The Commissioner of Railway Safety (CRS) has asked the state government for a report on how much wind force the proposed Majerhat bridge could sustain.

The CRS, which functions under the civil aviation ministry, will only approve the design of the bridge after it is satisfied with the report, sources at Nabanna said.

Commissioner of railway safety Mohammed Latief Khan had recently written to Sanjay Banerjee, the chief bridge engineer of Eastern Railway, seeking the report for better understanding of the design.

Banerjee later forwarded the letter to the PWD, which is rebuilding the Majerhat bridge. The earlier bridge had collapsed on September 4, 2018.

Railway and PWD engineers said the letter had wanted to know whether the design of the proposed bridge had factored in the risks that strong winds could pose to the structure.

Sources said the government would have to conduct a “Wind Tunnel Test” to ascertain how much wind force the proposed bridge could tolerate.

Structural engineers The Telegraph spoke to were divided on the necessity of the test.

No bridge or flyover in Calcutta, except Vidyasagar Setu, has undergone the test, some engineers pointed out. IISc, Bangalore, had carried out the wind tunnel test for Vidyasagar Setu.

The test is an aerodynamic study of a bridge where the stress on the structure caused by a certain wind velocity is mathematically mapped. “A prototype of the bridge will have to be made before stress-point calculations are made,” a PWD engineer said.

Data with meteorologists show that the city’s average wind speed is 7.4mph between March and early September. Between September and March, it is 4.9mph.

Many structural engineers this newspaper spoke to said the Majerhat bridge would not be so high that wind could impact its stability.

“Does Calcutta experience such wind force that can topple a flyover or a bridge? You can always ask for the test results but it can’t be the deciding factor,” said Samiran Sen, an engineer on the state government’s panel that is studying the health of bridges and flyovers across Calcutta.

Railway sources said the CRS had sought the report because the proposed bridge would be a cable-stayed structure over railway tracks.

The deck slab of the proposed bridge would be quite long. So, it is necessary to find out how the cables would sustain the bridge amid rough winds, railway officials said.

A section of independent engineers is in favour of the test. “The interaction of wind with a bridge is always worth calculating. The fact that it has not been done for other structures can’t be the reason why it shouldn’t be done for the proposed structure at Majerhat,” said Somnath Ghosh, professor of civil engineering at Jadavpur University.