It’s Never Just One Thing

The preliminary reports on the FIU bridge collapse are damning in a very specific way. The exact cause of the collapse is still unknown but it’s become clear that the bridge design was not driven by engineering concerns.

Bridges have a series of constraints that make them very different from the building structures we work on. They typically lack redundancy at the macro level, so that a suspension bridge with two main cables, for example, will not have a safe roadway if one of those cables fails. A building with a modern frame, particularly one designed to meet code robustness criteria, can withstand the loss of a single column. The structure of bridges is mostly or entirely exposed, while that of buildings is largely or entirely hidden. Bridges are designed closer to the bone, particularly in terms of dynamic loading: it’s rare when a building has a serious dynamic problem under ordinary load, but it’s an ordinary design concern with bridges. There are more constraints, but these are the ones that come to mind.

It is not only possible to for bridge designers to meet all of those constraints and create a beautiful structure, it’s the goal. Even ordinary highway overpasses can be pleasing in appearance if they are well designed. Good proportions, clean lines, and unobtrusive connections all help. One significant aspect, sometimes stated and sometimes not, is that the overall form should follow the structural requirements. This is where the LIU bridge design started to deviate from best practices:

• The “cable stays” were actually pipes that were not the primary structural members that they appeared to be.
• The truss, which was the primary support, was reinforced concrete. Trusses have a mixture of members with tension and compression as their main forces; reinforced concrete is not particular adept at handling tension.
• The truss and stays were on a single line down the center of the bridge, which meant there was zero redundancy on the macro level. It also meant that the bridge had less torsional resistance than it would have with two trusses, which might not matter that much for a pedestrian bridge, but would have provided a more robust structure.
• The truss geometry was driven by visual alignment with the stays rather than the forces within itself.

A safe design and proper construction could have followed even with these peculiarities, but they made it less likely. Truss geometry should be driven by the forces, as should the bridge design as a whole. Whether one prefers a truss bridge or a stayed bridge is a matter of taste, but there are beautiful examples of both. You get beautiful (and, of course, safe) bridges in either form by designing them honestly to the structural forces rather than to preconceived ideas of appearance.