President Eisenhower was personally and particularly interested because as a young US Army Lieutenant in 1919 he had been part of a logistics experiment in which the US military wanted to see how many soldiers how quickly could they ship from the East to the West coast in the event of war. The answer was a very disturbing not very many, not very fast.
The 50,000 bridges, 25% of which were built at the height of NASA/Interstate mania in the late 1960s, were mostly made of steel because it was desirable to support the US Steel industry where a lot of blue-collar voters were employed. This was back in the day when Americans built things and created hardware before this dirty dangerous and physically demanding work was peeled off to the Third World and the American economy, like that of Europe, started to subsist on shifting electrons rather than trainloads of coal. It was a huge pork-barrel and lots of engineering and construction firms got their forks out. Nothing lasts forever, except Roman aqueducts, and a lot of that hardware is now reaching the end of its useful life. In 2006 Presdent Bush signed a new law to authorise a spend of $300 billion for upgrades. in some cases this was boondoggle. There have, nevertheless, in recent years, been some catastrophic collapses and they make for interesting reading for scientists and engineers and anyone who does the best they can with the material world.
One visually obvious problem is spalling in reinforced concrete, not caused by heat but by the corrosion of the interior steel rods which expand as they oxidise and eventually pop off the exterior concrete which exposed more steel to the air and water. It is not helped by firing out salt and other de-icing agents every winter, the electrolytes of which greatly speed up the process. But Engineers build with redundancy and you can lose a certain amount of the volume of the supporting structures and still be within engineering specification. Another problem is that nobody in 1956 could imagine the volume of traffic at certain key bottle necks in the system. It's known by anyone who has watched Thelma and Louise or Paris, Texas (Whoop whoop Wenders alert) that you can go for miles and miles without meeting another car on much of the network but at places like Bridge 9340 in across the Mississippi in Minneapolis as many as 140,000 vehicles a day would be travelling along I-35W and most of those in the crush of rush-hour.
On the evening of 1st August 2007, Bridge 9340 was jammers as usual, actually worse than usual because they had closed 4/8 lanes of the deck to repair the surface. Like many bridges in the Northern states it took a pounding in the winter with the salt, grit and de-icing solutions. So resurfacing was a regular requirement but it required that 250 tons of additional weight (sand, cement, water, machinery) was also on the bridge; not to mention an extra 50mm of concrete above the original specs that had somehow accumulated on the road bed from previous repairs. ANNyway, at 1805hrs the centre span over the Mississippi collapsed and dumped more than 100 vehicles including a loaded school bus, their occupants and a score of construction workers 30 meters down into the river. The emergency services from city, county, state and surrounding municipalities responded with commendable speed and efficiency, not least because such a collapse was one of the exercises that they had all carried out together. So only 13 people died and 100 were seriously enough injured to go to hospital.