Bridge design must meet the demand of Padma

KMN Sarwar Iqbal
Bridging both banks of the Padma does not essentially limit to mean bringing both sides’ people together by means of bus or train and facilitate them with natural gas and electricity. It is a sacred approach to unify those southern people and bring them to a common society which will contribute the nation’s development in all aspects-both socio-economic and agro-industry. However, intuitively one may like to inquire-is it a bridge which the authority would be planning to erect like the one over Buriganga river?

The answer is lying within the relationship between these two rivers where the second one is being fed by the first one. Simply, in case Buriganga becomes thirsty; its thirst will be quenched by the flow of the Padma via Dhaleswari river.

But when things turn turtle which is normally true during the summer as the Mother Padma’s basin dries out, can she be revitalized with the reverse flow from Buriganga? Obviously it is an opposition to the rule of nature and impossible. Since the Padma holds the most significant water flow which keeps our water born botanical, zoological and agricultural diversity alive, the vitality of the Mother Padma must be kept uncompromisingly intact and secure. This should be the main criteria of designing the bridge over the great river Padma and no other wise.

Till now, the architecture of the bridge known to us is it is going to be a two-tire bridge which will allow traveling automobiles through the upper deck while the lower deck will allow the rail cars. Supporting the decision, the authority reasoned that the rail cars usually would need an extended ramp to embark on as well as to depart from the bridge until reinstituting on the plain. Moreover, building a two-tire bridge will cut the need of some extra lands which will be acquired from the nearby farmland, the authority added. Before making such a conclusion on land economy, the authority must justify the technical feasibility of the project. Constructively speaking, the loads applied onto the columns and beams of a parallel structured (horizontal) multipurpose (i.e. automobiles and rail cars) bridge are not the same as it happens to those of a two-storied (vertical) bridge. According to the definition of pressure (P = W/A), the columns and beams of the vertical bridge will carry more than double load, caused by the load concentration, than those of horizontal one will unless their location and number are changed. As a result, taken all other design parameters are constant, the number of required columns should be doubled to uphold the same safety factor.

Although a significant part of the load may be proportionally redistributed by redesigning the columns and beams and reorienting them will reduce only a few; but the designer will find no opportunity to cut it down to the half or equivalently which the parallel one requires.

Upon constructing the two-tire bridge, the additional columns will significantly affect the flow of the river.

What makes the flow complicated is the Padma does not direct a single-phase flow of liquid alone. Instead, it carries a mixed flow of liquid and solid (i. e. mainly sand and alluvial) particles, and not to be misnomered as two-phase flow. Except the rainy season, the river keeps water reserve very low throughout the year contributing to the flow’s density to increase with respect to that of normal water. Eventually, such an immersion of series of columns across the river span will develop an unsteady flow soon after the bustling flow strikes the boundary of the columns which leads the mixed flow to become severely turbulent. Such an occurrence would eventually contribute to a higher degree of precipitation of solid particles, mainly sands, at “no slip” zone, namely the river basin in front of the column roots unto a long way upstream the river.

Simultaneously, because of the continuity principle; volumetric flow rate (Q = AV) of the passing-by mixed flow must be constant, thus the flow gains higher velocity towards the downstream which will create cavitation and keep continuously eroding the “no slip” zone or namely the river basin adjacent to the rear bottom of columns. Consequently, a hydraulic jump or two different velocity profiles before and after the column boundary develops. Such a phenomenon will bring no good either to the river or to the bridge and its only effect is erosion.

Naturally, we expect a high density of vehicle transfer through the bridge which causes the structure to face constant vibration problem of different merits. Besides the vibration caused by the locomotives, powerful gust of wind could also be a potential threat to the structure. Since the vibration related failure (fatigue) is quite unpredictable, without urgumenting its estimated life cycle, an ever alert sophisticated maintenance unit must be employed to receive the warning of oncoming failure and to take appropriate measures; but still no cent percent safety is assured. In the case of a two-tire bridge such problem becomes even more complex and its maintenance also becomes quite hectic than they can be in the case of a parallel one. A question arises when does a two-tire bridge become necessary?

Answer in a single statement is when the space is limited and, or the design restricts horizontal expansion of the structure. Building a bridge over such a great river does not fall into that category anyway. This two-tire bridge idea can be employed in the densely populated cities where spacing is concerned. For instance, the author witnessed such structures while traveling subway in New York, USA.

Remorsefully it is true, once the Padma shetu is erected, one will find too little or no value of playing that famous folksong ” sarbonasa padma nadi…..ki tor kul kinara nai”; because, its unstable banks will be given stability with a fixed span which is the mixed product of our feelings and needs combined with rudeness against her freelance career. Anyway, we will do whatever we feel to do or need to do. Since that giant concrete reptile will be standing across the flow of the Padma to takeover the load of our socioeconomic needs, couldn’t we be moderate in creating a reptile which will be strong enough to stand on its most minimum number of legs so that such a great river’s serene flow experiences a tolerable obstruction in its continuity?

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