How can you incorporate environmental factors into wind engineering stress analysis?

Wind engineering is a branch of structural engineering that deals with the effects of wind on buildings, bridges, towers, and other structures. Wind can cause various types of stresses, such as bending, torsion, shear, and vibration, that can affect the stability, safety, and performance of structures. To design and analyze structures for wind loads, engineers need to consider several environmental factors, such as wind speed, direction, turbulence, terrain, and climate. In this article, you will learn how to incorporate these factors into wind engineering stress analysis using some common methods and tools.

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1 Wind speed and direction

The first step in wind engineering stress analysis is to determine the wind speed and direction at the site of the structure. Wind speed and direction can vary depending on the location, altitude, season, time of day, and weather conditions. Engineers can use historical data, meteorological stations, wind maps, or wind tunnels to estimate the wind speed and direction for a given site. The wind speed and direction can then be used to calculate the wind pressure and force acting on the structure.

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Felipe Simões

Structural Engineer specializing in Glass Structures
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In order to perform stress analysis it is important to find basic wind speed, and consider enough directions in order to make sure the structure is analysed safely. For in-shore structures, many countries have codes with wind maps and coefficients for analytically derive expected pressures and suctions on the structure. For offshore structures, many locations around the world have meteorological stations constantly measuring wind speed and direction, and codes for offshore structures provide guidance and coefficients for determining wind loads. Structures in locations without established wind data, with complex shapes or vicinities may require wind properties measurements in-situ.

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2 Wind turbulence and gusts

Wind turbulence and gusts are fluctuations in wind speed and direction that can cause dynamic stresses and vibrations on structures. Wind turbulence and gusts depend on the atmospheric stability, the roughness of the terrain, and the presence of obstacles or other structures. Engineers can use statistical models, such as the Davenport method, to account for the effects of wind turbulence and gusts on structures. The Davenport method uses a gust factor to modify the mean wind speed based on the turbulence intensity, the natural frequency, and the damping ratio of the structure.

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Felipe Simões

Structural Engineer specializing in Glass Structures
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Wind turbulence and gusts are important variations of wind speed, and may also be associated with periods of harmonious loading. Most civil codes for buildings provide indications - for simple cases - on how to take into account these phenomena. That can be both in deriving factors that correct the pressure/suction from wind speed in order to find a static-equivalent pressure or suction value, as well as a loading frequency. Those depend on several variables (terrain and structure roughness, shape, etc) and on which wind load model is being adopted. Nonetheless, they are fundamental for adequately determining load cases and conditions in static and dynamic analyses.

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3 Wind-structure interaction

Wind-structure interaction is the phenomenon that occurs when the wind flow affects the motion of the structure and vice versa. Wind-structure interaction can result in resonance, flutter, galloping, vortex shedding, and buffeting, which are forms of aerodynamic instability that can cause large-amplitude oscillations and fatigue damage on structures. Engineers can use numerical methods, such as the finite element method (FEM), to model the wind-structure interaction and evaluate the dynamic response and stability of structures. The FEM involves dividing the structure and the wind flow into discrete elements and solving the equations of motion and continuity for each element.

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Felipe Simões

Structural Engineer specializing in Glass Structures
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When wind passes by a structure, it loads and deforms it. The structure, as an obstacle to the wind, changes its trajectory and speed gradient. Furthermore, you can have periods of harmonic variation of pressure in frequencies sensitive to the structure's own frequencies. That is fluid(wind)-structure interaction. Especially for slender, low stiffness and/or certain structure shapes, it is important to take that into account. The iterative nature of this interaction usually leads us to recurr to FEM software to understand the effects of this interaction on the structure. A cantilevered pole under wind load may be safe for wind in terms of strength, stability and deformation, but it may gallop side to side when dynamic behavior is considered

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4 Terrain and exposure

The terrain and exposure of the site can influence the wind profile and the wind load on the structure. The terrain and exposure can be classified into different categories, such as open, suburban, urban, or coastal, based on the roughness and topography of the land. Engineers can use empirical formulas, such as the ASCE 7-16 standard, to adjust the wind speed and direction based on the terrain and exposure category. The ASCE 7-16 standard also provides coefficients for different shapes and orientations of structures to account for the wind pressure distribution and drag effects.

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5 Climate and design life

The climate and design life of the structure can affect the probability and severity of extreme wind events, such as hurricanes, tornadoes, or thunderstorms, that can cause catastrophic failure or collapse of structures. Engineers can use probabilistic methods, such as the extreme value theory, to estimate the return period and the peak wind speed of extreme wind events for a given site and design life. The extreme value theory uses statistical distributions, such as the Gumbel distribution, to fit the historical data of maximum wind speeds and extrapolate the values for rare events.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How can you incorporate environmental factors into wind engineering stress analysis?

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2

3

4

5

6

1 Wind speed and direction

2 Wind turbulence and gusts

3 Wind-structure interaction

4 Terrain and exposure

5 Climate and design life

6 Here’s what else to consider

Structural Engineering

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How can you incorporate environmental factors into wind engineering stress analysis?

1

2

3

4

5

6

1 Wind speed and direction

2 Wind turbulence and gusts

3 Wind-structure interaction

4 Terrain and exposure

5 Climate and design life

6 Here’s what else to consider

Structural Engineering

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