Pamban Bridge - The one-off span to Rameshwaram

A bascule bridge or Scherzer span, is a moveable bridge with a counterweight that continuously balances a span/leaf, throughout its upward swing to provide clearance for boat traffic. It may be single or double leafed and Pamban bridge has two leaves.

The Scherzer span can be opened and closed by cranking wheels which in turn rolls the curved structure on the horizontal platform.

During the cranking operation for opening, the cantilever portion of with rail moves upwards and counter weight comes down.

Typically, a bascule bridge span consists a curved structure (like roller) with a long span on one side and a counter weight on other side. Rails are mounted on the long span. The curved structure rests on a horizontal platform and can roll on this platform. The counter weight is heavier and at a higher level than the span and creates unstable equilibrium (by design) resulting in a rolling effect on the
Scherzer span. Thus, a natural upward lifting tendency is created on the span such that the span does not fall into the sea. On the Pamban bridge, the span weight is 184 tons and the counter weight itself is 234 Tons.

To avoid the lifting up of span due to unstable equilibrium, the span is always kept in closed condition and ratchet mechanism (brakes) arrests upward lift of the span.

In effect, the Scherzer span is a rolling cantilever bridge and the point on which the curved structure rests is the fulcrum.

The fulcrum location is dynamic and changes to various points on the horizontal platform whenever the Scherzer span is opened or closed.

The outer pillars on which these platforms are fixed are specially designed to handle upward tensile forces created by vertical forces on the counter weight.

During train movements, cumulative weight of rolling stock /loco and span weight exceeds the counter weight on the other side. This creates reversal effect is and vertical force is created on counter weight.

Longitudinal, lateral and vertical alignment are to be precise to ensure safe train operations.

Longitudinal alignments are adjusted by local engineering staff, while vertical alignment can be a real challenge.

Scherzer span has male/female inter locking mechanism (called Fan wellage) and while closing the span, bridge operator ensures that this securely locked.

On the night of December 24, 1964, a super cyclone struck the area and the Pamban-Dhunskodi Passenger was washed away by a tidal wave killing all the occupants (around 115 passengers) on the way to Dhanushkodi. The cyclone caused many girders of Pamban bridge to be washed away but the Scherzer span withstood its fury.

After 1964 super cyclone, Pamban- Dhunskodi line was not restored and the village Dhunskodi was declared "unfit for living” by the Madras government.