Design Wind Speed Characteristics

1
Design Wind Speed Characteristics

• Jon A. Peterka
• Kirsten D. Orwig
• 2009 Structures Congress

2
Introduction

• Wind load provisions differ worldwide.
• How do they differ?
• What do they have in common?
• What are some unresolved issues?

3
Key Differences

• Averaging schemes
• Design speed definitions
• Load multipliers

4
Key Differences Contd
5
Key Similarity

• Assume Neutrally Stable Boundary Layer.
• Dominated by mechanically generated turbulence.
• Represented by Power Law or Log Law relationships.

where for neutral stability
6
Straight Line Wind

• Durst Chart Gust Factor for Straight Line Wind
  from ASCE 7.

ASCE 7
7
Unresolved Issues

• Non-neutral boundary layers
• Thunderstorm downbursts
• Tornados

8
Non-neutral BL

• Buoyancy and mechanically driven turbulence.
• term ? 0
• Example affects efficiency and durability of
  wind turbines

Stull, R.B., 1988.
9
Downbursts

• Transient
• Averaging difficult and event-dependent
• Unique events (depend on storm translation speed,
  angle of descent, etc.)
• Gust factors (3s/1hr) much greater than 1.53 used
  in ASCE

10
Downburst Contd

• Simulated downburst profiles.

Chay, M.T., et. al., 2006
11
Downburst Contd

• Averaging of full-scale downburst.

Orwig, K.D
12
Downburst Contd

• Angle of descent example.

13
Tornado

• Transient
• Complex wind flow (vertically and horizontally)
• Unique events (single vortex, multiple vortex,
  translation speed, size, etc.)
• 3s gust not 1.53 times 60min mean and Durst gust
  chart doesnt apply.

14
Tornado Contd

• Profiles change in time and space.

Bluestein, H.B., et. al., 2007
15
Tornado Contd

• Complex wind flow in and around tornado.

T
Fujita, 1971
Wurman, J., et. al., 2007
16
Tornado Contd

• Each tornado is unique.

Single vortex grew in size and intensity over a
few minutes
Courtesy Kirsten Orwig
Courtesy Kirsten Orwig
17
Conclusions

• Wind provisions differ worldwide.
• Similarities include straight-line wind and
  neutral BL assumptions
• Each is suitable due to associated multipliers.
• Averaging times are critical for regions where
  other wind types dominate.
• Downbursts and tornados may generate greater
  loads due to transience and complexity and remain
  to be addressed.

18
References

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19
References Contd

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20
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