Wind engineering

Wind loads on buildings

The design of buildings must account for wind loads, and these are affected by wind shear. For engineering purposes, a power law wind-speed profile may be defined as:

${\displaystyle v_z=v_g\cdot {\left(\frac{z}{z_g}\right)}^{\frac{1}{\alpha }},0<z<z_g}$

where:

${\displaystyle v_z}$ = speed of the wind at height ${\displaystyle z}$

${\displaystyle v_g}$ = gradient wind at gradient height ${\displaystyle z_g}$

${\displaystyle \alpha }$ = exponential coefficient

Typically, buildings are designed to resist a strong wind with a very long return period, such as 50 years or more. The design wind speed is determined from historical records using extreme value theory to predict future extreme wind speeds. Wind speeds are generally calculated based on some regional design standard or standards. The design standards for building wind loads include:

• AS 1170.2 for Australia
• EN 1991-1-4 for Europe
• NBC for Canada

Wind comfort

The advent of high-rise tower blocks led to concerns regarding the wind nuisance caused by these buildings to pedestrians in their vicinity.

A number of wind comfort and wind danger criteria were developed from 1971, based on different pedestrian activities, such as:

• Sitting for a long period of time
• Sitting for a short period of time
• Strolling
• Walking fast

Other criteria classified a wind environment as completely unacceptable or dangerous.

Building geometries consisting of one and two rectangular buildings have a number of well-known effects:

• Corner streams, also known as corner jets, around the corners of buildings
• Through-flow, also known as a passage jet, in any passage through a building or small gap between two buildings due to pressure short-circuiting
• Vortex shedding in the wake of buildings

For more complex geometries, pedestrian wind comfort studies are required. These can use an appropriately scaled model in a boundary-layer wind tunnel, or more recently, use of computational fluid dynamics techniques has increased. The pedestrian level wind speeds for a given exceedance probability are calculated to allow for regional wind speeds statistics.

The vertical wind profile used in these studies varies according to the terrain in the vicinity of the buildings (which may differ by wind direction), and is often grouped in categories, such as:

• Exposed open terrain with few or no obstructions and water surfaces at serviceability wind speeds
• Water surfaces, open terrain, grassland with few, well-scattered obstructions having heights generally from 1.5 to 10 m
• Terrain with numerous closely spaced obstructions 3 to 5 m high, such as areas of suburban housing
• Terrain with numerous large, high (10 to 30 m high) and closely spaced obstructions, such as large city centres and well-developed industrial complexes

Wind turbines

Wind turbines are affected by wind shear. Vertical wind-speed profiles result in different wind speeds at the blades nearest to the ground level compared to those at the top of blade travel, and this, in turn, affects the turbine operation. The wind gradient can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical. The reduced wind gradient over water means shorter and less expensive wind turbine towers can be used in shallow seas.

For wind turbine engineering, wind speed variation with height is often approximated using a power law:

${\displaystyle v_w(h)=v_{ref}\cdot {\left(\frac{h}{h_{ref}}\right)}^a}$

where:

${\displaystyle v_w(h)}$ = velocity of the wind at height ${\displaystyle h}$ [m/s]

${\displaystyle v_{ref}}$ = velocity of the wind at some reference height ${\displaystyle h_{ref}}$ [m/s]

${\displaystyle a}$ = Hellman exponent (a.k.a. power law exponent or shear exponent) (~= 1/7 in neutral flow, but can be >1)

Significance

The knowledge of wind engineering is used to analyze and design all high-rise buildings, cable-suspension bridges and cable-stayed bridges, electricity transmission towers and telecommunication towers and all other types of towers and chimneys. The wind load is the dominant load in the analysis of many tall buildings, so wind engineering is essential for their analysis and design. Again, wind load is a dominant load in the analysis and design of all long-span cable bridges.