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It is a heritage structure and a symbol of the former second capital of the British Empire, but Calcutta's Howrah Bridge has become a giant spittoon.

Pedestrians have spat half-chewed betel leaf, areca nut and slaked lime on its steel hangers, corroding the base of the mighty bridge.

To save the bridge, port engineers have come up with the idea of covering the steel hangers with fibreglass.

The bridge was built in 1937 and does not have a single screw.

It has 78 hangers bearing its 26,500 tonnes of steel, as well as the 500,000 pedestrians and half a million vehicles that use it every day.

Officials say corrosion is eating the hangers away, endangering the structure itself.

Thousands of people spit guthka - a chewable mixture of tobacco and slaked lime which often causes oral cancer - on the steel hangers. They also spit paan - betel leaf stuffed with areca nut, slaked lime and other ingredients.

The spitting has become such a menace that "the hanger bases have reduced to 50% of their original size in just three years", said ML Meena, chairman of Kolkata Port Trust, which built and maintains the bridge.

Amitabha Chatterjee, the engineer in charge of maintaining the bridge, pointed to the extent of the damage.

"Look at the base of this hanger. It has become red with guthka and paan spit. The base of the hanger was 6mm, now it is half," said Mr Chatterjee.

Port authorities tried to deter passersby from spitting but without much success.

"Whenever I see anyone spitting on the bridge, I book him for the offence. But it's impossible to keep a vigil on every pedestrian," says duty policeman Parbati Chowdhury.

The authorities said they regularly changed the steel plates and put on chemical coats.

"It didn't work. The steel bases continue to erode," said Mr Chatterjee.

Then came the fibreglass idea.

Mr Meena said: "We decided to put up fibreglass covers around the bases. It's washable. Whenever spit accumulates, we can clean the fibreglass covers. The entire project will be over in three to four months."

The covers had to be custom built. They will be 2m (6.5ft) high and cover the steel bases tightly, so spit cannot seep inside.

But there will also be some divine intervention.

"There will be 'do not spit' signs written on the fibreglass shields, and pictures of gods and goddesses will be added so that people won't dare to spit on those," one official said.