In the north-western province of Qinghaiin China, an arc bridge has collapsed during the construction phase: the accident caused the death of 12 workerswhile 4 are still missing. The infrastructure should have been completed by the end of August, becoming the largest double -track continuous railway bridge in the world, as well as the first bridge of this type to cross the yellow river.
The structural engineer Gaetano Cantisani he provided us with antechnical analysis of the accident, explaining the functions of the cable and the various factors that could have caused the release, causing one cascade reaction which determined the collapse of the central part of the structure.
What was the cable for and why was it so important for the construction of the bridge?
«First of all, it must be specified that the support cable he performed a function temporary And provisional: as can also be seen from the images, in addition to the aquamarine arch, the infrastructure is also composed of two brown pylons positioned in the support area, from which the ropes that keep the structure, which gradually are formed.
It is a technique often used for i Arco bridges Which, until they are complete, do not work as they should because the arch is not yet able to transfer the actions that are provided to it by the structure that hangs below. Probably during this phase it is not even able to hold his own weight.
As it is built, in fact, the arch becomes more and more heavy and therefore needs a support that it is provided to him by these cables, fixed on the pylon that I have a counterpart on the other side. Consequently, thehorizontal action it is discharged on the counterpart (fixed to the pylon), while thevertical action It is downloaded on the pylon itself.
In other words, the arc bridge is unable to support its weight until the suturethat is, that element that unites the two Arco stumps which, once completed, will constitute the arch.
The infrastructure, therefore, had been designed to be maintained provisionally From these cables: only in cases where the arched bridges are smaller in size (as in the case of some railway bridges built in Italy along the high -speed line) it is possible to build the entire structure of the bridge without the use of cables, and then place it directly on the spot. Otherwise, the presence of these additional structures is always necessary, precisely because not all the components of the bridge are performing their function. “
What were the consequences of the release of this cable?
«The moment the provisional cable yields one is generated cascade reaction: the cable that gave way to all the other following, because the whole part that was to hold up the broken cable is now held by next cablewhich in turn gives because the weight is excessive, until the structure gives it completely.
Among other things, from the video of the collapse it is possible to note “scie“Around the cables: most likely it is dustwhich could be raised at the time of the cable or anchoring on the bow. The cable, in fact, is in tension and when it comes out suddenly it generates a sort of whisk. “
What could have caused the release of the cable?
«Behind the collapse of the bridge there may be different causes: from the published images, it seems that all council They had been positioned, but probably the last one was not yet definitively attached. The most delicate phase of the whole construction was therefore taking place, because all the cables were at the time of maximum stress. The failure may have triggered due to a lack of assembly of the same at the arch, but it could also be linked to a previous damage of the cable (which, being a provisional element, is often reused).
At the same time it should be considered an additional important aspect: during the construction of a bridge of this type, a precise alignment Between the two pieces of bow, especially if you are in the final phase, as in this case. That’s why the workers were working at nightto try to limit the thermal changes and the consequent dilations of the structure due to the temperature variations between day and night. dilations of the structure due to temperature. At this stage, the altitude adjustments between the two sides can also be carried out by varying the tension in the cables, which adds to the possible contributing factors of the failure.
To be considered, then, there is the question of cable sizing (i.e. what is the diameter that the cable must have to be able to hold a certain weight), even if unlikely since they are very controlled objects and that require relatively simple project calculations, as well as security margins usually high precisely for their crucial role.
Finally, the hypothesis of thehuman error: it must be emphasized that in such a delicate constructive phase as the final one theattention is maximumbut clearly this possibility cannot be completely excluded.
In summary, behind the failure of the cable there may be a series of nonsensealso linked to environmental conditions, which may have caused it release and triggering the waterfall reaction ended with the collapse of the central structure. “
How common are accidents of this type?
«In reality, accidents of this type occur mainly during theconstruction phase (and this is true in general when it comes to bridges), when the arc bridge is not yet able to support its weight and needs a external support: once completed, however, the collapse of a bridge of this kind is quite rare and, if it happens, is mainly linked to the age of the infrastructure and the poor maintenance over the years.
In addition to the factors mentioned above, it must also be considered that it is a bridge of record size: being among the first large arch bridges, what happens during the construction can be more difficult to predict.
Today, however, these collapses are less frequent also thanks to new technologies of monitoring in real time that allow you to control the cables with tools that record their instant shooting. “
