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Our Critical Infrastructure:
A Bridge Builder’s View of the Minnesota Highway Collapse

By Craig Mellinger
Special to Serviam

Like trolls beneath a crossing truss, we work almost unseen every day. The public passes above without a thought of the sweaty, gritty figures who build, service, maintain, and repair the nation’s bridges.

Bridges are a crucial part of a nation’s critical infrastructure — the network of highways, waterways, airways, pipelines, power lines, and communication systems that keep society moving. When properly designed and maintained, they make everything run smoothly. When placed under too much strain, whether from handling more capacity than the architects intended or from not being adequately repaired or replaced, they can bring everything to a stop. With often deadly consequences. Which is why protecting our critical infrastructure is a core component of homeland security.

The August collapse of the I-35W Mississippi River bridge in Minneapolis, Minn., reminded us of the fragile condition of many of the structures we take for granted. This time the collapse was attributed to excessively heavy use and lack of proper maintenance, but terrorist attempts on our bridge and tunnel systems already have begun. Bridge contractors are now on the front lines.

From below, one can see thousands of high-strength steel bolts that hold together the trusses that support a steel bridge. These bolts, nuts, and washers are the big brothers of those found at the hardware store. The design process of using mechanical connectors takes into account the actual strength of the bolt in addition to the friction created between steel connecting plates. Bolts tightened to a specific torque cause the friction. Tightening often involves going back over bolts again and again until all slack is taken in. The contractors who fasten and adjust these steel connections are a special breed of workers—and not everyone would invite them home for dinner—but as the strength of our bridge systems proves, they do their job well.

As an accountability and quality control measure, inspectors watch over the builders and ensure that they do their job well into overtime, working as many as 11 and 12 hours a day. Americans overbuild their bridges. Engineers go well beyond what is needed to ensure structural integrity and longevity. All citizens bet their lives on the bridge builders’ skill and attention to detail, without even thinking about it.

Deferred maintenance can be deadly when it comes to bridges.

The I-35W bridge in Minneapolis was an eight-lane, 1,907-foot steel truss arch bridge that carried traffic across the Mississippi River. Completed in 1967 and maintained by the Minnesota Department of Transportation, the bridge was the state’s fifth busiest, carrying as many as 140,000 vehicles daily.

Suddenly, during evening rush hour of that pleasant August day, the main spans collapsed, falling into the river and onto its banks. Thirteen people died and about a hundred were injured. Fellow commuters, truckers, bus drivers, and search and rescue workers heroically brought others to safety. Search teams worked around the clock to find victims trapped in the rubble. Divers discovered others still strapped to their seats in the muddy waters.

Bridge construction has changed in the 40 years since the I-35W span was built. The ironworkers themselves were a different breed of engineers in the days when the safety equipment behind the work today wasn’t yet conceived. They were a fearless bunch, as proud and diligent about their work as the guys who are on the job today. Looking back at the architecture, some bridge contractors wonder why someone would design a structure that was so dependent on one section. In the Minneapolis case, when one section failed, the whole span crashed down. Maybe it’s because the designers never expected any portion of the structure to fail, or presumed that, as the years went by, the governments that built the bridges would maintain the crossings as enthusiastically as they had constructed them.

Unseen Contractors

Engineers have since taken unforeseen factors into account to design structural redundancies to avoid such catastrophic failure. The whole process of connecting steel members together has changed since the 1960s. Most of the structural steel assembled in I-35W was a riveted connection. Bridge rivets are large steel pins with a rounded head on one end. The process involves heating the pins until they glow red hot, putting them in the connection holes, and beating the other end into a mushroom shape with a rivet buster. The contraction of the cooling rivet draws the steel pieces together. This process prevents the bolt from ever backing out.

These designs put all the stresses of the bridge on the shear capacity of each rivet. The downside, as decades of use revealed, is that if a rivet shrank too much in cooling, the mushroomed end would crack, or even break off. Minnesota’s harsh winter climate caused the pins to expand and contract much more than would occur in more temperate areas. Cracked rivets are difficult to detect before they totally fail. Newer steel bridges are built with high-strength threaded bolts, welded connections, or both.

Building an entire bridge of steel is becoming a thing of the past. Maintenance is difficult and costly. All bridges in the U.S. are inspected regularly by the state or other government authority responsible for the safety of the structure. Inspectors spend hour after hour, usually at great heights, scouring each structure to ensure that everything is as it should be. On bridges over water, divers inspect underwater sections of the bridge structure. The obstacles these inspectors face include years of surface rust and multiple layers of paint, road crud, and marine growth, all of which can conceal weaknesses or flaws. Special x-ray equipment is sometimes required to overcome the visual obstacles, but height, limited access, and weather add other challenges.

The builders and inspectors, men and women often working as private contractors, perform risky and painstaking tasks so that commuters won’t have to think twice about the safety of the bridges they cross.

Local, state, and federal authorities that have assumed the responsibility for road and street systems have not dedicated the necessary resources to build the new bridges required to replace, strengthen, or expand those already standing. The Minneapolis bridge collapse focused attention on these often-neglected nodes of the nation’s critical infrastructure and reminded us to act before another collapse takes more lives.

Craig Mellinger has been in the bridge-building contracting business for more than 10 years.

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From the November/December 2007 issue of Serviam.