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RE: wind lines and jurisdictions

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Yes, the wind maps and seismic maps demark iso-lines of wind speed or
acceleration. 

The lines are set based on best available information and political
concerns. It is difficult for me to believe that, for example, to the right
of a wind iso-velocity line the wind speed is 90 mph and to the right it is
100 mph. It seems that some linear interpolation should be done to get a
better estimate.

But all of the precision to get estimated loads seems to be excessive when
the maps are based on predicted events. In Anchorage, the NFPA Code does not
require lightning protection based of past events. We have experienced a
great number of lightning strikes the past few years and the code is out of
date.

At best, loads we apply to our designs are estimates based on the consensus
of code committees. Seems that a structural engineer could give the client
better value by spending this time on better value work. This includes
details, load paths, structural system toughness, and redundancy.

Bill

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
Sent: Sunday, August 14, 2005 7:50 PM
To: seaint(--nospam--at)seaint.org
Subject: re: wind lines and jurisdictions


I would have to disagree.

For the IBC (and thus ASCE 7), the lines represent the contants values of
acceleration (for seismic) and wind speeds (for wind).  In otherwords, the
wind map is a series of contours that represent the wind speed values at
the contour lines NOT the areas in between.  This should be obvious from
the note on the map that says: "linear interpolation between wind contours
is permitted".  Now, it so happens that the overwhelming majority of the
continential US is enclosed within a 90 mph contour, which effectively
means that whole "area" is to be treated as 90 mph...but this is just
because there is no smaller wind speed contour inside of that area.

And in actuality, for the 1997 UBC, it is completely OPPOSITE of what you
state.  The wind map has contours that represent the wind speeds AT the
contours, while the seismic map has contours that represent the borders of
areas of "constant" seismic zones (not accelerations...at least
directly...the zones indirectly represent seismic parameters like
accelations, etc).

At least that is how I understand the maps...

Regards,

Scott
Adrian, MI


On Sun, 14 Aug 2005, Gale45man wrote:

>Comparing the seismic and wind maps is like comparing
>apples and oranges.
>
>The seismic maps use lines representing constant
>values of acceleration.  The wind maps use areas or
>regions of constant values of wind speed, not lines.
>
>The seismic acceleration won't change much if you're a
>hair on one side or the other of an isoline.  The
>design wind pressure changes by a factor of 20 percent
>or so on one side or the other of the dividing line
>between regions.
>
>d a v e e v a n s


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