The three long-span bridges, the Gerstle, the Johnson, and the Robertson were built during WWII using 1940’s load capacities and design standards. Their load rating, HS-15, was intended for a 60-70,000 pound, five-axle semi-truck. They were well built and obviously have heavier trucks routinely cross them. Today the Department of Transportation & Public Facilities (DOTPF) uses a calculation formula, called a “Bridge Law”, to permit heavier trucks by adding length and extra axles. But the point is, the weights allowed on these bridges is by calculation, not measurement. The state assumes these bridges will handle more weight solely because of the formula application. They do not know when the bridges’ capacity is exceeded and they will fail. DOTPF is gambling. Every load weight over the operating rating takes a little piece of life out of a bridge and when you permit extra heavy loads crossing repeatedly, then you accelerate that decline. These bridges have survived because of moderate weight and low crossing counts. Remember, these bridges are 80 years old and even with good maintenance, the “ole girls ain’t what they used to be”. The Kinross ore haul will change that stable dynamic. To protect the community and commerce lifeline these bridges represent, the Kinross haul should not be allowed unless the State can guarantee the bridges can handle the load and truck counts. They can’t and won’t so that leaves the matter in safety and what’s best for the citizens of Alaska.
These bridges are known as “truss bridges” by the interaction of the steel beams they are built with. Truss bridges have one serious flaw, it’s called “Fracture Critical”. The main beams hold each other in tension and rely on every beam to do its job. If one beam breaks or fractures, then it’s likely the whole structure will fail. There are no truss bridges built in America any longer and existing ones are carefully monitored and usually restricted by weight. By rules these bridges are inspected for fracture every two years but that is a very strict process basically requiring the inspector to see and touch every beam piece. On these bridges that requires a man-lift and numerous hours. Not only do they have to see the beam, they have to review what’s under the paint and rust to be sure there are no cracks developing. If a small crack is missed and the bridge is hit by a vehicle in the right place, is subject to dynamic forces of a heavy truck, or even subjected to earthquakes and wind, then that crack can turn into a break and lead to the bridge failure. Every truck will shake those beams, and the heavier they are the more they will, potentially causing the Fracture Critical situation. Again, the bridges are doing OK with the light, infrequent loads but if you add the Kinross ore haul to the mix you’re asking for potential failure.
Alaska is the only state in the union that doesn’t have a maximum truck weight limit for its roads and bridges. Remember the old adage, “we don’t give a damn how they do it on the outside”? During pipeline days there was pressure to haul the materials to build the Alyeska pipeline. Pressure from the oil industry and trucking companies caused Alaska to exempt itself from the Federal Highway Administration (FHWA) nationwide rules and go its own way. In the lower 48 the maximum legal Interstate Highway weight without a permit is 80,000 pounds. We are part of the Interstate system but with the calculated “Bridge Law” formula, trucks just add axles and length to haul as much as they want. That is what Kinross has done, 16 axles over 95 ft allows them 165,000 pounds, be damned the impact on the roads and bridges. This wasn’t as big an issue in the past as trucks had less horsepower and heavy loads were infrequent. This all changes with the Kinross ore haul and an unknown and unlimited number of trucks being put on the highway.
[Editors Note: Our thanks to Bill Ward of Delta Junction for this expanded article on bridges.]