3.1 High Wind Areas and Risk of Structural Damage

Simple concepts have frequently been used in estimating live loads for structural design. Now, however, live loads on buildings, such as wind, snow, earthquake and floor loads, are receiving increased attention to match more accurate structural analyses that are possible. Wind loads have become particularly significant because of increasing numbers of high-rise buildings. Other factors have also contributed to importance of wind design: lightweight low-slope roofs, curtain wall construction and appearance of special structures having aerodynamic shapes. Some tall buildings that extend into regions of high wind velocity have swayed excessively in strong winds. Wind forces have blown off improperly anchored lightweight roofs, and roofing materials have been lifted by high local suctions and eventually peeled from large areas of roofs. These and many other problems have emphasized the importance of a clearer understanding of wind and its effects. With the old simplified approach, merely a uniform lateral pressure on windward side of a building and suction on leeward wall often represented total effect of wind. Crossly simplified rules were also used to calculate pressures or suctions on roofs. Only horizontal shear and overturning moment were calculated. For low or medium height buildings, such simple methods may have been reasonably satisfactory, but for tall buildings, the greater importance of wind loading calls for more accuracy. Wind is not constant either with height or with time, is not uniform over the side of a building, and does not always cause positive pressure. In fact, wind is a very complicated phenomenon; it is air in turbulent flow, which means that motion of individual air particles is so erratic that in studying wind, one ought to be concerned with statistical distributions of speeds and directions rather than with simple averages or fixed physical quantities.
Architects and engineers are concerned with and responsible for not only structural design, but also the choice of exterior cladding materials and components, operation of mechanical services such as heating and ventilating equipment, and with details of openings to limit infiltration. Wind has important effects on each of these aspects of design; one might even conclude that of the manifestations of nature with which the architect has to contend, apart from gravity, the effects of wind are ubiquitous

Wind usually refers to movement of air parallel to the earth's surface. Driving forces for such movements are pressure differences caused by unequal heating of the air. For a steady wind, however, direction of flow does not follow the steepest pressure gradient from a "high" to, a low" as one might expect. In fact, direction of flow is more nearly parallel to the isobars (lines connecting points of equal pressure) rather than perpendicular to them. This is because every object moving across the earth's surface deflects to the right in the northern hemisphere (to the left in the southern) because of rotation of earth. This deviating effect, called the Coriolis force, is small and is usually disregarded except in the atmosphere and ocean. Pressure gradient causing wind, however, is also small. Normally, wind requires several hours to develop, and although flow begins perpendicular to the isobars, it gradually deflects to the right as time passes, so that when a steady state is attained, wind blows more nearly parallel to the isobars. The Coriolis force and frictional drag force then balance the pressure gradient, plus or minus centrifugal force if path happens to curve.

Every structure has a natural frequency of vibration, and should dynamic loading occur at or near it, structural damage out of all proportion to size of load may result. For example, bridges capable of carrying far greater loads than the weight of a company of soldiers have been known to break down under dynamic loading of men marching over them in step. Similarly, certain periodic gusts within the wide spectrum of gustiness in wind may find resonance with natural vibration frequency of a building, and although the total force caused by that particular gust frequency would be much less than the static design load for the building, dangerous oscillations may be set up. This applies not only to the structure as a whole, but also to components such as curtain wall panels and sheets of glass. A second dynamic effect is caused by instability of flow around certain structures. Long narrow structures such as smoke stacks, light standards and suspension bridges are particularly susceptible to this sort of loading the other side of the object, causing an alternating pattern of eddies to form in its wake. A side thrust is thus exerted on the object similar to the lift on an aerofoil, and since this thrust alternates in direction, a vibration may result. Side-to-side wobbling effect of a straight stick pulled through water is an example of this phenomenon. As research gradually provides a better understanding of the structure of wind and the complex interactions between wind and buildings, one can look forward to greater economy in the use of building materials through greater precision in estimating static load; and to greater safety because of the inclusion of dynamic load in design

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