My 2-cent engineering perspective on the Pich Bridge tragedy

Koh Pich suspension bridge (Photo; AFP/Xinhua)

Sunday, November 28, 2010
By P. from Long Beach

In view of the recent tragedy on Koh Pich Bridge where several hundreds of our compatriots died and were seriously injured, I would like to provide below, my personal perspective as a Civil engineer in this regards. While my specialty is not in Structural engineering, but rather in Geotechnical engineering, i.e. a specialty related to foundation engineering etc, I will strive to explain as best as I can, in laymen terms, the various aspects of bridge construction, and in particular the suspension bridge type that is used for Pich Bridge linking mainland to Koh Pich Island, and I will also provide my 2-cent input on the various aspects any investigation should consider.

The most basic method of constructing a bridge can be seen in the illustration above. First several piers are constructed as support for the bridge, next a bridge deck is placed in between two piers to provide a platform on which traffic can move over the bridge. In general, guard rails will be constructed next to prevent vehicles and foot passengers from falling off the bridge. While in concept, this construction method appears quite simple, the reality is not so. For one thing, if the spacing between two consecutive piers is very wide, then the bridge deck need to be designed thicker or more rigid so that when traffic moves over the bridge, there would be not too much vertical deflection (see black arrow in the illustration above).

Why do you want to widen the spacing between consecutive piers? The answer is very simple: for cost saving. Because the piers have to be installed under water, their construction cost can increase significantly the total cost of the bridge. Therefore, in the design of the bridge, the engineer must consider a tradeoff between the cost of the piers and the cost of reinforcing (i.e. strengthening) the bridge deck.

In some instances, when the load that the bridge may carry becomes quite heavy, a system of truss (or steel lattice) can also be used to distribute the load on the bridge.

Bridge truss is a steel lattice to distribute the load on the bridge

The type of bridge that was built in Koh Pich is known as “suspension bridge” and its construction technique differs somewhat from the conventional approach described above. In fact, the first concept of such bridge design dated back to 1595 and was attributed to Fausto Veranzio, an inventor and a catholic bishop from Croatia. However, the construction of such bridge did not really take place until the early 19th century.

To build a suspension bridge, first, two pylons or tower-like structure are constructed near both ends of the bridge. These pylons serve as vertical support for the suspension cables. The suspension cables are very strong cables anchored (attached) to solid ground (or rock) on both ends of the bridge. Next suspenders (vertical support cable) are attached to the suspension cable. The bottom of each suspender is attached to the bridge deck and suspends the latter over the body of water.

One advantage of such bridge design is to minimize the number of the support foundation (the foundation for the pylons) and thus cutting down the construction cost due to the wide spacing between the pylons. One of the most spectacular example of suspension bridge is no other than the Golden Gate Bridge linking the city of San Francisco to Marin County, California.

The Golden Gate Bridge in California is one of the most spectacular example of "suspension bridges" in the world.

Would a suspension bridge be less safe than a conventional bridge? Not necessarily, for example, in the case of the Golden Gate bridge, not only is it a massive bridge that serves to move traffic in and out of San Francisco, but it was also built in one of most earthquake-prone area in the world. So far, the bridge is behaving quite well although Caltrans, the California Department of Transportation, is in the process of strengthening it to make it more earthquake-resistant.

Would a suspension bridge be more prone to movement?

In the case of Pich Bridge, preliminary official investigation claimed that the stampede was initiated by the swaying of the Pich Bridge due to the presence of the large number of revelers on the bridge. So the first question that comes to mind is whether the suspension bridge is more flexible (i.e. more prone to swaying under heavy load) than a conventional bridge?

To answer this question, one has to keep in mind that any construction is prone to movement. This is a simple law of action-reaction, i.e. when a load is applied on a structure, that structure will tend to move slightly to respond to the applied load. Can’t the engineer design the structure to minimize the amount of movement? Yes, by designing the structure to be more rigid, then the amount of movement (or deflection) will be minimized, but this also comes at a cost. Therefore, a tradeoff needs to be considered between what is known as acceptable or livable amount of deflection and the cost to build such structure.

Returning back to the case of the flexibility of the suspension bridge, one of the first video shown to a Civil engineering student in a Structural course is the collapse of the Tacoma Narrows Bridge in Tacoma, Washington in 1940. The bridge was opened to traffic on July 1, 1940. Its main span collapsed into the Tacoma Narrows four months later on November 7, 1940, at 11:00 AM (Pacific time). The spectacular collapse was recorded on film and shown below.

Collapse of the Tacoma Narrows Bridge

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.

As always, I remain opened to any input and suggestion by the public, be they engineer or not.

I thank you for your attention,

P. from Long Beach