In the June 2012 issue of Ralway Age, page 8, the following news item appears:
PATH: Awarded a $10 million contract to Skanska Koch, a division of Skanska USA Civil, to install more than 175 steel ribs along the North-South Concourse of the new World Trade Transportation Hub.
What are steel ribs?
Steel shapes - like I-beams, "Wide Flange’ beams, H-piles, and others - that are used to support/ restrain the side walls and tops / roofs of tunnels and similar large and deep excavations, such as the ‘bigger than a tunnel’ needed for a station here. Usually curved and roughly circular/ arched in shape, but not always. Link to some decent photos of steel ribs, from some Puerto Rico tunnels in 2006:
http://deereault.com/tunnel-design/puerto-rico-highway-53-tunnels.php
Tunnel Steel Sets? … the term “ribs” is what seems odd. None of the ones I supervised the placement of was radial, all were angular multi-sided steel shapes. Ours started at the spring line on the tunnel wall and went up.
Paul,
Ah! A picture is worth a thousand words. Yes, I now understand what these ribs are and what they are meant for. Thanks.