Force Coeffeicient Cf>25 (ASCE 7-10 CH29) 1

[ROLaBere]

I have a few questions pertaining to Figure 29.5-1 in ASCE 7-10 using a square cross section for wind loading. When h/D>25, am I supposed to take a ratio to obtain a force coefficient value? Or is this the maximum value for the force coefficient when exceeding 25? Also, is the value of h the overall height above grade of the component, or the height of the member itself not depending on elevation above grade? If someone would like to clarify this for me it would be much appreciated! Thanks!!

 

[JAE]

When h/D>25, am I supposed to take a ratio to obtain a force coefficient value?
[red] Per footnote 2, you can extrapolate out above h/D = 25. To do this you need to use the other two values (at h/D = 1 and 7) to get a proper extrapolation.[/red]

Or is this the maximum value for the force coefficient when exceeding 25?
[red]No. It is not a maximum.[/red]

Also, is the value of h the overall height above grade of the component, or the height of the member itself not depending on elevation above grade?
[red]It is the height of the structure in feet (you are in a Figure that is for Chimneys, Tanks, Rooftop Equipment) and the way I read it, h is the height of the particular entity, not the entire building it may sit on.[/red]



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[ROLaBere]

Thank you for your reply JAE. My main concern/question is the height of the member. The height of my structure is 130 ft. and my member extends above the roof height 20 ft. Making h=150 ft.? My h/D value is roughly 100. I am interested as in why my force coefficient gets greater as my members width decreases. This makes the design wind force increase quite a bit. I just want to understand that this wind force is correct even though it gets rather large.

 

[JAE]

I'm not sure I have comprehensive knowledge about why the h would equal 20 ft. in your case (vs. 150 ft.).

Why the force gets larger with thinner members is that with slender elements the total wind drag [bold]pressure[/bold] must go up to equate to the resulting [bold]force[/bold] on a thin member.

Think of it this way: A thinn cable in a strong wind would experience a force about equal to a larger round pipe. So the cable pressure used to get to that force is higher due to the thin projected area of the cable.

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[SAIL3]

Clear as mud.....the OP has not indicated clearly wheather this is a roof top item or not....in any event, IMO, h is the height of the member itself which seems to be 130ft in this case. Also I would take the Cf @ 25 as max value. The reason the Cf is increasing as the h/d increases is that it approaches an infinite value as far as the wind is concerned. The influence of press escaping over the top of the member will have less influence on reducing the overall wind drag on the member and reach a limit as h/d heads towards infinity. This is similar to lift on an aeroplane wing where the ideal wing would have an infinite length/chord ratio. If the OP has a h/d of 100 and h=130ft, then the d=1.3ft..looks strange to me..
One has to work really hard at it to get a drag coeff much greater than 2 in a normal wind situation. One example is a WF member with wind perpendicular to the web where the the drag coeff is 2.2, if I remember correctly, and this is due to the severe airflow seperation due to the fla extension.
Fig 29.5.1 is for Stacks, chimneys etc. and also can be used for roof top items with adjustments as indicated in the code and commentary.

 

[ROLaBere]

Thanks for your reply JAE and SAIL3. I have attached a sketch to show what I am talking about. I did not explain exactly what I am referring to hence some confusion. What I am trying to compute is wind analysis on a bucket elevator trunking that is rectangular in shape and braced to a concrete silo wall. The silo wall height is 130 ft. And I am looking at this top elevation to find the forces and reactions applied to the bracing. The sketch shows the trunking extending above the silo wall but that is not my concern, just the top of the wall elevation and below. Looking through the wind chapters in ASCE 7-10, I assumed this scenario would apply to chapter 29, Wind Loads on Other Structures and Appurtenances. The bucket elevator material is steel. My main question was what h value to use. I am looking at 20 sections for design of the bracing and using the top section design throughout. This does seem valid but produces larger h/D values when looking at the smallest side dimension. I would appreciate your thoughts and opinions on this.

 

[JAE]

I think SAIL3 is correct - upon reading the footnote 2 it does say "interpolation" which would negate the idea of going beyond the h/D of 25....so that may be a maximum value of h/D to use.


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[ROLaBere]

Upon further research that does look to be correct based on what I have found in an example found in Wind Loads - Guide to the Wind Load Provisions of ASCE 7-10. The example is of a bill board sign on flexible poles and uses that table for the pole wind loading force coefficient. The h/D value = 45 in which they use Cf=0.7 which is the maximum value under round cross sections with h/D = 25. So that does seem to be the maximum value to use based upon SAIL3 response and that example.

 

[SAIL3]

Sounds like you are on the right track....

 

[RFreund]

Yes the coefficient is maximum. The table is from TIA/EIA documents which state for an aspect ratio >= (greater than or equal to) 25, Cf equals... which is much clearer.

Yes, I would use the full height. Although there is probably some reduction due to the structure behind reducing drag.

EIT

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