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5.8 Example 8Cricket Sightscreen

A cricket sightscreen has been proposed for a cricket ground in Rockhampton,Queensland (Region C). Design wind loads are required to be determined.

Figure 5.16 Cricket sightscreen

The structure has a nominal solidity of 0.90.

Regional wind speed

The sightscreen will be exposed to winds for 40 days a year for 20 years. Then the

estimated total exposure time is (40/365)20 = 2.2 years.

With possible occupancy of 5,000-10,000 people in the ground: treat structure as Level 3

(AS/NZS1170.0 Table F1). Hence take average recurrence interval, R, equal to 500years (AS/NZS1170.0 Table F2).

From Table 3.1in AS/NZS1170.2, VR= Fc.66 = (1.05).66 = 69 m/s (rounded to nearest

1 m/s)

Wind direction multiplier Md= 0.95 (Section 3.3.2)

(Sightscreen can be at any orientation to wind)

Terr ain-height multiplier

z=h=6.1m, For Terrain Category 3, Mz,cat

= M6.1,cat3

= 0.82 (Table 4.1(B))

Shielding

Structure is shielded by the grandstand for all directions.

For this case take Ms= 0.80

5 m

6 m

6.1 m

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Topographic M ultipli er

Mt= 1.0

Site wind speed, Vsit,= 69 (0.95)(0.82)(0.8)(1.0) = 43.0 m/s for all (Equation 2.2)

Design wind speed

Vdes,= Vsit,= 43.0 m/s

Aerodynamic shape factor

Treat structure as freestanding hoarding/wall.

h = 6.1 m, c = 5 m, b = 6 m. (Figure D1)

c/h = 5/6.1 = 0.82b/c = 6/5 = 1.20

For normal winds (=0 deg),Cp,n= 1.3 + 0.5(0.3 + log101.2) (0.80.82) = 1.30 (Table D2(A))

Eccentricity of load, e = 0

For oblique winds (=45 deg),

Cp,n= 1.3 + 0.5(0.3 + log101.2) (0.80.82) = 1.30 (Table D2(B))

Eccentricity of load, e = 0.2b = 1.2 m from vertical centerline

Net porosity factor, Kp= 1- (1-0.9)2

= 0.99 (Equation D1)

Then Cfig= (1.3)(0.99) = 1.29 (Equation D2)

Dynamic response factor

Take Cdyn= 1.0 (natural frequency greater than 1.0 Hertz) (Section 6.1)

Design wind pressur e

p = (0.5 air) Vdes,2CfigCdyn= (0.5)(1.2) (43.0)

2(1.29)(1.0) = 1431 Pa = 1.43 kPa

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Design l oadings (ul timate limit states for overtur ning)

Figure 5.17 Design loads cricket sightscreen

Serviceabil i ty limi t states

The sightscreen is required to operable at more common wind speeds. For thiscalculation take average recurrence interval R to be equal to 20 years (consistent withAS1170.0, Appendix C)Then Regional wind speed = V20= Fc.45 = 45 m/s

(Note that Fc= 1.0, for R < 50 yearsSection 3.4)

For serviceability limit states, terrain-height multiplier is obtained from Table 4.1(A)not

Table 4.1(B).

Mz,cat= M6.1,cat3= 0.83

Vdes,= 45(0.95)(0.83)(0.80)(1.0) = 28.4 m/s

Ultimate limit states loads can then be factored by (28.4/43.2)2= 0.43

Net load across sightscreen for serviceability limit states = 1.44 0.43 = 0.62 kPa

1.43 kPa

1.43 kPa

1.43 kPa

1.43 kPa

1.2 m