Internal wind pressure
Internal wind pressure
(OP)
What's the science behind the internal wind pressure as given by ASCE 7? Does the enclosed building classification assume you have some amout of air leakage and that is what is pressurizing the inside of the building or I supposed sucking some air out if the leak is not on the windward side?
In other words, if you had a hermetically sealed box, would you need to consider internal pressure?
In other words, if you had a hermetically sealed box, would you need to consider internal pressure?






RE: Internal wind pressure
RE: Internal wind pressure
Internal pressure is the inward or outward force due to differences in the atmospheric pressure between inside and out. Is not directly related to wind movement, which is what the other wind pressures are based upon.
RE: Internal wind pressure
I would call this the force due to the net pressure.
RE: Internal wind pressure
RE: Internal wind pressure
No, but you cannot achieve a sealed box. Even with all doors and windows closed it is leaky. Do you imagine if you turned your building upside down and tried to float it that it would float? Of course not, it would leak like a sieve.
RE: Internal wind pressure
RE: Internal wind pressure
RE: Internal wind pressure
RE: Internal wind pressure
RE: Internal wind pressure
RE: Internal wind pressure
So when you get a net difference between a set internal pressure and a different exterior pressure, you get a net pressure applied to the walls of the box.
This is not necessarily the full internal pressure effect used in ASCE 7. It's factors do adjust depending on the number of openings .... more openings resulting in more internal pressure - my assertion above, I think, is somewhat wrong in that the internal pressure of a "box/building" is more dependent on moving wind, flowing into a partially enclosed space vs. net differences in atmospheric pressure.
RE: Internal wind pressure
RE: Internal wind pressure
I agree with JAE's line of reasoning.
RE: Internal wind pressure
RE: Internal wind pressure
After listening to everyone's comments and doing some of my own research I'm convinced that I do not need to consider internal pressure as defined by Figure 6-5 of ASCE 7-05.
Yes, I realize something besides wind may happen to cause a change in internal pressure, but that's a separate load case and we consider this.
RE: Internal wind pressure
FYI Unless I'm wrong, internal pressures only affect components and cladding loading. The mainforce resisting system should see the same loads regardless if it is enclosed or partially enclosed.
RE: Internal wind pressure
In non cyclonic (hurricane) regions the code specifies different wind speed from different directions as the prevailing winds are usually from one or two specific directions.
You can then figure out the internal pressure in either a simplified or a detailed procedure. For the detailed procedure you need to fugure out the size of dominant openings (e.g. roller shutter doors or similar) on each wall and then the effective area of 'leakage' on each wall and the roof (taken as up to .05% of surface area for industrial type buildings and as low as .001% for sealed buildings) .
The effective internal pressure depends on:
1: which wall the 'dominant' opening is on compared to wind direction. i.e. what is the external pressure at this opening.
2. the ratio of the area of the openings on this side compared to the total amount of leakage on all other surfaces.
For example if you had a large opening on the windward side you would get an internal pressure equal to the windward external pressure, if it was smaller you would get some percentage of this. If it was on the sides or leeward you would get negative pressures.
Some companies will actually design roof vents to provide a specified amount of internal pressure to help prevent wind uplift.
As with all of the major wind codes, the Australian code is based on statistical analysis of wind tunnel tests. Actual wind loads fluctuate widely and are hard to predict exactly.
RE: Internal wind pressure
This is true up to a point; the net horizontal wind load is the same. However loads on individual members such as columns and rafters of a portal frame are affected, and uplift also varies with internal pressure.
RE: Internal wind pressure