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RE: ASCE 7-05: Ice Loads Due To Freezing Rain (Chapter 10)

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Bill,
Your concern is justified.  Ice is a very definite problem for which we do not give proper consideration.  The ice that is listed in the ASCE 7 is radial ice.  Each bar will have up to 0.75 inches of radial ice which would be a total of 1 1/2" of ice between the bars of bar grating that could completely close the spaces in bar grating. 
 
This is a common problem and design consideration in the telecommunications industry.  The EIA TIA 222G addresses the ice issue with a bit more detail in how ice can seal up and change the wind profile of a single shape from a series of shapes without ice.  Keep in mind that the probability of wind and the wind velocity will change with the icing event.  You will not often have the extreme design wind at the same time for the extreme icing event.   It can happen in places like Alaska and Vermont
 
We should also consider ice shards as falling hazards.  Telecom towers even place shields above wave guide to protect it from falling shards.  Buildings constructed adjacent to broadcast or telecommunications towers often protect the roof with concrete pavers. 

Regards, Harold Sprague



 

From: bill(--nospam--at)polhemus.cc
To: seaint(--nospam--at)seaint.org
Subject: ASCE 7-05: Ice Loads Due To Freezing Rain (Chapter 10)
Date: Tue, 24 Feb 2009 19:18:57 -0500

I have been reading up on this, realizing that it is probably pertinent to all our industrial / refining stuff we do, since we've got a lot of bare steel frame structures sitting around outside.
My specific question is regarding bar grating.
Even in the South, Figure 10-2 shows between 0.25 and 0.75 in of "uniform ice thickness due to freezing rain." If you have the typical 1-1/4" x 3/16" bar grating with bars spaced at 1-3/6", the gap between the bars will just about be closed, completely filled with ice. That would be an additional dead load of something like 8 or 9 psf to the structure. For some extensive platforms that's gonna be a LOT of weight!
Am I looking at this right, or is there some other way I should consider it?
Thanks.




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