|EXTERNAL AND DIFFERENTIAL PRESSURES
The measured local pressures are external pressures suitable for the design of externally ventilated cladding. For a nominally sealed building, internal pressures are estimated by assuming that pressures on the exterior surfaces leak into the building. Differential pressures across curtain walls or glazing are determined from the difference between the external and the internal pressures. For building components that are exposed to air on both sides; such as canopies and parapets, net differences in pressure loads may be determined by instantaneously differencing the pressures on both sides of the building components. For buildings with operable windows, the effects of the transmission of the external pressures into the interior volume of the building should be considered. In this case, where there is extensive partitioning of the interior space (e.g. hotels, offices) it is likely that the dynamic component of the external pressures will also be transmitted into the interior space. The critical areas of concern may be found at corners of the building where one side of the corner experiences positive pressures while the other side experiences negative pressures. The transmission of the positive pressure through an opened window on one wall will create positive internal pressure. This internal pressure will add to the external suction already acting on the other side of the corner. Upper bound estimates of this effect can be provided based on the predicted peak pressures on the building walls, with allowance for the non-coincidental occurrence of the peaks on different walls.
The effects of operable windows or accidental openings on internal pressures may be studied in more detail with information obtained from the simultaneous measurements of pressures on the building. The scope of such studies depends on additional information on opening size, building volume and internal partitioning of the building. This must be determined on a case-by-case basis.
In some cases, various window breakage scenarios may need to be considered. Upper bound estimates of the net pressures and suctions, similar to the operable window cases, can be provided fairly simply; however, if these prove overly conservative, then a more detailed study as discussed in the previous paragraph may be worthwhile.