Tacoma Narrows: An example of ontological risk


Inspecting Tacoma Narrows (Image source: Public domain)

We don’t know what we don’t know

The Tacoma Narrows bridge stands, or rather falls, as a classic example of what happens when we run up against the limits of our knowledge. The failure of the bridge due to an as then unknown torsional aeroelastic flutter mode, which the bridge with it’s high span to width ratio was particularly vulnerable to, is almost a textbook example of ontological risk.

Of course recognising  that there’s such a thing as ontological risk is not necessarily all that helpful as knowing about things we don’t have any idea about is a peculiarly difficult undertaking. However engineers do seem to have a remarkable facility for advancing designs in the absence of knowledge, so having some way of measuring ontological risk would be useful in such circumstances. Now if we can’t directly measure ontological risk, then measuring the inverse, that is the degree to which a design is based on ‘faith’, might be a useful metric of how much risk we are carrying. In the case of Tacoma such a measure is the span to width ratio of Tacoma versus those of a number of contemporary bridges.

Tacoma Narrows vs contemporary bridges span to width ratio

As this simple bar chart indicates the Tacoma bridge stands well outside the experience (e.g evidence) based designs of preceding and contemporary bridges. Simply, this distance represents ontological risk. The further out we go the less our design is based upon empirical evidence and the more we rely on theory usually with a large dollop of subjective judgement (i.e. belief).

The real lesson from Tacoma is not that engineers shouldn’t take risks in their designs, nor that failure in such circumstances is inevitable, but that when you do advance a design that stands outside the experience of those that have gone before you should always look for some measure that provides an indication of your risk exposure.


F. B. Farquharson, et al., Aerodynamic stability of suspension bridges with special reference to the Tacoma Narrows Bridge. University of Washington Engineering Experimental Station, Seattle. Bulletin 116. Parts I to V. A series of reports issued since June 1949 to June 1954 (cited by Wikipedia).