Case Analysis of Bridge Construction Failure

Introduction

Construction failure has been recognized as one main cause of bridge collapse. However, it does not attract public attention because approximate 80% of bridge collapses caused by construction failure happened in the course of construction. It is understandable in that this type of collapse would not affect the contemporary traffic situation and would not pose threat to the citizens. Unfortunately, the lives of construction workers will be jeopardized, and the expense of demolishment and reconstruction will be significant.

Also, it should be noted that there is still roughly 20% possibility that the bridges collapse during their service life. As we all know, most catastrophic consequences attribute to the collapse during service life. Therefore, the deficient construction should not be neglected, and engineers can learn how to avoid it from relevant cases.

The purpose of this assignment is to highlight the importance of construction, illustrate two construction failure cases, and present what can be learned from them. The cases discussed below include West Gate Bridge and Seongsu Bridge.

West Gate Bridge

Background

West Gate Bridge, located in Melbourne, Victoria, Australia, is a steel box girder cable-stayed bridge, which spans the Yarra River separating the inner city and the suburbs. Its height above the water is 190 ft. The width is enough for eight lanes, and the total length reaches 8473 ft. the span between piers consists of 67 meters long concrete slabs and five steel box girders. The daily traffic flow is 160000. Due to the unique curving shape, the West Gate Bridge is one of the most remarkable architectures.

Collapse

On 15 October 1970, after two-year construction, the 2000 tons of span between the pier 11 and 12 buckled and then fell to the ground and the river. Beneath the span, there were several huts for the construction workers. When the collapse happened, some workers had a lunch break at their huts, and some still working inside the girders. Ultimately, this failure caused 35 fatalities and 18 injured. The collision of the ground and span induced the explosion of dust and mud, which spread out and shook the houses hundreds of meters away. The deafening sound could be heard 20 km away.

Cause

After the collapse, a royal commission had been assigned to investigate this disaster, and finally, they attributed this collapse to the construction method. According to Sean Bardy (2016), the construction method adopted was to halve each span along its length, prefabricate these half-spans, then lift them to the top of piers. In this approach, the lift load had been reduced by half, but the number of lifts had been doubled in the meantime.

Nonetheless, things did not go as well as they expected when this approach put into practice. Between Pier 10 and 11, one of the spans on the east-side encountered a problem. When the half-span was located on the pier, losing the support of the temporary trestles, the buckle was induced along the top flange of the span since the centerline of the bridge moved down when the two half-spans joined together.

Then, contractors thought the buckle would be mitigated when the span was in the ultimate position, so they kept lifting and sliding the span into the final position, instead of lowering the span to the temporary trestles to eliminate the buckle. However, once the span was put in the final position, there was nothing can be done to move back.

Afterward, the buckle was not reduced significantly when contractors finished the lifting and sliding work. Therefore, the decision was made to relieve some bolts of transverse splices on the top flange, thereby alleviating compression stress strength of top flange locally. Thus, the two top flanges can connect to one another after reducing the compressive strength of the top flanges to flatten out the buckle.

To avoid the occurrence of the same buckle, contractors took measures to stiffen the top flange for the west-side span. To achieve this goal, longitudinal stiffeners had been used, and the diagonal supports linking bottom flanges and top flanges also had been placed. This method proved efficient.

However, another problem came out that the vertical gap between the east-side span and the west-side span was so large that they cannot connect to each other completely. The next action taken to fix this problem was to place large mass concrete blocks on the half-span to shorten the gap. Unfortunately, this approach turned out to be wrong because it made the entire top flange of the west-side span buckle. Although the preventive measures had been taken, the extra massive load of the concrete blocks exceeded the limit of them.

One month passed away before contractors made a decision to remedy. They decided to adopt the same method as that used in east-side span, saying removing the bolts of transverse splices on the top flanges, but there was one significant condition they forgot: massive concrete blocks were still standing on the span. The neutral axis and the area of cross-section kept going down with the removal of bolts. Ultimately, the west-side span cannot carry the large load on it anymore, therefore dropping straightforward. The east-side span collapsed subsequently due to partially connected to the west-side span.

Seongsu Bridge

Background

Seongsu Bridge, located in Seoul, South Korea, is a cantilever bridge that has 4 lanes width and 1160 meters length. It is an important transportation junction linking the Seongdong and Gangnam districts and spanning over Han River. The construction was started in April 1977 and finished in October 1979.

Collapse

In the early morning, the rush hour on October 21, 1994, 15 years after putting into use, Seongsu Bridge collapsed during service life. One of the slabs at the middle of the bridge suddenly fell in Han River. Unfortunately, there were cars and a bus dropping down with the slab. The falling height is approximately 20 meters. This collapse caused 32 fatalities and 17 injured. Afterward, the government decided to rebuild this bridge since the deficiency of this bridge was too large to be repaired.

Cause

After investigation of this bridge’s failure, several factors were responsible for the collapse. First, poor welding is the main cause. Specifically, the structure beneath the slabs consisted of steel trusses, where welding was an integral role. However, the welding had been found insufficient, saying the strength of the welding did not meet the standard. Therefore, the fatigue crack initiated at the welding toe and kept propagating until rupture. At first glance, the fatigue crack was attributed to the collapse, but the truth is that the welding was not strong enough to resist the development of crack.

Moreover, the contractors were supposed to inspect whether the welding is sufficient by using X-ray or other techniques. Second, to prevent the road from freezing in winter, a significant amount of calcium chloride was spread out on the slabs. Chloride accelerated the deterioration and crack propagation of steel structure. Additionally, lack of regular and comprehensive maintenance promoted the occurrence of cracks.

Lessons learned

From West Gate Bridge case, we can learn that the method using two half-spans is considerably hard to control, which specifically means the continuous problems come out in the course of construction. The different level of the buckle of spans is an obstacle preventing spans from connection fully.

The remedy methods the contractors used in West Gate Bridge also presented to us that using longitudinal stiffeners and diagonal support linking bottom flanges and top flanges is an effective way to mitigate the buckle of top flanges. From the case of Seongsu Bridge, we know that the quality of the welding, which sometimes does not catch people’s attention, could control the fate of the building. We would pay an extremely expensive price for neglecting the welding work. Moreover, regular maintenance is required for every architecture. The omission of the inspection is fatal.