Wind Forces

Question 1

Building drift:

Answer 1

the distance a building moves in wind

Question 2

StraightLineWind:

Answer 2

most common wind type,blows in a straight line

Question 3

Down Slope Wind:

Answer 3

wind that flows down the slope of a mountain

Question 4

Special Wind Regions:

Answer 4

mountainous areas in the continental US

Question 5

Downburst:

Answer 5

An area of significantly rain-cooled air that, after reaching ground level,
spreads out in all directions producing strong winds. Associated with thunderstorms

Question 6

Northeaster:

Answer 6

cold, violent storm that occurs along NE coast and last for days

Question 7

Exposure:

Answer 7

classification for the characteristics of the ground roughness and surface
irregularities in the vicinity of a building

Question 8

Basic Wind Speed:

Answer 8

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

Question 9

Aerodynamic Pressure:

Answer 9

the interaction between the wind and the building

Question 10

Hurricane forward movement (translational speed) varies between 5 – 25 mph

Answer 10

• 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

Question 11

When wind interacts with a building, both positive and negative (suction) pressure
occurs simultaneously.

Answer 11

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

Question 12

•Fujita scale categorizes severity based on

Answer 12

observed damage
• F0 (light damage) – F5 (incredible damage)
• F0/F1 are most common, but F2/F3 frequently occur

Question 13

• Exposure Categories:

Answer 13

• 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)

Question 14

• Damage typically begins with peak gusts

Answer 14

of 70 – 80 mph

Question 15

• The max drift allowed =

Answer 15

1/500 x height of building

Question 16

• For purpose of calculation however wind is considered a static force based on:

Answer 16

• 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

Question 17

Proper Siting Methods Include:

Answer 17

• 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

Question 18

Compared to hurricanes, tornadoes are

Answer 18

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.

Question 19

Wind presssure on a building is affected by what factors

Answer 19

wind speed

terrain surrounding the building

building height

building shape

Question 20

In the event of a tornado, why are its effects not covered in building code?

Answer 20

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

Question 21

True or false. Drift causes overturning moment and torsion in buildings.

Answer 21

False

Question 22

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?

Answer 22

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

Question 23

Name the gradient heights for

1) urban metropolitan area
2) open country
3) suburban areas

Answer 23

15000 ft for urban metrolpolitan areas

900 ft for open country

1200 ft for suburban areas

Question 24

The moment resisting frame is an appropriate system to use for resisting wind forces in what type of building

Answer 24

Low rise buildings and high rise buildings of 30 stories or less

Question 25

In wind design, the maximum permissible drift of one story relative to an adjacent story is

Answer 25

0.0025 times the story height

Question 26

In designing cable roof systems, the forces that require special consideration are …

Question 27

What is used to measure hurricane intensity?

Answer 27

Saffir Simpson scale

subjective

Question 28

What is used to measure tornadoes?