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?Pontoon bridges need design improvement ?

india Updated: Nov 06, 2006 00:17 IST

A SPOT visit to the Ardh Kumbh mela site by a Motilal Nehru National Institute of Technology (MNNIT) expert and the study of the recent pontoon bridge on Ganga that got washed away has established that the design of  make-shift pontoon bridges being built urgently need to be improved.

Sound technological advancements need to be incorporated to help prevent injuries to the common man and also aid in their weight shedding that would make their handling easy and cost effective.

Assistant professor in the department of Mechanical Engineering Department, Dr HS Goyal, who carried out the study, says that only a sound technological support in the construction and maintenance of the pontoon bridges and understanding of design and manufacturing limitations will ensure fail-safe structure.

"Pontoon resting on river water keeps on floating freely as compared to pontoons resting on wet sand beds. Sufficient water-buoyancy effect allows a damping to the pontoons and the pontoon load behaves as a sprung mass. Reaction to pontoons by mud and sand is different as compared to reaction by the flowing water and with a moving vehicle, the possibility of unequal displacement at various support points of a particular pontoon resting on wet sand bed can not be ruled out," he said.

Dr Goyal said that the unequal load leading to difference in displacement can ultimately lead to over-stressing of the connections joining pontoon with I-channels and that could even lead to the brittle fracture from unknown locations.

"Moreover the rectangular irregular and shaped rough-edged weld cut made slot too behaves differently as compared to a smooth drilled circular hole with smooth surface under stress.

Besides the propagation of the crack from the edge of the slot with the application of load will also be different. The irregular shaped rough edged surface along with heat affected zones created by weld cut marks provide an ideal environment for the fracture to occur under dynamic loading irrespective of the so-called safe oversized dimensions," he warned.

Among the top suggestions to improve the situation, Dr Goyal suggests the engineers concerned to ensure that all the joints between pontoon and steel channels are flexible enough to allow hinge type of connection which demands proper lubrication at the joints.

"Care needs to be taken to avoid fretting fatigue in all the cases besides taking metallurgical aspects too under consideration at every stage. The present fasteners need a fresh look in the long run as a metallic rope fastener will absorb shock and will also allow flexibility in the joints preventing the over-stressing of the holes and thus minimise the possibility of a fracture," he said.

Dr Goyal said that only machine drilled holes should be allowed in place of irregular shaped weld cut slots being used at present.

"Over and above the design limit of safe vehicle speed on the temporary bridge should be compulsorily checked and the on-site engineers need to remember that synthetic fibre based ropes behave differently as compared to natural fibre ropes,” he said.

Other than breaking strength, they also need to take into consideration surface friction and flame retardation characteristics," he added.