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.