Dynamic wind pressure

For a building with a flat roof (pitch less than 10°) it is normally assumed that reflection does not occur when the blast wave travels horizontally. Consequently, the roof will experience the side-on overpressure combined with the dynamic wind pressure, the same as the side walls. The dynamic wind force on the roof acts in the opposite direction to the overpressure (upward). Also, consideration should be given to variation of the blast wave with distance and time as it travels across a roof element. The resulting roof loading, as shown in Figure 3.8, depends on the ratio of blast wave length to the span of the roof element and on its orientation relative to the direction of the blast wave. The effective peak overpressure for the roof elements are calculated using Equation 3.11 similar to the side wall. 

Take dynamic wind pressure as 1280N/m, corresponding to 160 kph (100 mph) ... 

Computer program Dynamic wind pressure on cooling tower (NAZAM-4)... 

C Prepared by Liu, checked by Y. Bangash C THE SUBPROGRAM FINDS THE DYNAMIC WIND PRESSURE AT ... 

The forces on a structure associated with a blast wave resulting from an external detonation are dependent upon the peak values and the pressure-time variation of the incident and dynamic wind pressure action, including characteristics of the reflected blast wave caused by interaction with the structure.28... 

FIG. II-4. Additional side-on blast parameters for TNT U shock front velocity (m/s) u particle velocity behind the shock wave (m/s) Q dynamic wind pressure (Pa) b decay constant. 

As the wave front moves forward, the reflected overpressure on the face of the structure drops rapidly to the side-on overpressure, plus an added drag force due to the wind (dynamic) pressure. At the same time, the air pressure wave bends or "diffracts" around the structure, so that the structure is eventually engulfed by the blast, and approximately the same pressure is exerted on the sides and the roof. The front face, however, is still subjected to wind pressure, although the back face is shielded from it. 

This blast effect is due to air movement as the blast wave propagates through the atmosphere. The velocity of the air particles, and hence the wind pressure, depends on the peak overpressure of the blast wave. Baker 1983 and Hvf 5-/300 provide data to compute this blast effect for shock waves. In the low overpressure range with normal atmospheric conditions, the peak dynamic pressure can be calculated using the following empirical formula from Afewmark I956 ... 

The paper outlines a general procedure for predicting the dynamic response, including resonance, of hyperbolic cooling towers to turbulent winds. Pressure spectra on the tower surface were measured in a boundary layer wind tunnel. Application to full-scale tower is examined. It is concluded that while the quasi-steady response increases with the wind velocity squared, the resonant response increases faster than wind velocity cubed. 10 refs, cited. 

Atmospheric features which are smaller than the mesoscale have pressure fields in which wind acceleration is a significant component (which is referred to as the dynamic wind). The pressure gradient which causes this dynamic wind is called the nonhydiostatic pressure. 

The main purpose of this approach is the identification of major dynamic parameters which can be used for estimating and predicting tank behavior when subjected to severe loading conditions such as earthquake, blast or drastic wind pressure. 

Reliable modal information can be obtained by output-only dynamic measurements, i.e. accelerations are dne to ambient influences. In the case of bridges, traffic under or on top of the bridge, besides wind pressures can be the cause of the induced vibrations. So closing bridges to apply controlled force excitation is not necessary. This makes ambient vibration monitoring suitable for continuous condition assessment. 

Detonations in solid material are characterized by a sharp rise in pressure which expands from the centre of the detonation as a pressure wave impulse at or above the speed of sound in the transmission media. It is followed by a much lower amplitude negative pressure impulse, which is usually ignored in the design, and is accompanied by a dynamic wind caused by air behind the pressure wave moving in the direction of the wave. 

II-5. When the blast wave impulse encounters an obstruction it results in a reflected wave typically two to four times the magnitude of the side-on peak pressure, but of shorter duration, impinging on obstructions perpendicular to the free field or side-on blast wave s direction of travel. As the positive blast wave traverses a building structure, in addition to the reflected pressure on the windward side, it exerts a positive pressure on all walls and the roof of the structure as it passes. Dynamic winds following the blast wave exert a positive pressure (inward) on the windward wall and negative pressures on the side and leeward walls and roof. 

Table 6.10 presents some damage effects. It may give the impression that damage is related only to a blast wave s peak overpressure, but this is not the case. For certain types of structures, impulse and dynamic pressure (wind force), rather than overpressure, determine the extent of damage. Table 6.10 was prepared for blast waves of nuclear explosions, and generally provides conservative predictions for other types of explosions. More information on the damage caused by blast waves can be found in Appendix B. 

Other properties of tlie blast wave are tlie shock velocity, wliich is tlie rate or speed of tlie blast wave as it travels tluough tlie air, tlie particle velocity (or peak wind velocity), tlie peak dynamic pressure, and tlie peak rejected overpressure. 

In a Japanese plasma wind tunnel, SPA specimens were tested up to 3.8 MW/m2 at 0.7 bar aerodynamic pressure (Fig. 12). After a test duration of 60 s, no obvious damage was visible. The surface temperature of about 2600°C was reduced to 100°C within 20 min. Further analysis showed a maximum charred depth of the ablator of 15 mm. The carbonization process did not change the geometric dimensions, the new heat protection system can be considered absolutely stable to deformation. The carbonized layer still has a noticeable pressure resistance and transfers the load applied by the dynamic pressure to the structure. 

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