System design, Wind and seismic effects, Figure 1-30 – Rice Lake Weigh Modules/Mount Assemblies User Manual
Page 28
SYSTEM DESIGN
1-26
Wind and Seismic Effects
■ Exposure: The exposure conditions at the site must be known.
Built up or rough terrain can cause a substantial reduction in wind
speed. The United Building Code (UBC) defines 3 exposure catego-
ries:
Exposure B: has terrain with buildings, forest or surface irregu-
larities 20 feet or more in height covering at least 20% of the area
extending one mile or more from the site.
Exposure C: has terrain which is flat and generally open, extend-
ing one half mile or more from the site in any full quadrant.
Exposure D: represents the most severe exposure in areas with
basic wind speeds of 80 mph or greater and has terrain which is flat
and unobstructed facing large bodies of water over one mile or
more in width relative to any quadrant of the vessel site. Exposure
D extends inland from the shoreline 1/4 mile or 10 times the vessel
height, whichever is greater.
■ Importance Factor: An importance factor of 1.15 is used for
essential facilities which must be safe and usable for emergency
purposes after a windstorm in order to preserve the health and
safety of the general public. Such facilities include medical facili-
ties having surgery or emergency treatment areas, fire and police
stations. A factor of 1.0 is used for all other facilities.
With this information, the wind forces can be calculated in accor-
dance with methods described in the UBC. This information may
be used to verify the stability of the vessel using standard mounts,
or to design additional restraints if deemed necessary.
100
90
100
90
80
70
80
70
70
80
70
80
70
80
90
80
100
90
110
110
110
110
100
90
80
70
90
80
70
70
70
90
90
80
70
NOTES:
Basic wind speed 70 mph
Special wind region
1.
Values are fastest mile speeds at 33 feet above ground for Exposure Category C and are
associated with an annual probability of 0.02.
2.
Linear interpolation between wind speed contours is acceptable.
3.
Caution in use of wind speed contours in mountainous regions of Alaska is advised.
4.
Wind speed for Hawaii is 80 and Puerto Rico is 95.
5.
Wind speed for Alaska varies from 70 inland to over 110 in coastal areas.
6.
Where local records or terrain indicate higher 50-year wind speeds, they shall be used.
7.
Wind speed may be assumed to be constant between the coastline and the nearest inland contour.
Figure 1-30