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Wind Analysis Evolves
Your discussion of the numerical "virtual wind tunnel" in
the article "Catching the Wind," in which the topic of computational
wind engineering for flows around buildings was discussed,
suggests that this technology was suitable for showing wind
patterns, surface pressures, pedestrian-level winds and snow
drifting (ENR 9/2 p. 31). This sort of technical misinformation
is truly frightening.
To date, there have been three international conferences on CWE. On rare occasions
that CWE results have been compared to full-scale data or wind-tunnel data,
that comparison has not shown the computational values to be ready for routine
design use. This sort of validation is a minimal requirement for any new design
procedure. One of the pioneers in the field, Professor Shuzo Murakami, points
out that the method of modeling turbulence on a computer needs to improve substantially
before the work described will produce results close to reality. Without this
skill, even mean flows around buildings, which are influenced by the mechanical
turbulence created by separation and reattachment, will not be correctly predicted
by CWE.
To be fair, this is a growth area of research and gross flow predictions of
pedestrian-level wind conditions and snow drifting in complex anthropogenic
environments may be viable in the not-too-distant future. However, validation
attempts of even pedestrian-level wind conditions with full-scale or wind-tunnel
data in the recent peer-reviewed literature have indicated that discrepancies
between 50 and 200% are not uncommon. To suggest that CWE is ready to be used
for building surface pressures (presumably for cladding design, which is governed
by peak pressures) is very misleading and could cause an uninformed engineer
with a CFD package to make some serious design errors.
The "virtual wind tunnel" ENR discussed is certainly the way of the future,
but it is not currently ready for use in routine engineering design. To assess
structural or cladding loads via this approach would be imprudent, and in fact,
quite wrong. Even the more nebulous assessment of pedestrian-wind conditions
(requiring mean and gust flows) is not yet ready for CWE.
Solving wind-related issues on a computer would be a lot easier and cleaner
than building physical models and collecting the data in a wind tunnel. But
the technology is not at that point yet. Thus, your quote that "a digital picture
is worth a million words" is an incomplete statement. A digital picture that
is wrong or misleading is worth no words, and could be dangerous.
Unprotected Bents
As a structural engineer, I find it hard to believe that any engineer or highway
department could be so incompetent as to design an overpass that could be knocked
down by a single errant truck as described in your article "Hit Hard, Bridge
Collapses After Truck Rams into Two Columns," (ENR 9/16 p. 7).
Did the designers assume that no one would ever run off the freeway? Didn't
it occur to them that one might and it would be prudent to provide protection
for bridge supports in the form of guardrails, barrier walls, or columns strong
enough to resist normal freeway vehicles?
The remaining columns shown in the photo appear to be prime targets for a similar
future accident. How many other weak, unprotected bents are
there in Texas and elsewhere that are subject to such failure?
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