In light of the I-5 bridge disaster over the Skagit River in Washington State, whose cause was a truck hitting the portal bracing of the Warren through truss bridge, questions have flown around as to whether a witch hunt to eradicate them is worth all the billions of dollars to be spent, or if it makes more sense to restrict them further for a fraction of the price and if so, how?
What about the usage of height detectors? This concept may be unusual to many engineers and politicians, but they are being used in many regions of the country today.
Like this one in Le Sueur, Minnesota. Located along the Minnesota River, the largest community of Le Sueur County with 4,000 inhabitants is the birth place of William Mayo, one of the founders of the world-renowned Mayo Clinic in Rochester, two hours to the east of town. While the town has a nice city center to the east of the railroad, it does have a thorn on its side, something city officials are hoping to get rid of when the railroad decides to abandon its line between Mankato and Shakopee, which is this deck plate girder bridge:
This railroad bridge crosses Hwy. 93 and has provided numerous headaches for drivers for two reasons: 1. The hill leading to the underpass is steep and after going underneath the bridge, you end up on the Minnesota River bridge and 2. The underpass is too narrow and too low for trucks to pass, forcing many truck drivers to use the exit to the north of town (at Hwy. 112 and US Hwy. 169). As the railroad is very active on this line, state and local authorities for many years have tried to place warning signs on both ends of the crossing, even warning drivers from as far away as US Hwy. 169 to consider other options if their load is too wide or even too narrow.
This concept was found just east of the interchange on Hwy. 93. How it functions is simple: if an overheight truck passes by the detector, an alarm will activate warning drivers of the danger ahead forcing him to change his course. A secondary alarm is activated warning police and other officials of the danger ahead so that they can act quickly and stop the person before reaching the underpass or bridge with a low clearance.
Given the lack of ability of some drivers to pay attention to weight and height restrictions of many bridges in the country, with the resulting factor being damage or destruction of the structure, this concept may be the best solution to the problems involving bridges with these handicaps. With millions of bridges with height restrictions on America’s highways, the cost for replacing every single structure would be so exorbitant that it would put the entire country back into an economic recession that would be worse than the one we just saw recently in 2008/09. This is not counting the cost for environmental impact and mitigation surveys and the design of the structure. In Minnesota alone, at least two dozen of these bridges are still in operation on the state’s highways, many of which still have some years of service left, like the Hwy. 7 Bridge west of Montevideo. This Parker span has spanned the Chippewa River since 1959 and is in tip-top condition.
This leads us to our question for the forum:
Do you think that height and weight restriction devices like the one in Le Sueur would be the most effective way to keep overweight and/or oversize vehicles from utilizing bridges with such restrictions? If so, how can we ensure that these people obey these restrictions without damaging or destroying the bridge? If it is not a viable solution, what alternatives would you recommend?
You can place your comments here on this page, or on the Chronicles’ facebook or LinkedIn pages. By doing so, you might have some ideas to share with others, who might find them interesting and useful. Furthermore it will help many people who think replacement is the only option to look at more reasonable options which can save money and force many people to think common sense. Looking forward to your thoughts on this innovation, which seems to be a very effective solution to our height and weight problems on the roads.
Note: The Skagit River crossing reopened to traffic on 15 September after crews replaced the temporary Bailey truss spans with concrete beams spans, built at the site where a section of the truss bridge collapsed. More information can be found here.
The Minnesota River crossing featured a 700 foot truss bridge (400 foot Pennsylvania, 200 foot Parker and 100 foot Warren pony) built in 1923 by the Wausau (Wisconsin) Bridge and Iron Works Company, replacing an iron Post through truss bridge. It used to carry US Hwy. 169 before it was relocated to the west and served as a bypass in 1967. The bridge was replaced with a current structure- a concrete slab bridge- in 1984. A photo of the Post truss can be found here as well as the 1923 span (here).
The Bridgehunter's Chronicles 
The red light cameras at intersections suggest a possible solution. Place warning signs, a detector and issue tickets to violators identified by the detector. A road sensor could also detect if weight-speed were over limit and issue tickets. There are some legal problems with red light cameras but with properly written state laws such a system could be used for administrative fines.
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We have something similar to that in Germany for traffic violators- red light cameras photographing license plates and driver with the driver receiving a ticket a week later. That might even be a better idea, if you ask me. 🙂
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Steel “Headache Bars” offer a low-cost, low-tech way to Absolutely Ensure that an offending over-height load never physically reaches a jeopardized span, regardless of the level of alertness of the driver. A simple pair of robust steel upright beams or pipes with an equally-robust steel beam crossbar spanning the roadway and berms between them, situated to precisely match the maximum allowed clearances of the truss superstructure, makes the bridge clearance envelope impenetrable.
This not only provides fail-safe protection for the trusses in the even that a ‘merely inattentive’ driver gets a load beyond the detector and warning devices, but also totally prevents a terror act where an over-height vehicle is deliberately driven into a truss to either cause mass casualties or sever a critical transportation artery for commerce and commutation to extract maximum negative economic impact (or both).
Some might argue that there would be legal issues if a vehicle too high for the bridge clearances were to be abruptly stopped short a few dozen or hundred yards short of a span, with resultant debris, chaos and possible consequential mishaps at that location. It could be argued as well, though, that having a ‘mess’ at the Headache Bar location is still a Far Better Outcome, in EVERY case, to dropping an entire bridge and its vehicle occupants into the spanned chasm or body of water.
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