BAS Wind Flashcards
Near-surface wind is the most variable of all meteorological elements. The portion of the air in which speed is affected by the earth’s surface is called the …
…“boundary layer,” (which extends several hundred feet above the Earth’s surface.)
Wind speed increases with height
above the Earth’s surface (Figure 2.1). Due to
friction, wind speed nearly disappears at ground level.
The recommended height to measure wind speed it –1– ft above the ground
1) 33 ft
True or false
Hurricane forecasts often refer to wind speeds measured by aircraft at heights well above the atmospheric boundary layer.
True
The effects of ground roughness are not accounted for and the actual near-surface winds may be significandy less (65 percent to 85 percent of measured speeds). Thus, structures that “survived” a reported 125 mph wind may have only actually been exposed to a 95 mph wind. Such variations in reported and actual wind speeds can lead to a false sense of security about the wind resistance of buildings.
In general, wind damage in typical built-up areas will begin at gust speeds of about –1– mph.
50 mph.
Some shingles and siding will come off, trees will begin to lose limbs or be uprooted, and overhead traffic lights and signs may come down.
The Saffir-Simpson Scale, which is based on site observations and instrument read- ings, is the preferred preliminary measure of hurricane intensity because..
Only from careful post-storm assessments of land-based or ocean-surface anemome- ter records and wind damage can the true distribution of surface wind speeds be ascertained.
A 50-year storm means that a storm of this intensity or greater would be expected to hap- pen about once every –1– year.
1) 50 years, or that there is a 2 percent chance each year that such a storm will occur.
What does ACSE stand for?
American Society of Civil Engineers
The flow behind a long cylinder held perpendicular to wind is charac- terized by the periodic shedding of vortices (whirling air flows). This is termed what?
Vortex shedding
It creates periodic lateral forces that can cause vibration of slen- der structures such as towers and tall buildings.
Vortex-shedding vibration takes place when the wind speed is such that the shedding frequency becomes approximately equal to the natural fre- quency of the cylinder—a condition that causes resonance.
When reso- nance takes place, further increase in wind speed by a few percent will not alter the shedding frequency. This phenomenon is called “lock-in.”
Classical flutter (or simply flutter) is…
… a two-degrees-of-freedom vibration involving simultaneous lateral (across-wind translational) and torsional (rotational) vibrations. It occurs in structures that have approximately the same magnitude of natural frequencies for both the translational and the rotational modes. Similar to galloping and torsional divergence, flut- ter is produced by aerodynamic instability completely unrelated to vor- tex shedding.
The four primary damage mechanisms associated with severe wind- storms involve:
(1) aerodynamic pressures created by flow of air around a structure;
(2) induced internal pressure fluctuations due to a breach in the build-
ing envelope;
(3) impact forces created by wind-borne debris; and
(4) pressures created by rapid atmospheric pressure fluctuations (associ-
ated primarily with tornadoes).
As winds increase, pressure against objects is added at a non-linear rate. How so?
Pressure force against a wall mounts with the square of the wind speed so that a three-fold increase in wind speed, for example, results’ in a nine-fold increase in pressure. A 25 mph wind causes about 1.6 pounds of pressure per square foot. Therefore a 4x8 sheet of plywood will be pushed by a force of about 50 pounds. In 75 mph winds, that force becomes 450 pounds, and at 125 mph, it becomes 1,250 pounds.9
Windnd pressures acting on buildings are distributed loads that are assumed to act normal to the building surface. Positive wind pressures–1–and negative pressures (suction) act –2–.
1) act toward the surface of the building element
2) away from the building surface
The fundamental char- acteristics of wind pressures are described below based on the building component affected and the orientation of the building in the wind environment.
As stated previously, the windward wall is subjected to positive pressures and the leeward wall to negative pressures. As the wind flows upward and over the windward edge of the roof, the flow is accelerated and there is a tendency for the wind flow to separate from the roof. These flow characteristics result in the roof being subjected to negative pressures. In addition, locally high negative pressures can occur at both the windward eaves and roof ridge. The magnitude of the local pressure excursions depends on the slope of the roof.
depicts the pressure distribution acting on a high-sloped roof with the wind blowing perpendicular to the roof ridge. The wind- ward slope is subjected to positive pressures while the leeward slope is subjected to negative pressures. In addition, locally high pressure excursions are to be expected at the roof ridge. The magnitude of the ridge pressure fluctuations will depend on the slope of the roof. Shed roofs experience the highest loads (worst geometry) for wind parallel to the slope and directed toward the high wall.
If either a pitched or flat roof has an overhang, the roof will be subject- ed to high positive pressures on the windward overhang as depicted in Figure 3.10. If the overhang is associated with a flat roof or a low-slope roof, these forces will combine with negative pressures and add to the overall roof uplift that must be resisted.
If breaches occur in the exterior building envelope during a windstorm, the internal building pressure is changed. If the breaches occur primarily on the windward wall, ….
…the internal pressure of the building will be increased and the walls and roof of the building will be forced outward
The most common source of breaches in a building during windstorms is the failure of doors and windows. Debris impacts, as discussed in the following section, are a major cause of such failures.
If the breaches occur primarily on the side walls or the leeward wall the internal building pressure is reduced and the walls and roof of the building are pulled inward.
For current purposes, wind-borne debris can be divided into two groups:
1) small missiles:Small wind-borne missiles also can be blown about in relatively moderate wind storms when measured wind speeds only marginally exceed 60 mph
- include objects such as roof ballast, small pieces of building materials, and natural materials such as pine cones and tree limbs
2) large missiles: include pieces of timber, sheet metal, plywood, trusses, roof-top HV AC equip- ment, large glass panels, and siding
While wind speeds on tall buildings are magnified at the base, they increase in strength at …
…the building corners. Architects should keep these wind dynamics in mind in terms of pedestrian comfort when locating entrances in tall buildings (Figures 3.22 and 3.23). Corner entrances at the base of a high rise will be sub- ject to the highest wind loads
When wind blows from a diagonal across a corner, it creates…
… conical vortices, which can be seen in wind tunnel testing (Figure 3.27). By placing a vertical element at the corners of the form, the diagonal wind is split and the conical vortices broken up and dissipated. There are still high localized loads going over the top of the configuration, but they don’t compound at the corner, and a relatively small element at the corner will interrupt it.
Damage investigations show that unreinforced masonry walls are a common structural failure point even when they are subjected to winds well below the design level.
Detail for roof hold down
Detail for roof at lintel
