ASCE 7 Wind load

[LoadSlayer]

Hi,
Why does the internal wind pressure coefficient have two sign values?

 

[structSU10]

there are instances where a slight vacuum is produced in the building creating the negative internal pressure, and instances of the opposite. These values will generally cancel out but do affect the magnitude of windward, leeward, and sidewall pressures.

 

[LoadSlayer]

Can you explain how and why those instances occur? if we assume the wind hits in a constant direction.

 

[Heaviside1925]

OP,
If leakage areas are symmetrical or close, as structSU10 noted, they should cancel each other out, but in an instance that the windward side don't have, let's say, many doors and windows, i.e. leakage points, whereas the leeward side does, the building could experience negative pressure. Notice the next line down with even a higher coefficient for a partially enclosed building, consider the previous same scenario but with a large opening, meaning, a partially enclosed structure could experience even higher negative and positive pressures. With taller structures/slenderness, this can have an even more detrimental effect when the wakes on either side induce vibration into the structure.

 

[KootK]

It's a fine question. This can be confusing at times because there's really no such thing as "suction" or "negative pressure". Both of those things are really just a reduction in assumed positive pressure at the surface being considered. Air pressure is just molecules of air bumping into the affected surfaces and transferring momentum to them by way of collision after all.

An interior pressure co-efficient of 0.00 would represent the amount of positive pressure expected within the space when no air is leaking out of, or being pumped into, the enclosed volume. It generally would represent a steady state positive air pressure rather than the utter absence of absolute pressure.

When air leaks out of the building, you get a reduction in positive pressure within the enclosed volume (-ve coefficient). This is akin to attempting to empty a balloon without reducing it's volume. When air makes its way into the building, you get an increase in positive pressure within the enclosed volume. This is akin to attempting to inflate a balloon without increasing it's volume.

This is one of the reasons why the positive and negative pressures are often symmetrical about the zero value. There is rarely much certainty in these values and they basically just represent a reasonable oscillation about the no leakage, "zero" value.

 

[LoadSlayer]

Thanks everyone.