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


Additional Notes:

  1. It should be noted that the structures being considered are rigid moment frames, where by rafters and columns form a single structural unit. It is not conservative to adopt a high internal pressure coefficient of +0.7 for such buildings as it results in near zero load on the windward side of the frame. For more quality robust assessment a variety of internal pressure coefficients should be considered including 0.0 and +0.7, however for economy also need to give consideration to the situations which give rise to such internal pressures.
  2. Stick frame construction, as typically used for timber frame and cold-formed steel house framing, is pinned construction and is prevented from swaying by cross-bracing rather than rigid moment connections. In this type of construction rafters and wall studs can be designed in relative isolation from one another, as simply supported beam-columns. Since the structural member is considered in isolation it would not ever be designed for zero load. In such case the conservative approach would be to combine highest likely positive pressure on inside with highest external suction: so that have +0.7 inside and -0.65 on outside of wall, or -0.9 on outside of roof. Then for opening in roof, windward wall would have 0.7 outside and -0.9 inside, with side wall opening would have -0.65 inside and 0.7 outside to windward wall. The wind can change direction so all walls can be windward at some point.
  3. All structures have the potential to be overloaded by environmental loads, designing to the building codes does not make the structures resistant to the loads will actually experience. Most buildings in the built environment are not to the current code, and it is not practical to upgrade them all to resist the higher specified loads: loads which may still be exceeded. A more quality robust design approach would be to pay more attention to the mode of failure at over load and the human response to such over load, rather than attempting to resist ever larger environmental loads.

Revisions:

  1. [03/09/2015] ; Original Post
  2. [21/10/2015] : Added Notes section