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RE: Wind Uplift on Awning (venting)

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Donald Bruckman wrote

To me, the most compelling piece of information out of that Katrina report
from UO was the simple and, in hindsight, rather obvious notion that hip
roofed structures did better than gable roofed ones and that eave vents did
far better in limiting damaging water intrusion than gable end vents..

<end quote>

Are you aware of a USAID/NDFD guide for strengthening of housing in the
Carribbean. I believe it precedes Katrina.

The observations are really reinforcing what is already known: but such
things get published in research papers and lost in time. Plus there is
resistance to incorporate changes in codes, or find appropriate code
clauses, because prescriptive solutions are not always applicable or
acceptable. People would tend to object if told they have to have a hipped
roof so it is left as an option.

My previous post: was that the venting can produce different internal
pressures depending from which direction the wind blows relative to the
opening. It can produce high internal suction which can reduce uplift on
roof, or high internal pressure increasing roof uplift. ASCE7-05 simply
adopts some conservative design pressures: which don't help explain what
happens.

By appropriate internal partitioning, and location of external vents, the
internal pressures can be controlled. The airflow needs both entry point and
exit point: preferably entry on windward face of building, and exits on
faces experiencing suction. But the wind can change direction, and thus the
pressure coefficient and flow through will be different, than first trial.
Which is all too complicated and impractical for small house structures, and
so hopefully use conservative estimates of pressures for design.

The other issue is the magnitude or meaning of the design actions. The
design actions used for allowable stress design are relatively meaningless.
At the ultimate limit state design load, materials will have permanently
deformed, and whilst the house may not have collapsed, it is unlikely that
the owners can move back into the house after the design event: the building
will no longer meet all the serviceability and habitability requirements of
the building code. At some lower load, a more common and frequent storm or
earthquake event, the house will remain habitable after the event. For
example at the unfactored allowable stress design wind speed the house may
be expected to remain usable, but at the ultimate limit state wind speed
then will have to rebuild.

At the ultimate limit state event, the house is not the place to be taking
shelter: at lower load events and then house may be suitable. Loading above
the serviceability loading and the house is expected to experience a certain
amount of damage and require repair.

In Australia for example: the recent cyclone Larry, has indicated many
designers are not applying appropriate internal pressure coefficients and
they are not using appropriate topographic factors. Cyclone Larry was not
the design level event, so there should have been few houses destroyed.
Further debris from older houses and backyard sheds caused damage to newer
houses. This reinforces the insurance industry program to upgrade tie-downs
in older houses: upgrade your house and your insurance costs go down. Plus
increased attention to design of other structures around a residence: roller
doors were blown off leading to failure of sheds which weren't designed to
be open. How they managed to get approved in Queensland I don't know, since
codes insist design for open condition in cyclonic regions unless can prove
otherwise. And few door manufacturers have any comprehension of wind
loading.

The following may be of interest:

Report on findings after Cyclone Larry Available here:
http://www.bom.gov.au/weather/qld/cyclone/tc_larry/

JCU Cyclone Testing Station
http://eng.jcu.edu.au/cts/research.htm


Regards
Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
mailto:sch.tectonic(--nospam--at)bigpond.com
Adelaide
South Australia
 



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