I can add to the news item on the glass bottom bridge at Kanyakumari, and I will

 

I can add to the news item on the glass bottom bridge at Kanyakumari, and I will

Per Google, there are at least three glass bottom bridges in India. Therefore, you may think that the chest-thumping that is going on in Tamil Nadu is, in Shakespearean terms, Much Ado About Nothing.

You would be wrong. That play was a comedy of errors, but the bridge is not a comedy of errors but of doing things right. I know that the general public would not be interested in how the loads are carried but only that they are carried to the supports. As a structural engineer, I hope to interest a few structural engineers.

Look at the picture below (ignore the tower on the left end of the bridge, a requirement during construction; nothing more):

In a typical arch bridge one would see the deck being supported by vertical hangars from the arch. There are no vertical hangars here. Then, how does the rib of the arch takes the load on the deck? There are arch strings, which are not vertical. The best part is the tension strings connecting the deck and the arch are so thin, they become almost invisible – the best architectural feature of this type of arch bridges.

What are these called? Network Arch Bridges. I will try to explain in simple terms how the Network Arch Bridge, conceptualized in the 1950s, differs from the conventional arch bridge in which vertical hangars are used. In the latter, if a load, say a truck on highway is on near the left end of the bridge, only the segment of the arch around the load point(s) on the bridge will take the load. Because of this, the part far away from the load will not only not carry any part of the truck load but may lift up, relieving the tension in the hangar. Though the strings are given an initial tension (called pre-tensioning), this structural system is neither effective nor inefficient.

Now, Network arch bridges are substantively different, though they may not appear so. Given below are pictures of and also diagrams to bring in conceptual clarity.

One may take in merely the hint of the slant strings towards the right end of the bridge. Even the bridge itself would vanish if one were to look at from the middle of the water-way! The deck is slim and so also the ribs. The figure below gives the schematic of the structural system.

The critical points are on the arch rib where the strings attach to it.

I will try not to go too deep into the mechanism. Each nodal point on the rib carries two strings instead of one as in the conventional arch (except at the very ends).

In the above sketch showing the deflected shape, the bottom cord does not lift up anywhere along the span and hence all strings will be in tension, even at or near the supports. The rib’s dimensions are also much reduced (from basic engineering mechanics course).

This is all I want to say about the glass-bottomed arch bridge joining two rock outcrops (of significant size) out in the middle of the sea. This is truly the first for India, and thanks to some state department officials (Punjab and Haryana) to whom the network arch bridge was mooted but did not bite. Hence Tamil Nadu has the first Network Arch Bridge that is glass-bottomed.

Raghuram Ekambaram