Wind Pressures

DOE-STD-1020-2002

enclosed structure, except on windward walls and on steep windward roofs. External pressures

include pressures on windward walls, leeward walls, side walls and roof.

Internal pressures develop when air flows into or out of an enclosed structure through

existing openings or openings created by airborne missiles. In some cases natural porosity of the

structure also allows air to flow into or out of the building. Internal pressure acts either inward

or outward, depending on the location of the opening and the wind direction. If air flows into the

structure through an opening in the windward wall, a "ballooning" effect takes place: pressure

inside the building increases relative to the outside pressure. The pressure change produces

additional net outward-acting pressures on all interior surfaces. Openings in any wall or roof

area where the external pressures are outward acting allows air to flow from inside the structure:

pressure inside the structure decreases relative to the outside pressure. The pressure change

produces net inward-acting pressure on all interior surfaces. Internal pressures combine with

external pressures acting on a structure's surface.

With systems and components, interest focuses on net overturning or sliding forces,

rather than the wind pressure distribution. The magnitude of these forces is determined by wind

tunnel or full-scale tests. Also, in special cases associated with aerodynamically sensitive SSCs,

vortex shedding or flutter may need to be considered in design. Typical wind sensitive SSCs

include stacks, poles, cooling towers, utility bridges, and relatively light-weight structures with

large smooth surfaces.

Gusts of wind produce dynamic pressures on SSCs. Gust effects depend on the gust size

relative to SSC size and gust frequency relative to the natural frequency of the SSC. Except for

tall, slender structures (designated wind sensitive structures), the gust frequencies and the

structure frequencies of vibration are sufficiently different that resonance effects are small, but

are not negligible. The size (spatial extent) of a gust relative to the size of the SSC contributes to

the magnitude of the dynamic pressure. A large gust that engulfs the entire SSC has a greater

dynamic effect on the SSC than a small gust that only partially covers the SSC. In any event,

wind loads may be treated as quasi-static loads by including an appropriate gust response factor

in calculating the magnitude of the wind pressure. Straight wind, hurricane or tornado gusts are

not exactly the same, but errors owing to the difference in gust characteristics are believed to be

relatively small for those SSCs that are not wind sensitive.

The roughness of terrain surrounding SSCs significantly affects the magnitude of wind

speed. Terrain roughness is typically defined in four classes: urban, suburban, open and smooth.

Wind speed profiles as a function of height above ground are represented by a power law

relationship for engineering purposes. The relationship gives zero wind speed at ground level.

The wind speed increases with height to the top of the boundary layer, where the wind speed

remains constant with height.

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