During "Ice Storm Elvis" in Memphis in 1994, the ice layered up to over 4" thick. It was solid ice. It rained and sleeted for three days. When the precipitation started, it was 15°F outside, but the temperature warmed to 28°F to 30°F and stayed there during the whole three days.

Then came the sun and a 40°F day. Those chunks slid off and crushed the cars they hit. If I'm calculating right, they were maybe 1200 lb each. Nightmare.

Best to you,

Goober Dave

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[SteelPE]

I just remembered this video I saw a few months ago enjoy:

http://www.youtube.com/watch?v=pfBp2QYOIbc

 

[btrueblood]

Mike,

Terminal velocity of a pointy icicle would be a lot higher than 120 mph, unless it tumbled. A skydiver can get over 200 mph if falling headfirst rather than belly-down.

 

[msquared48]

Yes, but the shapes I am dealing with would have a lot of wind resistance - not the typical ice cycles you see on houses, but a coating of ice around the members of the latticework, forming to their shapes. So I would expect some tumbling and breakup on the way down.

If you have a pointy projectile, all bets are off... (How low can you go? ... into the ground?)

Mike McCann
MMC Engineering

 

[DRWeig]

Giant slabs of ice are more fun to watch, but not as fast.

For those who are activists, please don't let the architects aim the roof at the parking stripes like the Google Earth capture below. This is the apartment building where I lived. I lived on the top floor, right across the hall from the swimming pool. It was a tiny apartment, 400 sf, but my mailbox said "Penthouse 6"

All of us tenants got small dents in our cars every winter from small ice chunks. I started parking across the street at a hospital when icy weather was in the offing. 1994 was immense, though, and caught me by surprise. Entire county was without power for several days, and it was five weeks before some folks got lights back on.

Best to you,

Goober Dave

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[racookpe1978]

Interesting topic!

I like the idea of the double protection (fine particle solid cover plus heavy mass penetrant trap, but wounder about the order of the two.

Armor plating for tanks now uses a series of thinner plate to break up the entry projectile and then spaces between the plates to allow each penetrating remnant to shatter apart and spread out. The next thin plate then spreads that load out even more, etc.

So, for the same mass of steel against the same mass and velocity of the ice comet (icicle spear seems to easiest way to calculate mass ?) a grating system slightly angled could be "disguised" as an "architecture walkway feature" (a sunscreen even) while allowing a given amount of dollar of steel and structure to cover a wider area of parking lot and sidewalk.

So, I'd imagine a grating of 1 inch deep, 3/4 inch, or 1/2 deep parallel bars + cross round bars - kind of like the walking gratings you see over sidewalk openings - at angle 25 to 35 degrees to 45 degrees (45 probably too steep) so almost every size of icicle would hit a steel bar or cross rod. Then, under the grating, I'd suspend a lightweight net like a bird or insect gauze so the wind and rain could get through. or be sloped to allow the rain to run off into drains.

The problem with a net or gauze under the grating is cleaning leaves and debris - trapping those creates squirrel, mouse, rat, or elephant nests - depending on the size of your local rodent.