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If the wind velocity increases from 0 to 60 mph (27.0 m/s), the frequency of vortex excitation will rise from 0 to a maximum of 0.16 Hz. Since this frequency happens to be very close to the natural frequency of the building, and assuming very little damping, the structure would vibrate as if its stiffness were zero at a wind speed somewhere around 60 mph (27 m/s). Note the similarity of this phenomenon to the ringing of church bells or the shaking of a tall lamppost whereby a small impulse added to the moving mass at each end of the cycle greatly increases the kinetic energy of the system. Similarly, during vortex shedding an increase in deflection occurs at the end of each swing. If the damping characteristics are small, the vortex shedding can cause building displacements far beyond those predicted on the basis of static analysis.

When the wind speed is such that the shedding frequency becomes approximately the same as the natural frequency of the building, a resonance condition is created. After the structure has begun to resonate, further increases in wind speed by a few percent will not change the shedding frequency, because the shedding is now controlled by the natural frequency of the structure. The vortex-shedding frequency has, so to speak, locked in with the natural frequency. When the wind speed increases significantly above that causing the lock-in phenomenon, the frequency of shedding is again controlled by the speed of the wind. The structure vibrates with the resonant frequency only in the lock-in range. For wind speeds either below or above this range, the vortex shedding will not be critical.

Vortex shedding occurs for many building shapes. The value of S for different shapes is determined in wind tunnel tests by measuring the frequency of shedding for a range of wind velocities. One does not have to know the value of S very precisely because the lock-in phenomenon occurs within a range of about 10% of the exact frequency of the structure.

1.3.5. Dynamic Nature of Wind

Unlike the mean flow of wind, which can be considered as static, wind loads associated with gustiness or turbulence change rapidly and even abruptly, creating effects much larger than if the same loads were applied gradually. Wind loads, therefore, need to be studied as if they were dynamic in nature. The intensity of a wind load depends on how fast it varies and also on the response of the structure. Therefore, whether the pressures on a building created by a wind gust, which may first increase and then decrease, are considered as dynamic or static depends to a large extent on the dynamic response of the structure to which it is applied.

Consider the lateral movement of an 800-ft tall building designed for a drift index of H/400, subjected to a wind gust. Under wind loads, the building bends slightly as its top moves. It first moves in the direction of wind, with a magnitude of, say, 2 ft (0.61 m), and then starts oscillating back and forth. After moving in the direction of wind, the top goes through its neutral position, then moves approximately 2 ft (0.61 m) in the opposite direction, and continues oscillating back and forth until it eventually stops. The time it takes a building to cycle through a complete oscillation is known as a period. The period of oscillation for a tall steel building in the height range of 700 to 1400 ft (214 to 427 m) normally is in the range of 5 to 10 seconds, whereas for a 10-story concrete or masonry building it may be in the range of 0.5 to 1 seconds. The action of a wind gust depends not only on how long it takes the gust to reach its maximum intensity and decrease again, but on the period of the building itself. If the wind gust reaches its maximum value and vanishes in a time much shorter than the period of the building, its effects are dynamic. On the other hand, the gusts can be considered as static loads if the wind load increases and vanishes in a time much longer than the period for the building. For example, a wind gust that develops to its strongest intensity and decreases to zero in 2 seconds is a dynamic load for a tall building with a period of, say, 5 to 10 seconds, but the same 2-second gust is a static load for a low-rise building with a period of less than 2 seconds.

1.3.6. Cladding Pressures

The design of cladding for lateral loads is of major concern to architects and engineers. Although the failure of exterior cladding resulting in broken glass may be of less consequence than the collapse of a structure, the expense of replacement and hazards posed to pedestrians require careful consideration. Cladding breakage in a windstorm is an erratic occurrence, as witnessed in hurricane Alicia, which hit Galveston and downtown Houston on August 18, 1983, causing breakage of glass in several tall buildings. Wind forces play a major role in glass breakage, which is also influenced by other factors, such as solar radiation, mullion and sealant details, tempering of the glass, double- or single-glazing of glass, and fatigue. It is known with certainty that glass failure starts at nicks and scratches that may be made during manufacture, and by handling operations.

There appears to be no analytical approach available for a rational design of curtain walls of all shapes and sizes. Although most codes have tried to identify regions of high wind loads around building corners, the modern trend in architecture of using nonprismatic and curvilinear shapes combined with the unique topography of each site, has made experimental determination of wind loads even more necessary.

Thus it has become routine to obtain design information concerning the distribution of wind pressures over a building's surface by conducting wind tunnel studies. In the past two decades, curtain wall has developed into an ornamental item and has emerged as a significant architectural element. Sizes of window panes have increased considerably,

Figure 1.6. Distribution of pressures and suctions.

requiring that the glass panes be designed for various combinations of forces due to wind, shadow effects, and temperature movement. Glass in curtain walls must not only resist large forces, particularly in tall buildings, but must also be designed to accommodate the various distortions of the total building structure. Breaking of large panes of glass can cause serious damage to neighboring properties and can injure pedestrians.

