Net Pressure on Buildings

[Xaviera]

Hi everyone,

I have a question in regards to estimating the effects of openings on a building to the pressures acting on buildings. To calculate the effects of locating openings on a building do we get a pressure resulting from the net pressure coefficient of the Cpe and the Cpi? To calculate the pressure resulting from the net pressure, do I just try to get the aerodynamic shape factor using the Cpn? Thank you in advance for your help. I will appreciate it very much.

 

[JAE]

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[Settingsun]

The question is a bit vague. If you're still interested, maybe providing some details of the building and what load you're trying to calculate would help us understand.

Since the question is about openings, I'm assuming it's an enclosed building (and that you're talking about the Australian Standard). The Cp,n coefficient doesn't apply to enclosed buildings.

The design pressure on any external surface is (external pressure minus internal pressure). I've said it that way instead of (Cp,e minus Cp,i) since there are other factors that apply to external pressure but not to internal pressure: Ka, Kl & Kp.

Whether an opening is big enough to cause the 'full' internal pressure tabulated in the standard depends on its size in relation to the enclosed space. The 'no wind' pressure is about 100kPa on both sides of a surface (ie standard atmospheric pressure; resultant is zero since the same pressure acts on both sides). To raise the pressure on the internal surface by 1kPa, an extra volume of air has to blow into the room to increase the total mass of air inside the room by 1%. Examples below:

- Room volume is 150 m3 [alternative example: 1000 m3]
- Open window is 0.5 m2 [alternative: 0.05 m2 - one window just left open a crack]
- Wind speed is 60 m/s [alternative: 40 m/s]
- Design wind gust is 3 seconds duration.

- Maximum volume of air through window = 60m/s * 3 seconds * 0.5m2 = 90m3. Easily enough in comparison to 150m3 room volume to achieve the full internal pressure.

- Alternative example: 40m/s * 3 seconds * 0.05m2 = 6m3 compared with 1000m3 volume. Maximum internal pressure increase = 0.6kPa, and probably a lot less since the speed at which the wind blows into the room will slow as the internal room pressure increases.

Edit: This calculation shows the general principle but also assumes that the design wind gust comes out of nowhere. In reality, there will be a sustained wind at a lower speed, then the design gust is a short-term increase to maximum speed. It doesn't take much of an opening to achieve full internal pressure.