Wind Forces Flashcards
Building drift:
the distance a building moves in wind
StraightLineWind:
most common wind type,blows in a straight line
Down Slope Wind:
wind that flows down the slope of a mountain
Special Wind Regions:
mountainous areas in the continental US
Downburst:
An area of significantly rain-cooled air that, after reaching ground level,
spreads out in all directions producing strong winds. Associated with thunderstorms
Northeaster:
cold, violent storm that occurs along NE coast and last for days
Exposure:
classification for the characteristics of the ground roughness and surface
irregularities in the vicinity of a building
Basic Wind Speed:
the wind speed with a 50 year average recurrence interval
measured at 33’-0” above grade in Exposure C (flat, open terrain) It is a peak gust
speed
Aerodynamic Pressure:
the interaction between the wind and the building
Hurricane forward movement (translational speed) varies between 5 – 25 mph
- Saffir-Simpson Hurricane Scale rates intensity of hurricanes
- Category I (weakest) – Category V (strongest)
- Hurricanes have the greatest potential for devastating a large geographical area
When wind interacts with a building, both positive and negative (suction) pressure
occurs simultaneously.
positive pressure occurs on the windward side of a building and Negative pressure (suction) occurs on the on leeward side & roof
• Pressure is greater at corners, overhangs & parapets
•Fujita scale categorizes severity based on
observed damage
• F0 (light damage) – F5 (incredible damage)
• F0/F1 are most common, but F2/F3 frequently occur
• Exposure Categories:
- Exposure B: rough terrain, urban, suburban, and wooded areas
- Exposure C: flat open terrain with scattered obstructions and areas adjacent to oceans in hurricane prone regions
- Exposure D: smoothest terrain, areas adjacent to large water surfaces outside hurricane-prone regions, mud flats, salt flats, and unbroken ice• The smoother the terrain, the greater the wind load. (eg: office buildings in exposure D would receive higher wind loads than those in Exposure B)
• Damage typically begins with peak gusts
of 70 – 80 mph
• The max drift allowed =
1/500 x height of building
• For purpose of calculation however wind is considered a static force based on:
- Velocity: pressure on building varies as the square of the velocity • Height of wind above ground
- Surroundings: other buildings, trees and topography
- Size, shape and surface texture
Proper Siting Methods Include:
• Don’t locate a building in Exposure D if possible
• Avoid locating on upper half a hill
• Trees with 6”+ diameter, light/flag/power poles shouldn’t be placed near
building
• Provide at least two means of site egress for office/public buildings
Compared to hurricanes, tornadoes are
generally smaller in diamter and have faster wind speeds
A tornadoes consists of a funnel shaped rotating column of air that emerges from a cumuliform cloud. The average base diameter is 500 ft across. While wind speed average 250 mph. They can exceed 500 mph.
Hurricanes also consist of a rotating column of air but must form over water. THey may be 10 mi to 50 mi wide. Wind speeds must be excess of 74 mph to be classified as a hurrican. Winds have been reported to reach 190 mph.
Wind presssure on a building is affected by what factors
wind speed
terrain surrounding the building
building height
building shape
In the event of a tornado, why are its effects not covered in building code?
Because the effect of tornadoes are difficult to represent and consider, they are not covered in building codes or considered on the wind speed map. Wind velocity and direction change very quickly in a tornado, and wind researchers have difficulty modeling tornadoes in wind labs. The basic riules of design for high winds are generally applied for tornado prone regions
True or false. Drift causes overturning moment and torsion in buildings.
False
The maps of basic wind speeds given by the IBC ans the ASCE/SEI 7 both mention some special wind regions. What are these regions?
On the map of basic wind speeds, special wind regions are areas that are excluded from the map and for which wind speeds must be determined from local records. These region have a topography and conditions that are quite variable, and they are often very difficult from tehir surrounding areas.
These regions include the Pacific Northwest coast, the shores of the great lakes, and the mountains and canyons of southern California
Name the gradient heights for
1) urban metropolitan area
2) open country
3) suburban areas
15000 ft for urban metrolpolitan areas
900 ft for open country
1200 ft for suburban areas
The moment resisting frame is an appropriate system to use for resisting wind forces in what type of building
Low rise buildings and high rise buildings of 30 stories or less
In wind design, the maximum permissible drift of one story relative to an adjacent story is
0.0025 times the story height
In designing cable roof systems, the forces that require special consideration are …
What is used to measure hurricane intensity?
Saffir Simpson scale
subjective
What is used to measure tornadoes?
