Wind pressures

A smooth coal pile surface, coupled with the gradual slope, minimi2es the differential wind pressures and consequent oxygen penetration. A 4-6 X 10 t lignite stockpile from the excavation for the Garrison Dam in North Dakota has been stable for many years as a result of this storage method. 

When tanks are built having an open top, the wind pressure may cause buckling of the shell. A wind girder of sufficient section modulus is used to stiffen the open top according to... 

Wind pressures exerted on the exterior building surfaces, which can influence air movement indoors... 

This section will describe general features of airflow patterns and then present information on the dimensions and locations of recirculating (stagnant) zones around the building envelope, which determine wind pressures and contaminant dilution. This knowledge allows one to select the locations of stacks and air intakes and to calculate infiltration and natural ventilation rates. 

One of the effects of airflow or wind around buildings is the exertion of wind pressure forces on rhe surface of the building, which contributes to natural ventilation of the building and infiltration of outside air into the building. As discussed above, pressures tend to be positive (into rhe building) on upwind surfaces and negative (suction) on lateral, downwind, and roof surfaces. 

Pressure at a given location on the building surface is usually expressed as a pressure coefficient times a reference wind pressure at the building height without the building in place ... 

Natural ventilation is the controlled flow of air through doors, windows, vents, and other purposely provided openings caused by stack effect and wind pressure. Natural ventilation is used in spaces with a significant heat release, when process and hygienic requirements for indoor air quality allow outdoor air supply without filtration and treatment. Natural ventilation cannot be used when incoming outdoor air causes mist or condensation. Natural ventilation allows significant air change rates (20 to 50 ach) for heat removal with ntinimal operation costs. 

The airflow network (Fig. 11.41 is composed of nodes, interconnected by links, representing individual airflow paths. Internal nodes represent the individual zones of the building, and external nodes represent faqade locations, related to a specific set of wind pressure coefficients. Each link represents a specific airflow conductance type. 

Airflows are determined basically by a steady-state calculation for each time step. At each time step, first, pressures at external nodes are calculated on the basis of the wind pressure coefficients and the actual wind speed and direction. Then, for all conductances, the local pressures at each side of the link are calculated. At internal links, this pressure is dependent on the (unknown) zone pressure p and the aerostatic pressure variation due to the height of the link with respect to the zone reference height. At external links, this pressure is dependent on the external node pressure and the aerostatic pressure variation due to the height of the link with respect to the stack reference height. For the aerostatic pressure, the air density is determined considering the temperature, the humidity, and (if relevant) the contaminant concentrations in the zone or in the outside air, respectively. From this, the pressure differences across each conductance can be calculated, and from this the mass airflow tor each conductance /. 

Wind pressure distribution Set of wind pressure coefficient data for each external node... 

Data are available only for simple building geometries. In Allard," a tool for the calculation of wind pressure coefficients for simple geometries is made available, and another tool is described in Knoll et al. Existing wind pressure data have to be examined carefully, because many data represent peak pressure values needed for static building analysis. Real cases with obstructions and buildings in the close surroundings are difficult to handle. Wind-tunnel tests on scale models or CFD analysis will be required. 

Nevertheless, in many cases, mean wind velocities can be assumed. In ventilation-system reliability studies, e.g., where minimum ventilation rates are to be determined, a calm situation with little wind must be assumed anyhow, and the need for accurate wind pressure coefficient data is not so obvious. 

The factory is modeled as a two-zone network with door, horiztmtally pivoted windows, and roof shed windows as airflow elements. The extract tan and the duct and hood are modeled as additional airflow elements. Wind pressure coefficient data are taken from literature for a simple rectangular buihl-ing shape surrounded by buildings of equal height. 

Wind pressures The resulting positive or negative pressures due to the wind velocity set up on the walls and roof of a structure. 

Wind stop A flat plate or a cone fitted over an outlet or inlet duct in order to reduce the possibility of flow reversal due to wind pressure. 

Wind-druck, m. (Metal.) blast pressure wind pressure, -dtise, /. blast nozzle, twyer, tuyere. 

Outlets should not be provided in constantly recirculating systems, particularly where close control of humidity is required. The overpressure developed is far less than that exerted by the wind, and for this reason any system which does have both intake and discharge ducts should have them on the same face of the building. While care is necessary to prevent short-circuiting, this alleviates problems arising from the considerable wind pressure difference that can develop on opposite sides of a building. 

For a smooth cylindrical column or stack the following semi-empirical equation can be used to estimate the wind pressure ... 

At any site, the wind velocity near the ground will be lower than that higher up (due to the boundary layer), and in some design methods a lower wind pressure is used at heights below about 20 m typically taken as one-half of the pressure above this height. 

The loading per unit length of the column can be obtained from the wind pressure by multiplying by the effective column diameter the outside diameter plus an allowance for the thermal insulation and attachments, such as pipes and ladders. 

Any horizontal force imposed on the vessel by ancillary equipment, the line of thrust of which does not pass through the centre line of the vessel, will produce a torque on the vessel. Such loads can arise through wind pressure on piping and other attachments. However, the torque will normally be small and usually can be disregarded. The pipe work and the connections for any ancillary equipment will be designed so as not to impose a significant load on the vessel. 

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. 

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. 

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