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Recently, an engineering accident becomes the hot spot of the mass media and Internet social networks in China. In the morning of August 24, a span of the Yangmingtan Bridge fell down when four large trucks were driving on the decks of this span. This misfortune caused 3 fatalities and 5 injured according to the news report. Here is the news report about this engineering accident on the New York Times. Immediately, this bombshell spread over Chinese social network websites. Every netizen began to talk about it and most of them accused this tragedy to the imaginable black case work and poor management of this engineering project. Since in China, it is now a common sense that an engineering accident must be caused by some governmental corruptions, as I have discussed in this article Traffic lights and engineering accidents. However, besides these social factors, what is the technical reason for this engineering failure?

The photo of the collapsed bridge and the image of the simulation model made by me

After this accident, some Chinese engineering experts stated that the four overloaded trucks might be the main reason. Many netizens choose to laugh at this statement. In their opinion, it is impossible for only four trucks to collapse a big bridge. They just treated the experts’ opinions as a weak pretext. Nevertheless, it is possible, especially when the four trucks are illegally refitted ones. According to the official news report from the local police, the total weight of these four trucks was over 400 tons and it is clear that at least three of them were illegally refitted to bear more cargoes.

So here is the question. What is the allowable load of the bridge? Or in other words, when the engineers were designing this bridge, what is the maximum load in their consideration? Well, according to Chinese National General Code for Design of Highway Bridges and Culverts, the standard linear load is 10.5 kN/m for one traffic lane and a 360 kN point load per lane on a single span. The collapsed span has two traffic lanes and is about 120 meters long. So the total design load is 3240 kN, which equals 324 tons. The total weight of the four trucks is larger than this. Also, the four trucks were actually on the same traffic lane when the accident happened. The design load for one traffic lane is only 162 tons. In conclusion, the four trucks exceeded the load limit of this bridge.

Moreover, there is the problem of standard truck load. In the General Code, a standard truck is used to check the detailed design of structural components. This standard truck is 15 meters long and weighs 55 tons, which is much smaller than the four trucks in this accident. In fact, there are many heavier trucks in the real world. And again, like many other social issues in China, there is no proper system to deal with this problem. On an Internet forum, an engineer who is working abroad talked about his own working experience. A big heavy truck was waiting under the bridge and he was called to the site to check out if it was safe for the truck to go over this bridge or if it is necessary to stop other vehicles. But what is going on in China? Do we have such prior examination systems? In the year 2009, a bridge over the Li River in Henan province was collapsed by a truck that was as heavy as 260 tons. It seems that nobody cares until a bridge falls down. Or nobody cares even if a bridge falls down.

Maybe you would ask, why can’t we make the design load much larger? That is a complicated problem. On one side, the national General Code should consider the overall condition of the whole country. If the design load is too big, it will cause nationwide unnecessary waste of investment in bridge engineering. Based on the information from this blog article, if we compare this code with the American AASHTO LRFD 2007, we will find that the design load of these two codes are at the same level. On the other side, because of the high price of petrol and many other expenses in China, truck drivers can earn little money if they do not illegally refit their trucks to carry more cargoes. Some trucks on the Chinese road are even more than 250 tons although their legally allowed load is only 50 tons, such as that truck in the Li River Bridge accident.

The simulation of the process of collapse made by me

Back to this accident, we have known that the load of the four trucks exceeded the limit of the bridge. So the next question is, how did the collapse happen? Which part of the bridge broke down at first and caused the whole collapse? When all the 400 tons of trucks were on the same lane of this deck, it caused a disequilibrium in the transect of the bridge deck. Like you put a very heavy box on one side of a seesaw, the bridge deck began to rotate and fall down. What is worse, as the displacement of the whole deck increased, the disequilibrium of moments became even bigger because of the second-order effects of gravity.

The failure of the reinforced concrete capping beam

Normally, this overturning moment was controlled by the capping beams which lie between the bridge deck and the pier column. Unfortunately, the two middle pier columns of this span have no capping beams. As a result, the huge overturning moment had to be absorbed by the capping beams of the two side pier columns. Since the designer did not consider such extreme situations, the capping beams of the side pier columns simply cannot bear the huge shearforce cause by the overturning moment. With the failure of the RC capping beams of the two side pier columns, the bridge deck lost its final support and fell down to the ground.

If this accident is caused by the overloaded trucks, then many people would ask that why was this span and why other spans were still safe? These four trucks drove along on this long bridge and they did not collapse the former spans until they reached this span. The reason is that this span is a special one. If we look at the photo, we will find that the form of a steel box girder is only used in this span. The beams of other spans are all prestressed concrete T beams. The concrete T beams are much heavier than the steel box beam and thus less likely to be toppled down by eccentric loads. Imagine you put a big plate on a small table and your cute fat cat is lying on the edge of the plate. If it is a heavy stone plate, the cat will enjoy his time. But if it is a light wood plate, the cat may press the plate down and fall off the table. By the same token, the heavier concrete beam naturally has better stability than the light steel beam.

The example of the fat cat and the plate on the top of a table
thanks to my wife for drawing this lovely illustration

Also, other bridge spans have relatively closer pile columns and all these columns have concrete capping beams. While for the broken span, it only has two middle columns which are 40 meters away. What is worse, they do not have capping beams. Thus, they are frailer to the overturning moment. Take the example of plate and fat cat again. If the supporting table is very big and the plate only overhangs a little out of the edge of the table surface, then it is hard for the cat to press down the plate. On the other hand, if the table is very small and the plate overhangs very long from the edge of the table, it will be much easier. In this accident, the cat is the four overloaded trucks and the light wood plate with a small table is the bridge. The lighter steel beam and the lack of concrete capping beam open the door for the engineering failure.

Then the final question comes. Why this span is different from others? Why cannot the engineers make it the same as others? If it uses the same concrete T beams and pile columns with capping beams, this tragedy may not happen then. Since there is no further information or first hand materials, we can only guess. From the photo, we can see that this span is over a T-shaped road junction. Maybe due to some technical or aesthetical reasons, the structure height of this span is limited and the engineer has to use a different structure form. It is normal that steel beams are used when the bridge needs to cross over a wide road since you cannot add middle columns just in the middle of the road below the bridge. However, the use of steel beam needs to be examined carefully to guarantee the safety. In this case, the stability in the transect direction is neglected.

I am not blaming the engineers for not considering carefully. Instead, I understand them. As a structural engineer in China, I know what is going on in this industry. According to an official news report, the construction time of this bridge is only 18 months. If it only takes 18 months to build it, then how long did it take to design it? When there is not enough time, the quality of design will become less satisfactory. But no one pays attention. Governmental officers and developers only care about the completion date because it is literally “Time is money”. I am not trying to exculpate anybody. It is just unfair to blame engineers since they did not get enough time. In China, there is even a term called “three together projects” which means “design, modify, and construct all together”. The speed of construction is extremely fast, but the cost is the low quality of both the design and the construction. In the long term, this pattern of development is not a good decision.

In the year 1940, the first Tacoma Narrows Bridge collapsed due to a small wind. This accident revealed the importance of aerodynamics in the structural field. From then on, every engineer takes the aerodynamics and resonance effects into consideration when designing tall buildings or long-span bridges. The fall of the Tacoma Narrows Bridge serves like a caution light in this certain field. I hope the collapse of Yangmingtan Bridge could play the same role to prevent such disasters in the future.