Wind Loads

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Besides Current Loads, wind loads exert significant forces on a seastead. This is important to quantify required anchor or propulsion strength.

Winds, like currents, are strongly dependent on geographical location. Even more so than currents, they are very time-dependent. Average and maximum wind speeds can be used to give a characterization.

The maximum wind speed in the clubstead report is 19m/s, during a hundred year storm. Winds of 10m/s are considered common, and they are taken as an average wind load figure.

A cylinder with a height of 20m and a width of 10m would experience 10kN of load in 10m/s winds. Velocity dependence is predominantly quadratic.

As such, wind loading and current loading are in the same ballpark, in terms of magnitude. Given the correlation between current and wind direction, we might consider these loads as reinforcing each other all the time, as a worst case scenario.

Wind Load Calculation

The generic formula for wind load is F = A x P x Cd where F is the force or wind load, A is the projected area of the object, P is the wind pressure, and Cd is the drag coefficient.

The simple formula for wind pressure P in imperial units (pounds per square foot) is {\displaystyle P=0.00256V^{2}}, where V is the speed of the wind in miles per hour (mph). To find the pressure in SI units (Newtons per square meter), instead use {\displaystyle P=0.613V^{2}}, and measure V in meters per second.

This formula is based on the American Society of Civil Engineers code. The 0.00256 coefficient is the result of a calculation based on typical values for air density and gravitational acceleration.[6] Engineers use a more accurate formula to take into account factor such as the surrounding terrain and type of construction. You can look up one formula in ASCE code 7-05, or use the UBC formula below. If you're not sure what the wind speed is, look up the peak wind speed in your area using the Electronic Industries Association (EIA) standard. For example, most of the U.S. is in Zone A with 86.6 mph wind, but coastal areas might lie in Zone B (100 mph) or Zone C (111.8 mph).

Drag is the force that air exerts on the building, affected by the building's shape, the roughness of its surface, and several other factors. Engineers typically measure drag directly using experiments, but for a rough estimate you can look up a typical drag coefficient for the shape you are measuring.

For example:

  • The standard drag coefficient for a long cylinder tube is 1.2 and for a short cylinder is 0.8. These apply to antenna tubes found on many buildings.
  • The standard coefficient for a flat plate such as the face of a building is 2.0 for a long flat plate or 1.4 for a shorter flat plate.
  • The drag coefficient has no units.