wind load on free standing xxwall_1

8/13/2019 Wind Load on Free Standing xxWall_1

1/23

Free walls, roofs and billboards

Wind loading and structural response

Lecture 22 Dr. J.D. Holmes


2/23


• free-standing walls

• elevated walls and billboards

• free roofs and canopies

2

ha21

Lw pn

Uρ

p pC

2

ha21

LU pn

Uρ

p pC


3/23


• attached canopies

• solar panels


4/23


free-standing walls• wind at 90o to plane of wall (lecture 8, Chapter 4)

CD = 1.2

TWO-DIMENSIONAL WALL

Ground

SQUARE WALL

CD = 1.1

Ground

reference U taken as Uh (top of wall)


5/23


free-standing walls• wind at 90o to plane of wall (lecture 8, Chapter 4)

reference U taken as Uh (top of wall)


6/23


free-standing walls• wind at 90o to plane of wall

Jensen Number (h/zo) = 50 to 160

Mean

Maximum

b

h

0.1 1 10 100

b/h

4

3

2

1

0

Cpn


7/23



Jensen Number (h/zo) = 50 to 160

0.1 1 10 100

b/h

4

3

2

1

0

Cpn

Mean

Maximum

b

h


8/23



Net pressure difference high for first 1-2 wall

heights from windward end

mean C pn

1.6 1.0

1.9 1.4 0.7

1.1 0.8 0.6 0.41.62.2

1.82.7 1.4 1.1 1.0 0.8 0.7 0.6

b/h=2

b/h=3

b/h=5

b/h=10


9/23



Effect of corner is to reduce largest net pressure

mean C pn

0.1 1 10 100

y/h

4

3

2

1

0

Cpn

no corner 45

corner 45

infinite 45corner 225

no corner 225

45

225

y


10/23


Parallel free-standing walls (noise barriers on urban freeways)

-3

-2

-1

0

1

2

3

4

5

-50 -40 -30 -20 -10 0 10 20 30 40 50

wall spacing/wall height

N e t p r e s s u r e c o e f f i c i e n t s

Mean

r.m.s.

Maximum

Minimum

Shielding

significant shielding effects up to 10 wall heights separation

s h • wind at 0

o to plane of walls


11/23


Billboards• wind at 0o to plane of board

effect of elevation : increase magnitude of mean net pressure coefficient

=

= Cpn 1.5

mean C pn


12/23


Billboards• wind at 45o to plane of board

45o 1.5 1.1

2c

c

c

Ground

mean C pn


13/23


s

B

h

SOLID SIGN OR

FREESTANDING WALL

GROUND SURFACE

CASE AWIND

NORMA L TO

WALL

New table proposed for ASCE-7-05• Solid freestanding walls and solid signs

• Force coefficients Cf given as function of clearance ratio, s/h, and aspect ratio, B/s

CASE B

WIND AT 45°

TO WALL


14/23


Walls on bridges• wind at 0o to plane of wall

0

1

2

3

4

5

0 1 2 3 4

s/h

Cp

upwind

wall

Mean

r.m.s.

Maximum s

Coefficients based on U at top of wall : little effect of s/h ratio


15/23


Free-standing roofs

Usual convention : positive net pressure is downwards

pnet flat

pnet pitched

pnet troughed


16/23


Free-standing roofs

pnet


17/23


Free-standing roofs

Effect of stored goods : flow stagnates underneath - pnet goes more negative

pnet


18/23


19/23


Free-standing roofs

C pn averaged over half a roof

pitched - model tests

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.40.6

0.8

0 30 60 90 120 150 180

angle of attack (degrees)

C p ( m e a n )

5 degrees pitch 10 degrees pitch

15 degrees pitch 22.5 degrees pitch

30 degrees pitch

high positive and negative values for

roof pitches of 22.5o and 30o


20/23


Attached canopies (over loading bays etc.)

when mounted near the top of the wall, uplift force is high

zero pitch - model scale

hhc

wc

0.0 0.5 1.0 1.5 2.0 2.5 3.0Canopy height-to-width ratio, hc/wc

hc/h =1

hc/h=0.75

hc/h=0.5

-4.0

-3.0

-2.0

-1.0

0.0

Cpn or 4.0, whichever is the lesser

c

c pn w

h1.31.0Cˆ-

or 4.0, whichever is the lesser

c

c pn w

h0.41.0Cˆ-


21/23


Solar panels

wind loads are affected by many parameters :

on roofs of buildings

l

d

c e

w

h1 h2


22/23


Solar panels

• ‘stand-off’ distance reduces net load normal to roof

• higher roof pitch produces less uplift force

• panel near eaves or gable ends experience higher loads

• generally better to mount parallel to roof slope ( = 0)


23/23

End of Lecture 22

John Holmes225-405-3789 [email protected]

wind load on free standing xxwall_1

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