1.3.6.1. Distribution of Pressures and Suctions

When air flows around edges of a structure, the resulting pressures at the corners are much in excess of the pressures on the center of elevation. This has been evidenced by damage caused to corner windows, eave and ridge tiles, etc., in windstorms. Wind tunnel studies conducted on scale models of buildings indicate that three distinct pressure areas develop around a building. These are shown schematically in Fig.1.6.

1. Positive-pressure zone on the upstream face (Region 1).

2. Negative pressure zones at the upstream corners (Regions 2).

3. Negative pressure zone on the downstream face (Region 3).

The highest negative pressures are created in the upstream corners designated as Regions 2 in Fig. 1.6. Wind pressures on a building's surface are not constant, but fluctuate continuously. The positive pressure on the upstream or the windward face fluctuates more than the negative pressure on the downstream or the leeward face. The negative-pressure region remains relatively steady as compared to the positive-pressure zone. The fluctuation of pressure is random and varies from point to point on the building surface. Therefore, the design of the cladding is strongly influenced by local pressures. As mentioned earlier, the design pressure can be thought of as a combination of the mean and the fluctuating velocity. As in the design of buildings, whether or not the pressure component arising from the fluctuating velocity of wind is treated as a dynamic or as a pseudostatic load is a function of the period of the cladding. The period of cladding on a building is usually on the order of 0.2 to 0.02 sec, which is much shorter than the period it takes for wind to fluctuate from a gust velocity to a mean velocity. Therefore, it is sufficiently accurate to consider both the static and the gust components of winds as equivalent static loads in the design of cladding.

The strength of glass, and indeed of any other cladding material, is not known in the same manner that the strengths of steel and concrete are known. For example, it is not possible to buy glass based on yield strength criteria as with steel. Therefore, the selection, testing, and acceptance criteria for glass are based on statistical probabilities rather than on absolute strength. The glass industry has addressed this problem, and commonly uses 8 failures per 1000 lights (panes) of glass as an acceptable probability of failure.

1.3.6.2. Local Cladding Loads and Overall Design Loads

The overall wind load for lateral analysis consists of combined positive and negative pressures around the building. The local wind loads that act on specific areas of the building are required for the design of exterior cladding elements and their connections to the building. The two types of loads differ significantly, and it is important that these differences be understood. These are

1. Local winds are more influenced by the configuration of the building than the overall loading.

2. The local load is the maximum load that may occur at any location at any time on any wall surface, whereas the overall load is the summation of positive and negative pressures occurring simultaneously over the entire building surface.

3. The intensity and character of local loading for any given wind direction and velocity differ substantially on various parts of the building surface, whereas the overall load is considered to have a specific intensity and direction.

4. The local loading is sensitive to the momentary nature of wind, but in determining the critical overall loading, only gusts of about 2 sec or more are significant.

5. Generally, maximum local negative pressures, also referred to as suctions, are of greater intensity than the overall load.

6. Internal pressures caused by leakage of air through cladding systems have a significant effect on local cladding loads but are of no consequence in determining the overall load.

The relative importance of designing for these two types of wind loading is quite obvious. Although proper assessment of overall wind load is important, very few, if any buildings have been toppled by winds. There are no classic examples of building failures comparable to the Tacoma bridge disaster. On the other hand, local failures of roofs, windows, and wall cladding are not uncommon.

The analytical determination of wind pressure or suction at a specific surface of a building under varying wind direction and velocity is a complex problem. Contributing to the complexity are the vagaries of wind action as influenced both by adjacent surroundings and the configuration of the wall surface itself. Much research is needed on the microeffects of common architectural features such as projecting mullions, column covers, and deep window reveals, etc. In the meantime, model testing of building wind tunnels is perhaps the only answer.

Probably the most important fact established by tests is that the negative or outward-acting wind loads on wall surfaces are greater and more critical than had formerly been assumed. These loads may be as much as twice the magnitude of positive loading. In most instances of local cladding failure, glass panels have been blown off of the building, not into it, and the majority of such failures have occurred in areas near building corners. Therefore it is important to give careful attention to the design of both anchorage and glazing details to resist outward-acting forces, especially near the corners.

Another feature that has come to light from model testing is that wind loads, both positive and negative, do not vary in proportion to height aboveground. Typically, the positive-pressure contours follow a concentric pattern as illustrated in Fig.1.7, with the highest pressure near the lower center of the facade, and pressures at the very top somewhat

Figure 1.7. (a) Block pressure diagram, in psf; (b) Pressure countours in psf.

less than those a few stories below the roof. Figure 1.7a shows a pressure diagram for the design of cladding derived from pressure contours measured in wind tunnel tests shown in Fig.1.7b. The block pressure diagram shown in Fig.1.7a gives zones of design pressures based on the building grid system, to assist in the cladding design.

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