# TechNote#006 Internal Pressure Coefficients Industrial/Commercial Buildings

{Strictly speaking this is the first technical note I wrote back in 1996, but didn’t start to present such writings as notes until much later.}

AS1170.2 Loading code : “Wind loads” is primarily set up for structures that have a single state-of-nature : permanently open to the atmosphere, or permanently sealed from the atmosphere. Many buildings however have more than one state-of-nature, the number of states being dependent, among other things, upon how many doors & windows there are, and in what combinations they can be opened and closed.

An industrial building with a single door has at least two states-of-nature : door open or door closed. Estimating that the building is only occupied and in use 10 hours per day for 5 days per week, and the door is open during this time, then at most the building is only going to be open for 30% of the building’s life; and if we assume that the door will only be open for 80% of the time the building is in use, then the door open situation will only occur for about 25% of the building’s life.

Since the state of the building is not random, but time dependent, that is the door is only open during working hours; and wind speed distribution is also time dependent, such that longer time frames witness higher wind speeds. It follows that the building is not going to experience the full distribution of wind speeds over a given design life, for each and every state of its existence. Following Woolcock and Kitipornchai’s argument from the “Design of Portal Frame Buildings” [AISC,1987], this 25% experience of the full distribution is equivalent to taking 25% of the return period. Which in turn will reduce the design wind speeds & pressures for both strength and serviceability limit states, which is equivalent to reducing all pressure coefficients.

An alternative, approach is to say that there is only a 25% probability that the internal pressure coefficient (Cpi) will be 0.7(open), and a 75% probability that it will be 0.2(closed), therefore the expected value is EV(Cpi)=(0.25×0.7)+(0.75×0.2)=0.33, but to be conservative assume 0.4