Information in this report was gathered from Christchurch Polytechnic Institute of Technology [CPIT]s website relevant to Civil Engineering, government webpage and Google Scholar. In order to explain the reasons for the collapse of the Mianus River Bridge, some relevant data and graphs are attached as well.
3.1 The Failure of Pin and Hanger Assembly Design
The Mianus river bridge was a pin and hanger assembly design. An upper pin and a lower pin (both were 2.5 inch thick) connected with the girders. The upper pin and lower pin were connected by one 1.5 inch thick steel hangers from inside to outside of the web(National Transportation Safety Board, July1984, p2). The suspended span had 4 groups of pin and hanger to support the weight of structure. In this system, when one of girders was cracked, the whole structure could collapse. If a cyclical load was continuous to effect on the pin in the joint, it would cause fatigue of steel connections between pins and hanger. The crack was generated by overload conditions and repeated pressures(State of Michigan, n.d. p II-2, p II-5).
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3.2 Inadequate Drainage System
Drains were blocked and the inspectors did not clear them up. As a consequence, the inadequate drainage of bridge could not work in rainy day. The rainwater led into pin bearings in a moist atmosphere. In this wet condition, the strong electrolyte was formed. The anode in the electrolyte offered the ferrous ion. Meanwhile, the hydroxyl ion was produced by oxygen-reduction on the cathodes. Because the ferrous ion met oxygen-reduction cathodes, electrochemical corrosion (Battery phenomenon) was set up. The rust layer is Fe3O4, which rusted the inside and outside of pin hangers and left track of corrosion(Lloyd, n.d. P2 ). The rust scabs on the pin bearings would damaged the pins and hangers. Thus, the inside of the end of the lower pin became weak in the southeast corner. The outside of the pin bearings could not support the entire weight of the outside hanger. The remaining pins had to surfer from the whole load. For a long time, the shoulder of the pins got fatigue crack, the pin and hanger assembly collapsed eventually(National Transportation Safety Board, July1984, p3).
3. 3 Inadequate Maintenance
Two engineers were sent to be responsible for monitoring about 400 bridges per year. To sum up, only 12 local professionals had to inspect up to 3,425 bridges. The total number of bridge inspectors was inadequate in ConnDOT. During the process of inspection, the stuff always ignore or inspect carelessly to the corners. They did not find the trace of rust on pin bearings in the southeast corner (NYcorad, n.d., para 25).
Considering about this accident, the collapse of Mianus river bridge was the failure of pin and hanger assembly design. In addition, The drain system could not remove the water immediately. The rust was generated in the pin bearings that was difficult to inspect by engineers. The inadequate investment on maintenance is the third reason to cause the problem. When the southeast pin bearings was rusted completely, it was not able to support the weight. The rest of bearings had to suffer from the extra pressure at a long time until they reached to the level of fatigue crack at a certain moment. To avoid this problem, there are three recommendations including. First,
Second, weathering steels are used on anti-corrosion of steel bridge. Third, the professional inspector training program was introduced to enhance the assessments of inspection and engineering reviews.
5.1 The inspection and maintenance plates
Pin and hanger assembly plays an critical role in the bridge structure. The pin bearings support the whole weight which is easy to wear over period. The hangers could suffer from the fatigue to crack due to the long-term tensive condition. In the example of Mianus river bridge, the pins were rusted by leaking water. The track of corrosion in the pin and hanger assemblies was difficult to inspect. Due to unbalance force on the hangers, the fatigue of hangers cause the collapse happened eventually. Thus, there is an important assignment that the whole structure needs a periodic inspection and maintenance. Wear often happens in the pin and hanger on the suspended span between top and bottom. The centers of pins should be located and left a certain gap which is easy to inspect and repair. The plate of inspection must be considered in the design (State of Michigan, n.d. p II-14).
5.2 Weathering steels are introduced on anti-corrosion management
To prevent the corrosion on the structure of bridge, the weathering steels is a good choice used in this field.
5.2.1 The advantages and disadvantages of weathering steels
Comparing with the normal carbon manganese steels, the weathering steels (low alloy steel) have the higher resistance to the rusting. It includes the alloying elements that could form a protective layer proofing oxidation in the variable climate. Also, weathering steels are less maintenance, which can reduce both direct expending on maintenance and indirect expending on traffic possession (CORUS, n.d. P16). The simple periodic maintenance reduce the 30% of cost on steel recycling. This durable facility can save the construction cost at 10%, because money does not need to cost for the protective painting on the surface. Thirdly, weathering steel is one type of environment-friendly material due to few volatile organic compounds from paints. However, it is not suitable to use weathering steels in the marine and heavy polluted environment (Virmani, August 2009, para1).
5.2.2 Adding alloy elements to improve the weatherability
The corrosion of steel is a electrochemical process. The process is effected by the PH situation and Potential difference for function of weathering steels. Some element could be useful in the compound of weathering steel to develop its performance.
W(tungsten)is added in weather steel. When the rate of PH reach to 5 or lower, the ion WO42- will be produced and combine with Fe cation. The inhibitor is formed to prevent the further corrosion.
Ti(titanium) can interrupt the rust development. The ion FeOOH will be plugged by it on the surface. This rust layer passivates the rust.
If 80% of Al(aluminum) is added in the steel compound, the rate of rust resistance will increase twice as high as the common steel.
RE(rear earths) can upgrade the weatherability of steel. These REs transform the corrosion into a positive way, which seems to cover by a rust layer on the surface (Virmani, August 2009, para8-10).
5.3 launch The National Bridge Inspection Standards [NBIS]
The collapse of Mianus river bridge reminded the importance of inspection. Thus the professional inspection program is necessary to enhance the inspection assessment for the structures to reduce the rate of potential failures. The training program should include latest research, engineering experience and technology transfer of knowledge (Federal Highway administration, 2002, para 7,8).
5.3.1 Previous information collection
The information should be gathered through the NBIS. Furthermore, some critical step are needed such as minimum data collection, identification of qualification, offered to agencies responsible for bridge management, assistance with bridge program. The information could support the subsequent of examinations for the inspection(Federal Highway administration, 2002, para 12, 13).
5.3.2 The Inspection report
On safe side, the inspection should be performed per 2-year period. After the periodic inspection, a professional report is requested on the components of a bridge. The inspection should cover the surface of bridge deck, foundation, superstructure and drain system for the crossing waterways (Federal Highway administration, 2002, para 13).
5.3.3 The inspection assessment
Depending on the result of inspection, there is a bridge inspector point system from 9 to 0 (check Figure 5.3.3). If the work is excellent, the inspector will have assessment about 9. Code 0 means a fail inspection, which has to be performed again. This assignment of the ratings could guide the basis inspection of the bridge condition (Federal Highway administration, 2002, para 17).
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