Thoughts on the Structural Efficiency of Cable-Stayed and Catenary Suspension Bridges

During a recent sail from the south coast of England to the Mediterranean, using the inland waterways of France, I had the satisfaction of sailing beneath two of France's most impressive suspension bridges. First, one passes under what I believe is still the world's longest main span cable-stayed bridge, Pont de Normandie, completed in 1994, and having a central span of 856m. Shortly afterwards one comes under the somewhat older conventional catenary suspension bridge at Tancarville, having a central span of 608m. Even allowing for the age difference of the two bridges, what immediately strikes one is the relative slenderness of all the superstructure of the Pont de Normandie compared with the rather heavy main cable and stiffening girder of the Tancarville Bridge. At a vantage point midway between the two bridges the network of stay cables of the Pont de Normandie were barely visible to the naked eye, whereas the main cable of the Tancarville Bridge stood out clearly; the road deck girder of the Pont de Normandie was also much less bulky than that of the Tancarville Bridge. This seemed in confict with my own intuitively based prejudices as to the relative efficiency of these two systems. Is it merely that the designers over the intervening 35 years since the completion of the Tancarville Bridge have become bolder, used higher strength materials, or perhaps pushed the bounds of design to their absolute limits?

Professor J.G.A. Croll
Like many, I had not had an occasion to