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






RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
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.
Or is this the maximum value for the force coefficient when exceeding 25?
No. It is not a maximum.
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?
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.
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RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
Why the force gets larger with thinner members is that with slender elements the total wind drag pressure must go up to equate to the resulting force 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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RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
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.
RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
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RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
RE: Force Coeffeicient Cf>25 (ASCE 7-10 CH29)
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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