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What create wind?

CE 248 - Behavior of plastic design of steel structures Topic: Wind Loads

Tony Yang University of California, Berkeley

As the sun shines on the earth different parts of the land and sea heat at different speeds. This results in high and low pressure areas and leads to the lift and fall of air passes across the entire globe. Due to the angle of the earth while rotating the majority of the heat falls upon the middle of the world (equator) and much less towards the ice caps of the northern and southern hemisphere this means that as the warm air rises on the equator the cold air is pulled in from the ice caps. This spreads the warmth across the globe and results in moving air patterns.

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What wind loads?

· Wind is air in motion. Structure deflects or stops the wind, converting the wind's kinetic energy into potential energy of pressure, thus create wind loads. · The intensity of the wind pressure depends on: Shape of structure. Angle of the induce wind. Velocity of air. Density of air. Stiffness of structure.

Wind velocity vs. height

· Wind velocity increases with the power of the structural height.

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Air flow

· The more the air is streamed, the less the reaction force exerted by the structure.

Air flow

Courtesy of Structural analysis 3rd edition by Aslam Kassimali

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Dynamic effects of wind

· Vortex shedding As wind moved at constant speed and suddenly the air particle are stopped by the surface friction. Small mass of the restrained air will periodically break off and created the process of vortex shedding. The change in velocity causes the change in pressure and creates excitation to the structure. · Resonance

Dynamic Resonance

· Equation of motions

mu + cu + ku = p sin (t )

u ss ( t ) = u0 sin (t - )

P k c , n = , = k m 2n u0 = ust ,0 1 1 - ( / n ) + 2 ( / n ) 2

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Let

ust ,0 =

mu + cu + ku = p sin ( t )

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Rd =

2

Dynamic Resonance

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Tacoma narrows bridge

= = = = = =

0.05 0.1 0.2 0.5 0.75 1

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Rd

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3 2

1 0

0

0.5

1

1.5

2

2.5

3

/ n

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Ways to prevent resonance

· Spoilers

Code provision

· Wind load ASCE 7-02 Section 6 (pg 23)

Courtesy of Structural analysis 6th edition by R.C. Hibbeler

· Dampers · Modify the natural period of the structure to be away from the resonance.

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Analysis Procedures

· Static: Wind force is replaced by equivalent static force. 1. Simplified procedure: ASCE 7-02 Section 6.4 2. Analytical procedure: ASCE 7-02 Section 6.5 · Dynamic: Real time air pressure testing. 1. Wind tunnel procedure. ASCE 7-02 Section 6.6

Analytical Procedure

6.5.1 Scope. A building or other structure whose design wind loads are determined in accordance with this section shall meet all of the following conditions: 1. The building or other structure is a regular-shaped building or structure as defined in Section 6.2. 2. The building or other structure does not have response characteristics making it subject to across wind loading, vortex shedding, instability due to galloping or flutter; or does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.

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Bernoulli equation for fluid flow

·

Code provision

· ASCE 7-02 Equation 6-15 (pg 31)

q=

1 V 2 2

q is the static wind pressure

is the mass density of air

V is the wind velocity

q = 0.00256 K Z K Zt K dV 2 I

· Where K Z is the velocity pressure exposure factor

· Where

K Zt is the topographic effect factor

K d is the wind directionality factor

·

q=

1 V 2 = 0.00256 V 2 2

V

[psf] [mph]

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I

is the importance factor

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Velocity pressure exposure factor

· Accounts for influence of both height above ground and exposure factor. · Exposure

Categories Zg [ft] 1200 7

Velocity pressure exposure factor

· For 15 ft < z < zg

K z = 2.01( z / z g )

· For z < 15 ft

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Urban, suburban and wooded area. The B terrain must prevail in the upwind direction for a distance of 2630 ft or 10 time the structure height, which ever is greater Applies to all building not in B or D C Flat, unobstructed areas and water surfaces D outside hurricane prone area. The terrain must prevail in the upwind direction for a distance of 5000 ft or 10 time the structure height, which ever is greater

900 700

9.5 11.5

K z = 2.01(15 / z g )

· ASCE 7-02 Table 6-3 (pg 75)

2 /

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Topographic effect factor

· To account for greater wind speed if structure is located on a hill. · ASCE 7-02 Section 6.5.7 (pg 29) · ASCE 7-02 Figure 6-4 (pg 47)

Wind direction factor

· The factor accounts for two effects: 1) reduced the possibility of maximum winds coming from any direction and 2) reduced the probability of maximum pressure coefficient occurring for any given wind direction.

K Zt = (1 + K1 K 2 K 3 )

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· ASCE 7-02 Table 6.4 (pg 76) · Only used with the load combination shown in ASCE 702 Section 2.4.2 and Section 2.4.3.

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Basic wind speed

· ASCE 7-02 Figure 6-1 (pg 36) · Measured by anemometer located 33 ft above the ground in open terrain and represents the wind speed that have 2% probability of exceeding in any given year.

Importance Factor

· ASCE 7-02 Table 6.1 (pg 73)

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Design wind pressure

· ASCE 7-02 Figure 6-6 (pg 50) ·

Gust effect

· Accounts for loading effect on wind turbulence on structures. · Rigid structure (T < 1 sec) : G = 0.85 · Flexible structure or dynamic sensitive structure:

p = qz , h G C p - qh G C pi

· G = gust effect factor · Cp = external pressure coefficient · Cpi = internal pressure coefficient

1 + 1.7 I g 2Q 2 + g 2 R 2 Q R Z G f = 0.925 1 + 1.7 g v I Z · ASCE 7-02 Section 6.5.8.2 (pg details

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30) for more

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Pressure coefficients

· ASCE 7-02 Figure 6-6, 6-7 and 6-8 (pg 51 ~ 53).

Design force

· F=pxA · Design wind force resisting system ASCE 7-02 Figure 6-9 (pg 54) · Design for signs

F = qz G C f Af

Where Cf = net force coefficient. See ASCE 7-02 Figure 6-18 to 6-22 (pg 68 ~ pg 72) · Minimum horizontal wind pressure for design building = 10 psf

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Wind tunnel test

Photos of damage structures

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Tall Buildings

Question?

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Information

5 pages

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