HURRICANE
BCN 4583/5584-Natural Hazards Conducted by Dr. Abdol Chini, P.E.
Instructor Information
• • • • • • Associate Professor Rinker, Sr. School of Building Construction Office: ARC 152 Telephone: 392-7510 (w), 337-1760 (h) e-mail: chini@ufl.edu Office Hours: MWF 2 to 5 pm
Course Objectives
• Understanding the phenomena associated with the actions of wind upon structures. • Design of structures for extreme winds using ASCE 7-98 standard. • Disaster mitigation where the protection of people and the preservation of building function are principal objectives.
Wind Engineering
• • • • • • • Wind Nature and its Interac. with Structures Wind Damage Experience ASCE 7-98 Provisions Designing for Hurricanes Designing for Tornadoes Case Study-Hurricane Andrew The South Florida Building Code
The Wind Environment
Wind, or motion of air with respect to the surface of the earth, is due to differences in the amount of solar heat received by the atmosphere over various areas of the earth’s surface.
Equatorial and Polar Regions
• The axis of rotation of the earth is inclined at approximately 66.5 degree to the plane of its orbit around the sun. Therefore, the intensity of terrestrial radiation and the temperature of the atmosphere will be higher in equatorial than in the polar regions.
Simplified model of atmospheric circulation.
Additional forces generated by the motion of air
• Coriolis forces due to the earth’s rotation • Centrifugal forces due to the curvature of the path of motion • Retardation forces due to the effect of friction at the earth’s surface
Wind forces in structural design
• Tropical cyclones • Thunderstorms • Tornadoes
Tropical Cyclones
• Originate over tropical waters (5 to 20 degrees latitudes). • Derive their energy from latent heat stored in vaporized ocean water and released as condensation of the water occurs at high altitudes.
Classification of Cyclones
Max sustained wind 38 mph or less 39-73 mph 74 mph or greater Classification Tropical depression Tropical storm Hurricane
Saffir/Simpson Hurricane Scale
• • • • • 74-95 mph 96-110 mph 111-130 mph 131-155 mph >155 mph Category I (Juan, LA, 1985) Category II (Bob, NC, 1991) Category III(Betsy, FL,1965) Category IV(Andrew, FL,92) Category V(Camille, AL, 69)
Thunderstorms
• Raindrops exert viscous drag forces on the air through which they fall and create a strong downdraft. • Strong winds occur when the downdraft spreads over the ground.
Tornadoes
• A vortex of air (of the order of 1000 ft in diameter) which develops within a severe thunderstorm. • Powerful explosive forces may be caused by the difference between the pressure within the structure and the lower pressure prevailing within the tornado funnel.
Wind nature and its Interaction with Structures
Characteristics of Wind
• • • • • Wind gust, turbulence mean and peak values of wind wind variation with height wind speed-up over hills wind climate; probability of wind
SNAPSHOT OF WIND
Data Assembled by Sherlock
• Wind is chaotic • Wind speed increases with height • Gust size along wind, across wind, and vertical • we have to make some sense out of this chaos
MEAN WIND SPEED AVERAGING TIME
Mean Wind Speed Averaging Time
• One hour mean is approximately 60 knots (1 knot = 1.15 miles) • 10 minute mean is about 65 knots • 2 minute mean is about 75 knots • One minute sustained value is 80 knots • Peak (3 second) value is 85 knots • It is important to know averaging time
THUNDERSTORM RECORD
Thunderstorm Record
• Mean wind speed for a long time average is difficult to assess • One minute mean or 3-second peak can be obtained
Ratio of Averaging Time to Mean Hourly
• Hurricane wind gust duration curve established by Krayer and Marshall (1992) • V t = wind speed of averaging time t seconds • V 3600 = mean hourly wind speed • If hurricane sustained wind speed is 120 mph, what is the 3-second wind speed? (sustained wind is one minute average) • V3 = (120)[(V3/V3600)/(V60/V3600) = 120 (1.67/1.32) = 152 mph
RATIO OF AVERAGING TIME TO MEAN HOURLY
VARIATION OF WIND SPEED WITH HEIGHT
WIND TURBULENCE
40
30
Wind Speed (MPH)
20
Vmean =19.7MPH
10
0 0 3 6 9 12 15
Time (Minutes)
Wind Turbulence
• Wind speed can be considered as two components; mean wind speed and fluctuating component • Fluctuating component (turbulence) is caused by ground roughness • Turbulence is higher in rougher terrain and decreases with increasing height above ground
TOPOGRAPHIC EFFECT
PROBABILITY FOR STRUCTURE
n P 1 (1 P )
a
PROBABILITY OF EXCEEDING DESIGN WIND P Design Life of Structure n (years) 5 10 25 50
Annual Probability of Exceedance Pa 0.04 0.02 0.01 0.005
1
100
0.04 0.02 0.01 0.005
0.18 0.10 0.05 0.02
0.34 0.18 0.10 0.05
0.64 0.40 0.22 0.10
0.87 0.64 0.40 0.22
0.98 0.87 0.64 0.39
Probability for Structures
• P = probability of exceeding reference wind speed during life of a structure • P a = annual probability of exceeding reference wind speed (reciprocal of Mean Recurrence Interval) • n = design life or reference period in years • If MRI is 50 years and design life of a building is 50 year, there is a 64% probability that the reference wind speed will be exceeded during the life of the building. • If MRI is 50 years and life n=10 years, there is an 18% probability of exceedance.
WIND-STRUCTURE INTERACTION
• Aerodynamics; Pressure and Force Coefficients • Buffeting; Along-Wind Resonance
• Aeroelastic: Galloping, Flutter
Aerodynamics
• Windward wall experiences inward acting pressures • Leeward and side walls and roof experience outward acting pressures
Aerodynamics
• Separation of flow at sharp edges of structures • Separation causes high turbulence and localized high pressures • Aerodynamics effects are very complex and can be ascertained only through experiments in wind tunnel or full-scale
Aerodynamics
• Special case for building when wind enters the building • External and internal pressures combine to induce high outward acting pressures on leeward, side walls and roof.
Buffeting
• When frequency of wind gust matches frequency of the structure, dynamic response results
Aeroelastic
• Tacoma Narrow bridge in 1942 was designed for 100 mph winds, but it failed in sustained 40 mph wind because of response shape