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Re: Wind Loads-UBC vs. ASCE

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The difference between the 2 ASCE codes is the speed over time.

Fastest mile is the time it takes for wind to travel 1 mile; hence for 90mph wind it would take wind 40 seconds to travel 1 mile. So at that speed wind is averaged over 40 seconds

3 second gust is averaged over 3 seconds. It is acually between 2 and 3 but I guess the committee said lets just call it 3 seconds for simplicity and practicality.

If you were look at ASCE 7-98 the Durst curve can be found in the appendix. They failed to name the graph anything it seems. The 40 second y coordinate is around 1.3 and the 3 second y coordinate is 1.53

1.53/1.3=1.2 so to convert from 90mph fastest wind to 3sec gust you would multiply 90x1.2=108mph

Note the time average for fastest mile changes depending on the wind speed. 40mph fw speed would have an average time of 90 seconds. It takes 90 seconds for wind to travel 1 mile.

You can also use similar approach to convert tornado (fastest 1/4mile) and hurricane (1 min average)into either 3 second gust or fastest wind speed. Fastest mile wind speed is almost equal to hurricane speeds.

Dr Kishor C. Mehta and others please respond if this is not correct. He was Chairman of the ASCE 7-95 Guide to the use of the Wind Load Provisions. Attended his two day seminar on ASCE7-98 Wind Loads sponsored by ASCE. Worth the money.

From: Padmanabhan Rajendran <rakamaka(--nospam--at)>
Reply-To: seaint(--nospam--at)
To: seaint(--nospam--at)
Subject: Wind Loads-UBC vs. ASCE
Date: Wed, 7 Mar 2001 06:16:24 -0800 (PST)

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?


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