Page 233 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.17

When there is little surface roughness, uniform wind speed is achieved relatively near
to the ground. Therefore, over open terrain and water, the gradient height is nearer to the
lower end of the height range. The gradient height may be taken as approximately 700 ft
(213 m) over water and 900 ft (274 m) over open land. 1,9
Severe local terrain and the built environment near a particular site will have a signifi-
cant impact on wind speeds near the ground. The wooded environment and suburban com-
munity development effectively slow the near-ground wind speed for most building heights.
For these conditions, the exponent for the height ratio normally is taken in the range of 1/7, 1,9
and the gradient height, at 1200 ft (366 m), is near to the high end of the range. For the most
developed urban environments, the exponent can be as large as 1/5, and the gradient height
can be at 1500 ft (457 m). 1,9 In cities with tall buildings, conditions near ground are influ-
enced not only by the general surface roughness of the suburban areas, urban areas, or water
that surrounds the city, but also by the extensive cityscape of the area itself. The urban devel-
opment effectively increases the nearby surface roughness, which tends to decrease wind
speeds near to the ground. However, the arrangement of buildings in cities, with streets and
squares that connect and tend to align, often will channel winds into streams that pass along
ground level at speeds that exceed the average speed of the approaching wind. In addition,
in cities with tall buildings and other broad and tall features, there can be shielding offered
by upwind structures, or buffeting by vortices shed by such structures.
As such, individual buildings in city centers can feel reduced or magnified wind pres-
sures, depending on the cityscape for up to 1 mi (1.61 km) in the upwind direction. Needless
to say, it is very difficult to estimate actual wind speeds adjacent to buildings in city centers.
Investigators dealing with very hilly terrain face similar difficulties.

Analytical Approaches
Wind Speed Adjustment. The analytical approach for calculation of wind pressures
during a storm usually starts with the calculation of the basic wind pressure that corre-
sponds to the 3-second gust wind speed at the standard height of 32.8 ft (10 m) above
ground. Wind speed data averaged over time intervals other than 3 second should be con-
verted before subsequent calculations are performed. When the speed is measured at non-
standard height, or when the terrain upwind of the measurement location differs from that
upwind of the site of interest, wind speed should be adjusted for these differences.
Figure 7.12 can be used to adjust wind speed averaging effects. To account for height
and exposure, the wind speed at the measured height, together with the proper gradient
height and exponent on the height ratio determined for the recording site, should be used to
calculate the wind speed at the gradient height. The wind speed thus calculated at the gra-
dient height then can be used with the gradient height and exponent appropriate for the
environment at the investigation site, to calculate the wind speeds at various heights on a
structure. This basic wind pressure is then determined 1,10 by the formula:
2
2
2
q = 0.00256V lb/ft 2 (q = 0.613V N/m )
z
z
z
z
where V is the calculated wind speed, as a function of height, in miles per hour (meters per
z
second) at the site. The coefficient in this formula is based on the mass density of air under
standard conditions of temperature and pressure. For conditions that differ significantly
from standard conditions—69°F (20.5°C) at 29.92 in of mercury (101.325 kPa)—the
coefficient should be adjusted by the ratio of the mass density of air at the site to that of
air at standard conditions.
When more accurate data are not available, wind speeds below approximately 15 ft
(4.5 m) should be taken as constant and equal to the wind speed at 15 ft (4.5 m).
For structures or wind data recording sites that are located near the tops of hills or
escarpments, wind pressures should be adjusted for wind speed-up that occurs over the
windward slope, and which diminishes down the leeward slope and with distance beyond