T

he collapse of Kutai Kartanegara Bridge shocked many, not only the public but also the engineering community. The bridge was relatively new, having been completed in 2001 and was the pride of the wealthy East Kalimantan regency of Kutai Kartanegara.

Looking at the accident and how it occurred, one may be tempted to analyze, speculate and start the blame game. But in the end, to take a leaf out of the forensic science book, investigations should start from the scene of the incident.

Many will compare the incident to that of the famous fall of Tacoma-Narrows Bridge, also a suspension bridge, in the United States in 1940. Speculation of wind-induced vibration failure has surfaced.

However, considering the bridge’s characteristics, with a center span of only 270 meters and steel truss as the stiffening girder, the two cases are hardly comparable.

The failure of the Tacoma-Narrows Bridge was caused by what is called “coupled flutter phenomena,” where the wind direction around the bridge deck is modified when the bridge moves, which results in “motion-dependent forces”.

These motion-dependent forces can lead to vibrations with very large amplitudes, causing structural instability and failure. These forces typically appear on long-span bridges with flat and very slim girders, which is different from Kutai Kartanegara Bridge.

Suspension bridges are unique structures. Apart from their foundations, they have five load carrying components: girders, hangers, suspension cables, towers and anchorage blocks.

The girders support traffic loads and the loads are transferred to suspension cables by hangers. The suspension cables further transfer the loads to the towers and anchorage blocks at both ends of the bridge.

This is the loading transfer mechanism. This unique loading transfer mechanism allows a suspension bridge to span over a wider area, such as large rivers or even sea straits.

In order to guarantee the loading transfer mechanism, all structural components and connections must perform satisfactorily. Compared to other types of bridges such as concrete girder and steel truss bridges, cable-supported bridges are more sophisticated in design and loading transfer, thus they require more careful maintenance.

For the bridge to perform to standard, it requires not only a good design in terms of structural components but also proper maintenance throughout its service life.

In many cases where bridges have failed, we have learned that there can be many sources of collapse, such as initial design flaws, poor materials and construction quality, poor maintenance, false design and maintenance procedures, excessive loading conditions and human error, in addition to the natural causes such as wind ambush, earthquakes or flood-induced scouring of the bridge’s foundations.

We have also learned that it is not necessarily a single cause that brings down a bridge. Instead, bridge collapse may come down to a combination of reasons.

For example, excessive loading has a big chance of causing bridges to fail, especially for poorly-designed structures.

Looking at news reports, one thing that seems to draw attention is the quick sequence of failure initiated by a problem connected to the bridge’s structure. This is known as a progressive failure in structure engineering and it must be avoided in a robust design concept. Therefore, we must investigate the collapse by gathering field evidence and collecting eyewitness reports rather than speculating.

At this point, it is imperative that a qualified investigation team is set up and the investigation report made open to the public to scrutinize. Without this, we may never know what happened.

The aim of the investigation is not only to find the responsible party but more importantly to learn from the incident and apply these lessons of improvement to future structures.

The writer is a researcher on long-span bridges at University of Tokyo.