Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Wind Uplift on Awning (venting)

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Donald Bruckman wrote:

Seemingly, one section of the code (attic ventilation) destroys what another
(ASCE-7) seeks to preserve.
<end quote>

Not exactly. Proper venting is often used to reduce internal pressures.
Though ASCE7-05 doesn't seem to provide much option there. 

To the Australian wind loading code (AS1170.2), the general principle is
that the internal pressure coefficient equals the external pressure
coefficient that would be generated on the surface if it was there. Which
often seems a crazy idea for large openings with stepped pressure
distribution: but still useful elsewhere.

So a windward gable end would have a pressure coefficient of +0.7, whilst a
leeward wall would have something around -0.2 to -0.5 depending on
dimensions. That is air is flowing in on one side and flowing out on the
other. By varying the size of openings on the various building surfaces a
lower mean internal pressure can be developed. Openings in the side walls
(-0.65) or roof planes (-0.9) could result in high internal suction, and
maybe make the downward pressure on the roof plane greater than uplift.

This also as other implications. Take a building with say 10 degree pitch
gable roof with roof pressure coefficients (-0.7,-0.3) and open on one side

To AS1170.2 we use theta for direction and alpha for roof pitch. Theta=0 is
the transverse loading, and theta=90 is the longitudinal loading.

If the wind is from the left (theta=0) then have:

  -0.7  / \ -0.3
       /   \
      |Cpi = -0.5

If wind from the right (theta=180) then have:

  -0.3  / \ -0.7
       /   \
      |Cpi = +0.7

And for theta=90, at the windward end:

  -0.9  / \ -0.9
       /   \
      |Cpi = -0.65

The location of an opening in building relative to the wind direction can
have a significant effect on the loads the building experiences. (NB: I took
(-0.7,-0.3) from ASCE7-05, the rest are from memory using AS1170.2)

Also an open building, or free roof, such as a carport, when blocked under,
by say a parked caravan also experiences an increase in uplift pressure.

If an attic space is vented, then the load on the roof and ceiling are
different. But the venting benefit will be diminished if the airflow is
blocked by storage of junk in the attic space. Then there are problems of
ceiling vents, venting to the attic space or outside.

With a complex maze of rooms, walls, doors, windows and vents the actual
internal pressures are difficult to assess. To a certain extent, the
traditional concept of a weather lock entrance is preferable to the modern
idea of walking straight into a large open room. By weather lock I mean a
two door entrance: open an external door and enter small room, shut the
weather out, then open internal door into main building.

But now we also have an increasing preference for indoor/outdoor living,
where the dividing line between interior and exterior of building becomes
blurred. Add to that passive air conditioning design. 

A house becomes more open building than enclosed building. However occupants
tend to get at least 24 hours notice with the approach of a hurricane, and
hurricanes are seasonal: so building occupants can take steps to seal and
shield their houses. Whilst thunderstorms are not so seasonal, and warning
time can be as low as 15 minutes.

Another issue is that a house is not intended to be a storm shelter. There
are multiple states of nature that a building will experience and for each
of those states we have different expectations of performance. A building
has a primary functional core, everything else is an optional luxury. After
a hurricane for example we could sleep in a tent, but still use the kitchens
and bathrooms: thus minimising the spread of disease.

A principle objective of the wind loading code is to keep the building
components anchored to the owner's property, and reduce the hazards of
airborne debris. The design load could be exceeded and the building may
collapse. So we want the building to collapse in a manner which is
considered "safe", for example occupants have warning and time to evacuate.
Secondly we want the collapsed structure to remain anchored to the property.
Economics is another issue concerning how often we are willing to accept the
loss of amenity. During the hurricane shelter should be sought elsewhere in
a purpose designed shelter which is serviceable at the storm loading.

So determining an appropriate wind load is more complicated than simply
applying the code. Codes are simply guidelines and often complying with the
codes can act against the welfare of the community, and be downright
dangerous. It is the role of designer to try and balance all the conflicting

Ventilation of the attic space maybe essential to the building to avoid
condensation and heat build up in an attic space. During a severe storm
however the rapid inflow of air maybe more harmful then beneficial, and the
water it carries is definitely unwanted, so as with windows some of the
vents need to be closable.

With an appropriate control of airflows, it maybe possible to design a
pneumatic closing system which automatically closes the vents when wind
speeds get too high, and likewise opens the vents when airflow is suitable
for ventilation.

Or more radically fully open the building, removing cladding from the
frames, and thus reducing the loads on the structure. Or placing the
building on hydraulic jacks, and lowering a building below ground and
covering during hurricanes and tornadoes, and elevating above flood waters
at others times. Cost probably beyond the needs of the typical person, but
mobility provides other options: be at the other side of the country during
the hurricane season. Textiles and cables also offer other options for
changing the shape of a building from season to season: during the hazardous
season simply fold up and store a large portion of the building. Though
problem is that want the extended indoor/outdoor living areas in summer: the
cyclone season.

Buildings as adaptive machines: now that would be a whole different kind of

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at:
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*   Questions to seaint-ad(--nospam--at) Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: 
******* ****** ****** ****** ******* ****** ****** ********