koh pich

The collapse of the bridge was attributed to strong gale wind blowing at 67 km/h (42 mph). In this case, the strong wind was able to initiate the fluttering of bridge deck, i.e. initiating the dynamic motion of the bridge. The fluke in this case was a dynamic phenomenon known as dynamic resonance, whereby the dynamic shaking generated by the wind coincided with the fundamental period the bridge (the time it takes for the bridge to sway up and down in one cycle when it is shaken). When such phenomenon occurs, the intensity of the bridge shaking is self amplified and could lead to collapse as it did in the Tacoma Narows Bridge case.

After watching such spectacular collapse one wonders if the Pich Bridge could sway under the massive human load on the bridge? From an engineering point of view and as I explained earlier, the bridge will deflect slightly in the vertical direction when a large number of people crosses it at once. However, considering the foot traffic on the bridge, since not all the people were dropped in instantaneously on the bridge, but they rather walked in as a stream, therefore, from a preliminary engineering standpoint, I have a hard time seeing the Pich Bridge starting to sway suddenly. In my mind, I can picture a plausible scenario where someone suddenly realizing that there was a large amount of people on the bridge, started to shout that the bridge could collapse, and thereby initiating the stampede.

As an engineer, I cannot solely rely on hearsay as the basis of my investigation, therefore, I was dismayed when a preliminary official investigation announced almost immediately that the stampede was initiated by the swaying of the bridge. In the US (as well as anywhere else in the world) for example, knowing or estimating the live load due to the foot traffic and the structural construction of the bridge, it is quite common to perform a structural evaluation through a computer analysis to evaluate the amount of deformation due to the presence of the crowd on the bridge. Such scientific evaluation would provide more meaningful credence to the official claim than the hearsays and rumors that government officials rely on as their basis. Short of funds to perform such analysis, vehicles loaded heavily enough to simulate to load of the crowd could also be used to perform a live experimental test to see whether the swaying may take place or not.

But even if swaying can be proved to take place, the question still remains as to why so many people are allowed to cross the bridge at once so as to create traffic jam, knowing full well that the bridge is opened to traffic in both directions? Where was the traffic control or was there any to start with? These are more fundamental questions that any investigation will have to answer. The answer to these questions would then pinpoint clearly who is to be blamed in this tragedy. If the government is not willing to support such scientific investigation, the OCIC company which is involved in the development of Koh Pich must take the initiative by launching its own scientific investigation if it wants to clear its name from this criminal case. That would be my 2-cent input into this case.