It is a monumental tragedy; there is simply no other way to describe it. Motorists were driving across interstate 35 yesterday when, without warning, the bridge began to fall apart under their cars and plunged dozens of cars into the muddy Mississippi river.
When it opened 40 years ago, the bridge was an engineering breakthrough because no mid-span support was placed under the bridge. This allowed for unimpeded boat traffic on the Mississippi river below. Bridges decay, stress, and become damaged over time. The LA Times reports that the last comprehensive inspection of this bridge was in 2001. State inspectors concluded that the bridge “should not have any problems with fatigue cracking in the foreseeable future.” Tragically, the foreseeable future must be something less than six years. The LA Times reported that:
“Inspectors recommended frequent inspection – as often as every six months – of the steel trusses that bore the most stress. But they concluded that the state “does not need to prematurely replace this bridge … avoiding the high costs associated with such a large project.”
Bridges are often exposed to chloride as a result of marine salt or the application of salt during winter road deicing. Chloride attacks the bridge surface and can begin causing corrosion of both the bridge surface as well as the support structures. The reinforcements found closest to the top or bottom surface of the concrete are usually the first to suffer structural decay. Once the decay begins, structural components suffer progressive decay and structural integrity failure.
Testing for structural integrity has typically involved hammer tapping the surface of the deck to discover hollow places in it. In addition visual inspection of the support components are made. The problem with detecting potential failure and correcting it is the enormous costs involved with conducting repairs. Bridge Design & Engineering reports:
“Highway authorities in the USA have become increasingly aware of the scale of the problem during the 1990s, and current estimates suggest a possible US$3 trillion bill for repair to damaged concrete bridge decks.”
“Deck testing has usually been squeezed into repair programmes, meaning that results are provided too late to enable advance planning of repairs and only the shallowest, most obvious defects have been picked up.”
Our sympathies go out to the families of those who tragically died in this incident and to those injured. It is our hope that a thorough and open investigation of this tragedy can be conducted to prevent other such tragedies in the future.