The state government is planning to set up a bowstring girder bridge, designed by the Research and Standards Organization (RDSO), to replace the old Tallah Bridge. Lucknow-based RDSO is a ministry of railways undertaking that standardizes and tests railway projects.
The state public works department (PWD), which will build the new bridge, had recently prepared its general arrangement drawing (GAD) or the master plan. The GAD, illustrating the steel-arch bowstring structure, has to be cleared by Eastern Railway (ER) since the existing Tallah Bridge stands over train tracks. The new one will be built along the same alignment.
Learning from the experience of the ongoing Majerhat road overbridge (ROB), PWD has decided to adopt a design standardized by the railways and thus, cut construction time.
The existing Tallah Bridge, a key link between Kolkata and parts of North 24 Parganas, has lost much of its load-bearing capacity and needs to be pulled down. The railways and PWD will start razing the 57-year-old structure in a month or so.
“Ahead of the demolition, the state PWD, in consultation with railways, has decided to adopt one of the steel-arch bowstring bridges designed by the RDSO, which has several standard drawings for road overbridges of different spans, in its Lucknow office,” a state official said.
A bowstring girder bridge — or tied-arch bridge — has an arch rib on each side of the roadway (deck) and one tie beam on each arches that support the deck. Tips of the arch of this bridge are tied together by a bottom chord. Sources said the new bridge could replicate one of the 50m-span RDSO models, depending on the ground situation and the site conditions. “The time needed for designing the girders, preparation of drawings for the girders and geeting approval of design and drawing by the railways is eliminated. So we are simply adopting a design already approved by RDSO,” a state official said.
With the design of the ROB part settled, PWD can concentrate on designing the foundation and the piers. Soil test of the location where the piers will come up has already been done.
While the existing bridge is three-lane, the new one will have four lanes with three spans. The spans will have composite girders (RCC deck on steel girders). The new bridge is likely to be at least 5m wider than the old one, said an official. The existing bridge has 39 shallow pre-tressed concrete girders placed side by side so closely that it appears as a slab if seen from the bottom. The girders are also laterally pre-stressed.
The new bridge will follow RDSO’s loading standards as well. When the existing one was commissioned in 1962, it was capable of bearing 55 tonnes of weight on each flank with a total load-bearing capacity of 250 tonnes. But the load it was taking in each flank was nearly 90tonnes.
V K Raina, an engineer hired by the state government to study the health of the bridge, recommended razing the structure in his report, saying small vehicles should not be allowed on it for more than two months. Buses, trucks and other heavy vehicles are no longer allowed to ply on the bridge.
Source: Times of India