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

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Harold Sprague wrote:

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. 

The ASCE 7-05 standard does include provisions for Icing + Wind with a greatly decreased wind velocity typically shown (e.g. 30-40 MPH on the "hurricane coasts" of the Eastern seaboard and Gulf). The wind increases quite a bit as you move westward and northward, so that in particular in the plains states you can have "very heavy" icing in addition to winds of 60 MPH or so.

The wind velocity effect is calculated using the provisions of Chapter 6, and the wind effective area is increased (sometimes dramatically) by the icing effect.

I don't think it's as detailed a procedure as EIA, but it's pretty thorough for the kinds of structures that we're designing, which are typically "naked" structural steel framing all exposed to the elements.

Of course, the "big deal" now is that IBC 2006 doesn't seem to recognize anything to do with the ASCE 7-05 loads or combinations including icing. So at this point you're well advised (IMO, anyway) to use the ASCE 7 loads in addition to everything else.

FWIW, in this one job I'm doing, on the Georgia coast near Savannah, the typical bar grating icing load is only about 6.25 psf, which is less than the typical live load, so I don't think it has any real effect. However, a tall tower-type structure might. Even there, it does, as you suggest, close up the spaces between the bar grating.

In the job we're slated to begin in Valdez, Alaska, I feel certain icing will be a significant load event.
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