A bridge too broken

Shock and awe were defined afresh last Wednesday, 1 August, at 18:02 US CDT when the I-35W bridge across the Mississippi collapsed under rush hour traffic without prior warning. Numerous eyewitness photo's of that catastrophic event can be found on Flickr. More chilling than the visual images are the consequent reports of well-established historic knowledge on the structural deficiency of the bridge. Apparently, over 70 000 bridges in the USA carry the classification of structural deficiency - a truly startling statistic.

The classification of deficiency can be awarded for any structural score below 80/100. The I-35W bridge had a score of 50 (or 4/9 according to the NY Times) before the collapse. This score had been given in 2005, yet the bridge remained in service. That was until last Wednesday, when it finally gave way. So to state the collapse came without prior warning is not quite holding up to the truth. I would call 50/100 on structural integrity for a heavily used bridge ample warning of impending doom. The disaster prompts the question: How is such a situation of infrastructural decay conceivable in the richest country on Earth?

The NY Times have reported that there are 756 bridges of this design in use in the USA. Most bridges were built during the construction boom of the 1950's and 1960's. Since the 1970's these bridges were merely maintained. Yet, there hangs a question over the original design and consequent construction. After all, bridges under heavy use elsewhere in the First World do not collapse in similar fashion. Could the pressures of profit and budget have compromised solid engineering wisdom?

Alas, the picture appears even grimmer upon taking a step back from the immediate calamity in Minneapolis. In Massachusetts, the recently commissioned I-90E tunnel suffered a collapsed roof in 2006, which incident caused the death of a passenger of a car hit by a piece of the collapsed roof. The so-called Big Dig that produced this tunnel was terribly over budget and late as well. The road system have been plagued by operational problems ever since. As it were, the government tender process for the Big Dig was a model of graft and first class lobbying in favour of local project managers and contractors.

The particular flaw at the root of the collapsed roof of the I-90E tunnel can only be attributed to amateurish design and cost cutting in which a cheap solution for tunnel ventilation was selected with undergraduate incompetence. Even a Third Year Civil Engineering student should know that one does not hang from tie rods screwed vertically into concrete, a heavy weight subject to vibrations and thermal cycling. Concrete does not take well to the prevalent stresses under such conditions - especially over time.

Yet there is further rot to be spotted. Only last week, there were reports of a manhole grid that had become unsettled on I-93 and seriously injured a driver when it was flung up by a passing lorry. This grid featured on a freeway, not a secondary town road. It defies reason how a freeway can be designed with manholes and grids on the road surface or even the emergency lanes. Instead, water drainage should be provided along the outer edges via gutters, not via manholes and grids on the road surface. Such proven safe design can be found on German autobahns. Again, cost is the likely motivation for the manhole and grid solution over more sophisticated and costly gutters along the road edges.

All in all, it has been estimated that the total cost of upgrading national infrastructure in the USA will amount to over $1 trillion dollar - a truly staggering amount of money [1], especially when facing a cumulative budget deficit of $741 billion for 2005 and 2006. Yet, Congress has already approved $500 billion for the futile Iraq war and it may cost another $500 billion to finish that mission unaccomplished. Could these imbalances and tragic consequences be symptomatic of an endemic fault line in the US culture of pragmatic paths to profit?

Perhaps what we have here is a bridge too broken to be fixed by sound bites, YouTube productions and quick fixes. One can only sincerely hope that it will not turn out a bridge too far.