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Re: Wind Loads-UBC vs. ASCE

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ASCE changed from "fastest mile" to "3 sec gust" wind speeds in ASCE
7-95.  In the commentary, there was a short explanation.  As I recall,
the reason for the change was that most new recording stations don't
measure fastest mile.  As you might guess, the "3 sec. gust" speeds are
somewhat higher than the "fastest mile" speeds, but I don't believe
there is an exact correlation since the conversion would depend on how
gusty the wind is.  At the same time, the equations were supposedly
reworked so that the wind pressures would remain about the same.
However, if I recall correctly, there were also changes in wind speed
contours made in ASCE 7-95 for some areas based on historical data.  I
do recall that the wind speeds for Central Florida were lowered around
that time.  That could explain the difference you found between 7-95 and
7-88.  I believe that the adjustment to the equations in 7-95 was made
in the gust factor, but I don't remember if that was when the gust
factor was made independent of 'z'.

Chris Willcox


Padmanabhan Rajendran wrote:

> ASCE 7-98 wind map shows maximum "3 sec. gust wind
> speed" of 150 mph, whereas according to UBC 97, the
> maximum "Basic Wind Speed" is limited to 130 mph. Is
> there any correlation between the two terms? If not,
> is there any formula for extrapolating wind stagnation
> pressure for basic wind speed in excess of 130 mph,
> while using UBC 97?
>
> 2. ASCE 7-88 showed the gust factors, gz, as a
> function of the height 'z' along the height of the
> structure.  However, ASCE 7-98 gives a single value
> for gz (Sec. 6.5.8). Quick calculation indicates that
> the force due to wind would be less, if calculated
> with 7-98 provisions. The reduction is not
> insignificant. Were the provisions of 7-88 overly
> conservative or is there something in error in 7-98?
>
> Rajendran
>
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