Structural Engineering & Statics
where the load goes
Generator

Where the wind's energy is, and where the structure can reach it

Rendered here at the parameters it defaults to, with every essay that calls it — which is the same list as the blast radius of changing it.
Where the wind's energy is, and where the structure can reach it. The gust spectrum at a mean speed of 30 m/s, plotted as n·S(n) against frequency on a logarithmic axis so that equal areas are equal energy. Its total variance is 27 m²/s², which the closed form 6·K·U² gives as 27. Most of the energy is below a hundredth of a hertz — gusts lasting a minute or more. A structure at 0.2 Hz sits far out on the tail, and still takes 90% of its response from there, because the resonant part is amplified by π·f₀/(4·ζ) and the damping is 1.2%.

Where the wind's energy is, and where the structure can reach it. The gust spectrum at a mean speed of 30 m/s, plotted as n·S(n) against frequency on a logarithmic axis so that equal areas are equal energy. Its total variance is 27 m²/s², which the closed form 6·K·U² gives as 27. Most of the energy is below a hundredth of a hertz — gusts lasting a minute or more. A structure at 0.2 Hz sits far out on the tail, and still takes 90% of its response from there, because the resonant part is amplified by π·f₀/(4·ζ) and the damping is 1.2%.

2 essays call wind-spectrum. The drawing above is what it returns with no arguments at all; every call below passes it something, because a placement that passes nothing draws whichever member of the family the generator happens to default to rather than the one its essay argues about.

Where it is called

Changing this generator changes every one of these figures.

Where the wind's energy is, and where the structure can reach it. The gust spectrum at a mean speed of 30 m/s, plotted as n·S(n) against frequency on a logarithmic axis so that equal areas are equal energy. Its total variance is 27 m²/s², which the closed form 6·K·U² gives as 27. Most of the energy is below a hundredth of a hertz — gusts lasting a minute or more. A structure at 0.2 Hz sits far out on the tail, and still takes 90% of its response from there, because the resonant part is amplified by π·f₀/(4·ζ) and the damping is 1.2%. A stiffer structure at 2 Hz takes 22%. Dynamics

The wind is a spectrum

A wind load is quoted as a pressure, which suggests something steady. It is not — the energy is spread across four decades of frequency, almost all of it in gusts lasting minutes, and a tall building takes ninety per cent of its response from the sliver of that energy sitting at its own frequency.

 The wind pushes on one face and pulls on three. A 30 × 20 m building in plan, with the measured pressure coefficient on each face and the arrows drawn in the direction the pressure acts. Only the windward face is pushed; the other three are sucked, and the side faces are sucked hardest of all at c_p = -0.7. The horizontal resultant is 842 kN at a velocity pressure of 0.9 kPa, and the arithmetic of it is the whole point: the leeward suction pulls the building downwind, so it ADDS, supplying 38% of the answer, while the two side faces cancel each other exactly and supply none of it. The coefficients are wind tunnel data; what is computed is the free body they are applied to. Equilibrium

Most of it is suction

A wind load is drawn as arrows pressing on the windward face, which is where about three fifths of it comes from. The rest is a pull on the back. The two side faces carry the largest suctions on the building and contribute nothing at all to the answer — and the inside of the building, which nobody draws, decides whether the roof stays on.

The library, page 7 of 7 — where wind-spectrum sits