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Wind Direction : Was Wind Load Topographic Effects

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Joe,

Not sure I would approach it that way. As I mentioned the pressure
coefficients given in the code (ASCE7-05) are based on the extreme, within
the quadrant.

To AS1170.2 we refer to direction measured by angle theta relative to
building orthogonal axes, and angle beta are the compass cardinal
directions. Theta=0 gives transverse, and theta=90 gives longitudinal. We
are required to check minimum of 4 orthogonal directions, theta =0, 90, 180,
270.

Referring to these angles then the implication is that the pressure
coefficients ASCE7-05 Fig6-6, are based on extremes for theta=0 (transverse)
taken with airflow between -45 and 45 degrees. Whilst those for theta=90,
longitudinal are taken between 45 degrees and 135 degrees. Since pressure
coefficients are not equal for both directions, the shared angle (45) only
provides the extreme for one of the directions: I've not read anything which
says which direction.

Taking this into account. Then I would draw the building plan, and the
orthogonal axes, and the 8 compass directions correct orientation to the
building. Then qz would be determined as previously mentioned: the maximum
in the quadrant under consideration. For any wind direction theta, I would
then draw a line normal to the direction and project the building onto that
viewing plane. That which is visible is the windward face, that which is
hidden behind is the leeward face. Apply windward, leeward, and sidewall
pressure coefficients accordingly.

So for example one short and one long wall may experience Cp=+0.8, and the
other short and long wall would experience Cp=-0.5 (max. mag.), that is no
wall experiences the sidewall coefficients (Fig6-6). The roof planes would
experience windward and leeward pressure coefficients. If both roof planes
are largely visible from the viewing plane then adopt the longitudinal
pressure coefficients. That is adopt pressure coefficients relative to the
standard quadrants (transverse (0),longitudinal(90)) the wind direction
angle theta lies within. I would guess that 45 degrees belongs to the
longitudinal quadrant.

The resultant distribution is likely to produce more of a twisting affect
compared to the typical theta=0,90,180,270. Which is generally the reason
why consider angles other than the orthogonal directions: if need be.
Depending on pressure coefficients given they can be adjusted using trig:
many of the drag coefficients for steel sections are typical of coefficients
adjusted for direction using sine and cosine functions. Most pressure
coefficients don't have this option.

The projected dimension (or diagonal) of the building would be a simple
approach, for a simple windward pressure on a projected area. But it doesn't
take into account the change in distribution of pressure over the surface of
the building. Also wind blowing against the long or short sides is not
relevant once calculated the dimension projected on the viewing plane normal
to the direction of the wind. Hence the tough question. For a non-orthogonal
direction cannot have bracing in planes parallel to that direction because
don't have any such walls: so how does the wind on that projected area
distribute to the bracing in the walls?

I don't believe need to answer that question because the air is a fluid and
acts normal to the surfaces of the building. Our first task is to estimate
the distribution of pressure over those surfaces. Which involves calculating
qz, determining Cp[external] and Cp[internal]. Once have the distribution
then can determine the effect.

Hope that doesn't over complicate things.




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



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