The actual set of formulas used in the EIA standard is:
F = qz * Gh * [ Cf*Ae + SUM(Ca * Aa) ]
qz = .00256 * Kz * (V^2)
Kz = [z/33]^(2/7)
1.0<=Kz<=2.58
Gh = 0.65 + [ 0.60 / (( h/33)^(1/7)) ]
1.00 <=Gh<=1.25
except for tubular towers where Gh = 1.69
Where
z is the height in ft of the section of interest
h is the overall height of the structure
v is the velocity in ft/sec
F is the force of the wind on item under investigation
qz is the velocity pressure
Kz is the factor to account for increasing wind velocity with increasing
height.
Gh is a gust factor that is also based on height and structure composition.
Wind zones are specified as the wind speed at 10 meters above ground
that would be expected to occur once in 50 years. The 100 year number
is about 10 MPH higher.
The .00256 jives with what I remember from aerodynamics, but the old
brain is getting a little fuzzy.
Ae (effective tower area) gets rather involved and is more than just the
projected area of the tower.
Aa (area of appurtanences) basically the flat plate area of antennas,
feed lines, etc.
The Cf and Ca are standard determined coefficients that appear to be
bit larger than what a classical aerodynamisist might apply, but they
were determined by an industry committee of individuals from various
tower manufacturers, academia and others who were trying to
take into account real world experiences. So I don't think there is anything
sinister. It is just the way that they all agreed was a reasonable
and understandable way to determine what was safe.
While I was not involved in the EIA standards development, I have been in
on other standards meetings. The industry debates get rather heated, but it
is from this that good standards are created.
The point is the standard exists, right or wrong. It is recognized by industry
as reasonable and usable, and compliance to it is required by many
governmental jurisdictions. I learned a long time ago that you don't often
win when you fight both city hall and a whole industry. But I have been
known to take a wack at city hall by itself.
Hope I answered your question.
73 de n0yvy
At 08:07 PM 8/23/98 -0500, Bill Aycock wrote:
>Steve- There is a reference several of our more knowledgible tower Gurus
>have used that has been bothering me. I have seen the equation for dynamic
>pressure given as q= 0.004 v^2, with an attribution to the specification
>EIA-222 Rev F, or one of the earlier revisions. One of the functions of a
>spec is to define terms and the way they are both used and determined. That
>is one of the problems we are having with the definition of area, as it is
>to be used to fit antennae to towers, by way of load calculations.
>
>My problem with the equation as given (more than once), is that it gives an
>answer about four times the accepted value. By accepted value, I mean the
>one that would be given by any competent aerodynamicist when asked for the
>equation for dynamic pressure. The problem comes from the value 0.004. In
>the classical equation, this term is about 0.00119, and is half the (mass)
>density of air at normal sea level conditions.
>
>If the value given in the spec is such that it represents a built-in safety
>factor, they should say so. I dont have ready access to the spec, so I cant
>say how the equation is given in context, which is why I am asking you. The
>problem may be one of units, but my checks there dont look that way.
>
>Can you clarify this for me ?
>
>Thanks es 73
>
>Bill
>
>Bill Aycock W4BSG
>Jackson County, AL
>EM64vr
>W4BSG is "vanity" this time, but was
>earned by exam in 1954, the first time.
>
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