huangyhg

local pressure factor
i was going to post this in the as/nzs code issues, but then it struck me that this isn't a code issue but more of a wind effect issue.
the code (as/nzs 1170.2) specifies a local pressure region near the edge of building in relation to either the height or width. for a 3deg roof this increase the local pressure coeff 0.9 to 1.8 to allow for variance in wind pressure and is only relevant to roof sheeting and purlins by the wind code.
i am reviewing some engineering calcs from an engineer in adelaide. he is adamant that that the (mean) wind pressure coeff is more than conservative enough to ignore the local pressure effects (peak). because of the certifying local requirements he wants me to certify his design and structure. i am in a cyclone (hurricane) area region c.
my question is, what is normally assumed at the edge of a building, for other codes in the world, with regard to mean pressure effect and peak pressure effects? are there any local pressure type clauses?
is it normal practice to ignore local pressure effects? i never have.

when in doubt, just take the next small step.

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when you receive wind-tunnel testing reports you will find that the code provisions for wind coefficients on buildings is highly conservative. saying that, under no circumstances would i ever say that the local pressures can be ignored just because the nature of the code is conservative.
i am assuming that the design you are reviewing is relating to purlins that are near an edge of the building. in our office it is practice the increase the size of your purlins within the zone which has local pressure multipliers.
for example, near an edge of the building we would call up purlins with a greater steel thickness and more bridging than would be called up in regions of low wind coefficients and no local pressures.
again re-iterating, i would not accept their answer for ignoring local pressures and re-direct them to the kitipornchai book published by the asi.
asixth,
the parts of their building i have most concern with is the purlins and sheeting, i generally as per your advice, provide heavier purlin to the local regions and closer spaced.
i think i will take you advice and send them the latest design guide from the australian steel institute. as the asi is trying to improve the practice of shed design and have developed minimum guide lines for the design of portal frame buildings.

when in doubt, just take the next small step.

asixth...yes, some localized pressures are conservative, some are not based on wind tunnel results. in urban areas, where we would generally increase pressures as we go higher on a building, we found in one recently that pressures actually increased in the lower floors because of tunnelling and shedding around lower height buildings.
i agree that the localized factors should not be ignored, if for no other reason than future liability!
rowingengineer,
i agree with you and asixth. the adelaide guy may doing his first job in region c. tell him that is not the way it is done where you are, and to stick to the code. sounds like he needs to take a trip north after the next big cyclone to see what happens. the roofing and/or purlin failures always occur near the edges or ridges.

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