Atmospheric Icing Load on Bar Grating

[RPMG]

How do you compute the ice load on bar grating? The bars are too closely spaced to use the ice load from one bar and take summations. It's not exactly a flat horizontal plate. And why does ASCE equation 10.4-2 have pi as a factor for ice volume on large flat plates?

Mapped Ice Th. = 3/4"
Design Ice Th. = 1 1/2"
Grating: 1 1/2"x1/8" 19W4

Or do you not bother with ASCE, and assume that there is a 3" sheet of ice?

Thanks!

 

[thaidavid40]

Our practice for exterior, exposed bar grating, is to assume that the ice builds up on the bars and bridges solidly across the global exposed expanse. The ice load rarely governs anyway, and this seems a logical, conservative application of the load.
Dave

Thaidavid

 

[RPMG]

So that's one for a 3" sheet of ice. This is an open tower structure with lots of grating and piping for ice to accumulate.

Does anyone understand why, for a large flat horizontal plate, the volume of ice is: Vi = pi * td * As (ASCE 7-10 10.4-2)? I'm pretty sure that if I measured it with a tape measure, I wouldn't multiply the volume by pi.

 

[racookpe1978]

pi x td (tower diameter ??) would only logically be the "flat area" of just the tower "top" plate.

So, a single "flat platform" around a tower would have to be larger than that:

( [(Tower_dia + 2 x platform radius) x pi - (Tower_dia x pi) ] x the ASCE_Factor ) x )nbr of platforms)

Then you'd have to add the intermediate stairs or ladder areas.

 

[Guastavino]

I'm with Thaidavid. First, if you have 3" of ice, you shouldn't have any live load, in the real world that is. And, even 3" of ice is less than 20 psf, and I assume that your platform is designed for 60psf or more of live load? I'm thinking it won't control much of your design. But, to answer your question, 3" of ice sounds more than rational to me.

 

[Lomarandil]

It may be less than your live load by inspection, but depending on the structure, it could add significantly to your wind area for lateral loads.

(Of course, note that when considering combined ice and wind, ASCE 7 (figure 10-2) as well as NESC allow for reduced wind loads)

 

[RPMG]

racooke: td is the design thickness of ice, but I think that you're right in that it is only logical that the formula considers only the top plate. It doesn't make sense for me to use that formula, so I will ignore it.

njiutzwe: I was looking at load combination 2 for ASD (D + L + Di), which includes live load. However, I agree that if there is 3" ice, there is no live load. I believe that this is within the code's intent. The platforms may be subject to "roof (environmental)" or "floor" loading conditions, but not at the same time.

Lomarandll: I have not forgotten about wind. Ice wind is about 1/8 of the normal wind force in this case.

Thanks everyone

 

[SlideRuleEra]

RPMG - Don't completely rule out the possibility of ice load combined with live load. During operation of industrial plants, it is very possible for heavy replacement parts and tool chests needed for upcoming repairs / modifications to be positioned on exposed grating in advance. If ice accumulation happens while these items are in place, the live load / ice load combination now applies. This occurrence is not hypothetical, I've seen it happen. Fortunately there have been no consequences.

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