Page 231 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.15

and disaster response facilities sometimes maintain anemometers and record useful data. In
general, facilities that record and report data to the National Climatic Data Center should
be consulted first about conditions during a particular storm.
Often, reliable data at a particular site are not available readily. The nearest reliable
anemometers often are far from a site of interest. Furthermore, the wind environment in a
storm often is highly variable, with gustiness and local wind cells that cause speeds to dif-
fer over relatively short distances. This may be particularly true in thunderstorms which can
be very severe, but highly localized. It is possible for winds during a thunderstorm to be
extreme at a site when, a short distance away, official records indicate nothing unusual.
Sometimes, circumstantial evidence, such as the general extent of damage to trees and
structures in the vicinity of a site under investigation and at the location where data were
recorded, will help to assess what appear to be major discrepancies between recorded data
and observed conditions. The Beaufort scale (Table 7.1) (adapted from Ref. 8) is helpful
for estimating site wind speeds based on nearby physical damage and eyewitness accounts.
Care must be taken when interpreting data from any source. The standard currently used
for design of structures is based on a particular storm return period (usually 50 years) for
the maximum 3-second gust wind speed measured at 32.8 ft (10 m) above grade. Other
time-averaging wind speeds, including the fastest mile, the 60-second gust, and the mean
hourly wind speed, sometimes are recorded at wind monitoring stations. The fastest-mile
wind speed is determined essentially by measuring the time necessary for a column of air
1 mile long to pass a recording station. The 60-second gust wind speed is the average speed
of the wind during a 1-min interval. Averaged over 1 min, a wind with a speed of 60 mi/h
(26.8 m/s) will have the same fastest-mile and 60-second gust speeds. The mean hourly
wind speed is the average wind speed recorded over a period of 1 h.
Typically, the 3-second gust wind speed will be higher than the fastest-mile, 60-second
gust, and mean hourly wind speeds measured in the same wind. Shorter time-averaging

TABLE 7.1 The Beaufort Scale
Beaufort Speed Speed
No. description (mi/h) (m/s) Land effect
0 Calm Under 1 Under 0.4 Smoke rises straight up
1 Light air 1–3 0.4–1.3 Direction shown by smoke drift
2 Light breeze 4–7 1.8–3.1 Wind felt on face; leaves rustle
3 Gentle breeze 8–12 3.6–5.4 Leaves in constant motion; wind
extends light flag.
4 Moderate breeze 13–18 5.8–8.0 Raises dust and loose paper; small
branches move.
5 Fresh breeze 19–24 8.5–11 Small trees in leaf begin to sway
6 Strong breeze 25–31 11–14 Large branches in motion; whistling
of telegraph wires
7 Moderate gale 32–38 14–17 Whole tree in motion; difficulty in
walking
8 Fresh gale 39–46 17–21 Breaks twigs off trees
9 Strong gale 47–54 21–24 Some damage to vegetation and
structures
10 Whole gale 55–63 25–28 Trees uprooted; considerable damage
11 Storm 64–75 29–34 Widespread damage
12 Hurricane Over 75 Over 34
Adapted from Ref. 8.