Primary infrastructure : The proposed Padma bridge
(Left) Godavari Road-cum-Rail Bridge at Rajahmundry in Andhra Pradesh, India. (Right) Yangtze River Cable-Stayed Highway Bridge, an example of Chinese technology.
INFRASTRUCTURE is the cathedral of modern civilization,” Dr. Wayne Klotz, President of the American Society of Civil Engineers recently uttered these words pointing to 590,753 deteriorated and decayed road bridges above 20ft. span of his country. More than four billion vehicles cross these bridges every day. He tried to draw the attention of the US government for replacement and rehabilitation of about one quarter of these bridges in no time to avoid disaster like Bridge No. I-35W in Minnesota in 2007 that collapsed and fell into the Mississippi river killing 112 people and 35 vehicles damaging. Infrastructure is also part of our built environment. Roads, Highways and Railways fall in the category of primary infrastructure which helps to transport man and material from one corner to another of a country. People need bridges to cross rivers or gorges and also the rough terrain. According to Structural Engineering researcher, Kym Wilkinson of USA steel bridges last around 100 years, prestressed concrete bridges 75 years and reinforced concrete bridges 50 years provided they are designed properly and constructed as per codes and standards. As roads, highways and railways fall in the category of Key Point Installation (KPI) they are designed to last at least 75 years. Hence, most of the highway bridges in the USA are made of steel or prestressed concrete. Cost of a steel bridge much exceeds the cost of a prestressed concrete one, span and other factors remaining the same. There are different types of bridge structures, Girder, Truss, Arch, Suspension, Cable-Stayed etc. Concrete girder bridges are very common in the highways of Bangladesh where the structures are almost invisible for the road users; under the road surface, the supporting girders are of reinforced or prestressed concrete. The Hardinge Bridge is an example of steel truss bridge; Sydney Harbour Bridge in Australia is a steel arch bridge. Golden Gate Bridge in San Francisco, California is a suspension bridge whereas the Bidyasagar Bridge of Kolkata is a cable-stayed bridge. These different types of bridges have their specific purpose and utility and hence different design and construction process. Proper selection of the type of bridge for the specific purpose is a very important factor for the sake of aesthetics, economy and durability. In China, small span concrete arch bridges are usually made to replace the old brick arch bridges; to maintain the harmony, they are made of the same shape of the existing ones. This is a great advantage that concrete structures can be cast in any shape as desired. However, for a large span, roads and highway authorities all over the world go for steel or prestressed concrete bridges obviously for the sake of durability and railway men go for steel bridges only. This is mainly for two reasons: (1) Motor vehicles on roads and highways are lighter in weight than railway trains. (2) The railway locomotive and the train are not only heavier than the road traffic but the train also exerts a high impact load due to vibration that frequently causes to loosen the applied prestress. This curtails the longevity of the bridge itself. Hence, steel girder, steel arch and steel truss bridges are very common for railways and suspension and cable-stayed bridges are exclusive for roads and highways. Recently we have seen several articles on the proposed bridge on the river Padma at Maowa in newspapers including one in the daily Samakal on 26 January, 2009. Veteran journalist and prominent writer, Mr. Ajay Dasgupta wrote an interesting article namely, “Build the Padma Bridge but Save the River” with an artist's replica of the would-be cable-stayed bridge. We very much appreciate these. Since, no railway line exists on any side of this bridge, it will be much cheaper to make a cable-stayed road bridge like the Vidyasagar Bridge of Kolkata in concrete avoiding the costly steel structure. The saving thus accrued may be diverted in future for a smaller road-cum-rail bridge at Aricha connecting the already existing railway on the western approach. It is to be remembered that railway line cannot be placed on any portion of the deck of a prestressed concrete girder road bridge. Bangabandhu Bridge was designed for a road bridge but the then President wanted rail connection also and the consultants laid the rail on one side. The prestressed concrete girders developed cracks and the durability of the bridge is curtailed. This will compel to abandon the railtrack on the bridge and make a separate bridge for railways only. Examples of large road-cum-railway combined bridges are available only in China and India. The Americans have plenty of money and materials; hence, they make separate bridges for roads and railroads. Indians have made combined road-cum-rail bridges on all their big rivers thus they have the expertise and experience. The same is true for China. Asia's largest rail-cum-road bridge on the river Godavari linking Kovvur and Rajahmundry constructed by the Hindustan Construction Company (HCC) is considered as a modern day engineering feat. It is based on 28 bow string arch girders with equal spans of 97.5 m, thus making the total length 2,730 m. The bridge is fit for 250 km/h rail services. It has a road deck over the single track rail deck below. It was commissioned by South Central Railway on 11 March, 1997. The Nanjing Yangtze River Bridge, constructed between 1960 and 1968 is considered as the longest road-cum-rail bridge by the Guinness Book of World Records. It is the first bridge in China solely designed and constructed by the Chinese. The significant double-deck construction is made of main bridge portion which spans 1,677 m. The upper deck of the bridge is for highway with a span of 4,589 m. There are sideways on either side of the highway and each measure over 2 m. in width. The lower deck holds a twin-track railway with a length of 6,772 m. and a width of 14 m. that allows two trains to run in different directions. The bridge has nine piers altogether among which the tallest one is 85 m. with base area of about 400 sq. m. There are 200 cast iron reliefs inlayed on the banisters on the two sides of the highway. Two bridge towers are located at either end of the bridge each with a height of 70 m. They have elevators which facilitate access to both decks of the bridge as well as the watch towers. Recently constructed cable-stayed bridge in Nanjing was opened on Friday, October 7, 2005. Nearly five kilometres cross river bridge was designed and constructed to ease the traffic jam in Yangtze River delta zone. The Vidyasagar Bridge of Kolkata is of the same type. Design and construction of large span combined road-cum-rail bridge on a meandering river cutting through sandy soil is quite complicated. To deliver a good work, it warrants engineers with post-graduate education and long field experience. Only then the country will be truly benefitted. It may, perhaps, be wise to make a long cable-stayed road bridge (Chinese model) at Maowa as mentioned earlier, saving a considerable sum of money. This saving can be utilised, as said for a shorter length combined steel rail-cum-road bridge at Aricha. (For any clarification, please consult the free Wikipedia in Internet). Engr. Dr. M Harun Rashid FASCE, FIE Aust, RPEQ, is a retired Professor & Head of Civil Engg., RUET and Faculty Member of BUET: also life Fellow of the American Society of Civil Engineers and the Institution of Engrs. Australia.
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