Clearly, urban bridges have to handle less traffic, and much less heavy traffic, than the busy Van Brienenoordbrug, a large motorway bridge spanning the Nieuwe Maas river in Rotterdam. Nevertheless, a few years ago all Dutch bridges were made subject to the same European safety standards. TNO developed a new bridge traffic load model to show that Dutch urban bridges are strong enough.
In the past, traffic passing over Dutch bridges had always been divided into three categories: light, medium, and heavy. Bridges on motorways, over which occasionally overloaded trucks passed day and night, fell into the heaviest category. Bridges in urban areas were seen as handling ‘lighter’ traffic: cyclists, cars, and small trucks.
New safety standards for bridges
With the arrival of the Dutch Buildings Decree 2012 and the Eurocodes – a set of European applicable standards – this categorization was replaced, meaning that every bridge in the Netherlands is now formally required to be able to handle the heaviest traffic: trucks weighing 50 tons, and specially-licensed traffic weighing up to 100 tons. “That’s a daunting prospect for Rotterdam Council,” explains Jaco Reusink, an urban development advisor to the area. “It affects hundreds of bridges in Rotterdam alone, some of which are pre-war. We would have to reinforce and partly renew them to meet these standards. The costs were estimated at half a billion euro.”
“The new traffic load model helps municipalities to determine which traffic loads should be used for which bridges”
Safe? Prove it!
To expect a small bridge over a canal to be able to handle the heaviest trucks as if it were the Moerdijkbrug, when such trucks are not permitted or even able to use it, is clearly ‘a bridge too far’. On the other hand, European standards and regulations were created for a purpose, and Dutch municipalities – almost 400 of them – cannot simply duck out of the situation. Those not wishing to use the applicable structural and dimension-based regulations have to be able to demonstrate that the structure of an existing bridge is absolutely safe. “Rotterdam Council regularly inspects all its bridges for wear and tear,” explains Reusink. “This tells us about the condition of their technical status, and which of them need some extra maintenance work. But how do we demonstrate beyond any doubt that our bridges are strong enough to carry their traffic, and that they therefore do not have to meet the new, extremely demanding European standards? That was the challenge we were facing.”
Traffic load model
Rotterdam contacted TNO. “We did some very precise research into what traffic weights can be assumed to apply to Rotterdam’s city bridges, in order to be able to talk about their safety,” explains Adri Vervuurt of TNO, who specializes in the structural safety of bridges. “This research resulted in the development of a new ‘traffic load model’ that was incorporated into the standards. Put simply, this model helps municipalities to determine which traffic loads should be used for which bridges. The starting point is that heavy trucks weighing more than 50 tons will not be allowed to use certain bridges. Transports involving more than 50 tons need a licence, and municipalities may deploy this licensing system to regulate which traffic goes over which bridges.” As is the case in most safety calculations, what matters is that the statistical likelihood that something actually goes wrong is negligible. Vervuurt: “In the case of bridges we work with a failure probability of about 10-4 during the life expectancy of the bridge. To translate this into concrete terms, this is the probability that an excessively heavy vehicle crosses the bridge once every 75,000 years.”
“We were able to demonstrate that 90% of Rotterdam’s bridges were safe enough and did not need to be adapted to meet the new standards”
The bridge problem is as serious in Amsterdam as it is in Rotterdam. Marten Klein, a director of Amsterdam’s municipal engineering bureau: “A lot of our bridges are very old. Some of them date from a time in which there was no motorized traffic at all. The traffic they actually have to handle is still minimal, certainly compared to bridges on busy provincial highways. But Amsterdam’s city bridges are now being asked to meet the new European standards.”
No unnecessary strengthening
Klein puts the number of bridges in Amsterdam at about 1700, and is very happy with TNO’s new traffic load model. “It means we won’t have to strengthen all those bridges, an operation whose total cost can only be guessed at. The credit goes to Rotterdam, by the way – Rotterdam took the lead. Every other municipality in the Netherlands is going to benefit from their pioneering work. Across the country as a whole there are over ten thousand bridges that fall under the new regulations. Thanks to this new traffic load model, these bridges won’t have to be unnecessarily reinforced.”
“It’s partly thanks to our constructive talks that TNO’s new traffic load model has been made more accurate and robust”
90% are safe enough
Reusink and Klein look back on their collaboration with TNO with satisfaction. “It was a long job,” says Reusink. “TNO understood that Rotterdam needed to address the issues quickly, and did their utmost to find the answers. We were finally able to demonstrate that 90% of Rotterdam’s bridges were safe enough and did not need to be adapted to meet the new standards.”
Making the load model more precise
“Over the course of this project we weren’t always in complete agreement with TNO’s vision,” notes Klein. “But the great thing was that there was always space for open discussion, and TNO really listened to what Rotterdam and Amsterdam had to say. It’s partly thanks to those constructive talks that TNO’s new traffic load model has been made more accurate and robust. The job’s not over yet, though. The next step is to get an even clearer picture of the amount of traffic that actually crosses all these urban bridges. TNO can then use that data to make its traffic load model more precise.”
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