Bill Aycock wrote:
Hi Bill,
> 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 references to this formula attributed it to EIA 222 Rev C and RS-409.
> 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.
In a second post you asked:>However, there is still a problem in these
equations- they only make sense
>if the velocity factor is in Miles/hour, not feet/second, as given.
>the conversion is the ratio 88/60 to convert miles per hour to feet per
>second-- this factor squared times the density/2 value, 0.00119, does give
>the 0.00256 value you show.
>Is it possible that the units given for velocity have been in error in this
>spec for YEARS? I know the wind speed factor is almost always given in
>mph, not ft/sec, in the graphs used to tell what is to be used in local
>areas, but this table uses ft/sec- ??
Both Steve Sawyers and Hank Lonberg have replied clarifying the Current EIA
222F formulas for calculating wind loads.
It seems we all feel comfortable with the Qs = .00256 V^2 foundation for
the calculations.
My previous references to the older EIA 222 Rev C spec. stated the formula in
that spec was .004V^2.
Here is why it is different. The Rev C spec included a built in 30% increase
for wind gusts.
If we start with the fundamental .00256V^2 and add the 30% increase in wind
speed we get a 69% increase in windload.
If we multiply the original formula by 1.69 we end up with .004V^2.
That's where that formula came from.
An interesting observation, made by Dave Leeson, W6NL, in a yet unpublished
paper on comparison of the various standards, is that they all get similar
values.
Here is an excerpt from the paper
Comparisons of calculated wind pressure at two heights above ground at the
specified min basic wind speed (70 Mph for 222-C). Units are Lbs/SqFt.
Spec h=33' h=100'
EIA 222-C 20 20
EIA 222-D/E 19 24
UBC 17 21
CP3 22 24
ASCE 17 20
Most readers are farmiliar with EIA & UBC specs. The CP3 is the British
standard and ASCE is the American Society of Civil Engineers, Guidelines For
Electrical Transmission Line Structural Loading.
I hope this helps.
73, Kurt
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