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.

Cermak Peterka Peterson Inc.
Fort Collins, Colo.


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?