Bridge Aerodynamics Mark Ketchum's
Bridge Aerodynamics Page

<-- Tour Mark's Home Tour -->

The science of bridge aerodynamics was born after the Tacoma Narrows bridge collapsed in a moderate wind storm in 1940. The Tacoma experience taught engineers that wind causes not only static loads on the bridge, but also significant dynamic actions.

A cable supported bridge is subject to wind-induced drag (the static component), flutter (the instability that occurred at Tacoma Narrows), and buffeting (where gusts "shake" the bridge). Adequate aerodynamic performance is required with respect to each of these effects.

  • For modest span bridges, drag generally controls the strength required to resist wind.
  • Flutter becomes critical when the wind acting on the structure reaches a critical velocity that triggers a self-excited unstable condition. The task in design is to assure that the critical wind velocity is high enough so that it has a very low probability of occurrence. This can be achieved by providing a stiff structure and/or an aerodynamically streamlined superstructure shape.
  • Buffeting influences fatigue of the bridge materials as well as users’ comfort. The magnitude of buffeting response under higher probability wind conditions must be controlled.

Addressing these issues in an engineering context requires the use of wind tunnel models. Current practice is converging on use of such models for the aerodynamic properties of the bridge shape only. The mechanical properties of the bridge, and the final wind evaluation, are performed using computer models that incorporate the wind tunnel results.

An interesting lay-technical discussion of the failure of the Tacoma Narrows Bridge and many common misconceptions about its physics can be found in the paper

Resonance, Tacoma Narrows bridge failure, and undergraduate physics textbooks
by Yusuf Billah and Bob Scanlan, (Am. J. Phys. 59 (2), February 1991)

Download it as a 345KB pdf or as 527KB of linked GIFs.

They tell us that many math and physics textbooks are wrong: the failure of the original Tacoma Narrows Bridge was not due to resonance.

From the preface to the paper:

" . . . in many undergraduate physics texts the (1940 Tacoma Narrows bridge) disaster is presented as an example of elementary forced resonance . . . Engineers, on the other hand, have studied the phenomenon . . . and their current understanding differs fundamentally from the viewpoint expressed in most physics texts. In the present article the engineers' viewpoint is presented . . . It is then demonstrated that the ultimate failure of the bridge was in fact related to an aerodynamically induced condition of self-excitation or "negative damping" . . . This paper emphasizes the fact that. physically as well as mathematically, forced resonance and self- excitation are fundamentally different phenomena.

I have scanned this paper into image files so it is available to read and print. I have created an Adobe pdf of all 7 pages (345KB) (recommended if you have Acrobat reader which you can download for free from Adobe), and also the same scans (of about fax quality) as GIFs linked into html pages (about 60-90K per page, viewable from a browser and printable from an image viewing / editing program).

For photos of wind tunnel test models of the proposed Third Carquinez Strait Bridge, look here. The bridge is a 728 m span suspension bridge, with a closed cell orthotropic steel deck, air-spun cables, and concrete towers. It will be built across the Sacramento River on I-80 between San Francisco and Sacramento, California.

<-- Tour Mark's Home Tour -->

This page has taken   no   hits.