Wind is a mass of air that travels from high-pressure zones to low-pressure zones having low and high velocities which exert pressures on buildings and causes resultant loads.
Figure 1.Showing Wind Pressure on Building
An instrument for measuring wind speed is called Anemometer.
Formula for wind-pressure is as follows
P
=K.V^2
Here
P=
Wind Pressure
K=
Coefficient
V=
Velocity of wind
Value
of "K" depends on the speed of the wind and shape of the structure bearing wind
pressure,
Using
wind velocity in Miles per Hour (mph) wind pressures may be calculated using
Coefficient of 0.00256 which will result in Pressure in Pounds per Square
(psf).
Now
we will calculate the intensities of pressure using various wind velocities.
World Record for Fastest Wind Speed
The fastest wind speed ever recorded comes from a hurricane gust. On April 10,
1996, Tropical Cyclone Olivia (a hurricane) passed by Barrow Island, Australia.
It was the equivalent of a Category 4 hurricane at the time, 254 mph
(408 km/h). So we will calculate pressures up to the wind velocity of 254
mph.
Wind applies three types of forces structures. Description of wind is different on different velocities with units of Km/hour or Mile/Hour .
Uplift load - Wind flow pressures exerts
uplift pressure as in buildings wind flow under the roof pushes upward.
Shear force load – Mostly wind flow direction remains horizontal having different angles
which exert pressure and creates Shear-Forces on structures.
Lateral load – Wind Pressure results in a push and pull forces which can Slide the structure or overturn the structure , these forces are called Lateral Forces. on:
Designing Factor for against Wind
Load.
The wind is a natural force that can
destroy and uplift any structure which causes high losses so always consider
few designing factors for the construction of structures.
1- Calculate wind loads and apply
on a minimum area of 100 Sft where width will be 9 ft this will give better
pressure impacts in the case of building designing.
2-Resultant pressure acts at the height
of h/2 or h/3 so reinforce the building accordingly.
3-Weight of the building should be
greater than wind loads a factor of safety of 2 should be adopted only for wind
pressure dealing.
4-weak points in building structures
are door and windows which can be sustained up to 170 mph, but if the shingle
pieces or high moisture content exists in the wind air it can easily break
windows even on lower velocity.
5-Buildings having steel roofs
faces large intensities of uplifting pressures so the building should be
designed having proper structural framing properly braced in footings, vertical
members, and horizontal members.
6- In no case Resultant line
created from the Center of gravity of any structure should go outside the
foundation because it will overturn the building.
7-Light structures get damaged at wind velocity of 47 mph, trees are uprooted on wind velocity of 55 mph and widespread structural damages accrue on 64 mph.
8-Building structures should be
design calculating pressures on outer
sides and also inner side of buildings, make sure the weight of the building is
enough to bear wind loads also ensure the stiffness of all structural elements
in buildings.
9-Design outer exposed walls monolithic
and with homogeneous materials to create uniform stiffness and strengthening the structure.
10-Lighting poles should be
designed after calculating horizontal pressure using wind velocity of that area
where light poles exist. Provide weight in foundations greater than the horizontal pressures. A Factor of safety
of 1.5 to 2 may be adopted comparing wind pressure and weight of foundations.
|
CLASSIFICATION
OF WIND VELOCITIES |
||
|
CLASS
OF WIND |
WIND
VELOCITIES in MPH |
|
|
Strong Breez |
25 |
28 |
|
Moderate Gale |
34 |
49 |
|
Strong Gale |
50 |
53 |
|
Whole Gale |
54 |
62 |
|
Strom |
63 |
72 |
|
Hurricane |
73 |
78 |
|
Violent Hurricane |
79 |
199 |
|
Tornadoes |
200 |
311 |
|
|
|
|
Tornadoes may have velocities more than 200 mph.
Tornadoes are ferociously
revolving columns of air that extend from a thunderstorm to the ground. Tornadoes can destroy
buildings, turn over cars, and create deadly flying debris. A tornado can happen
anytime and anywhere. carry strong winds, over 200 miles per Hour.
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