The new West Thames Street Bridge is, in terms of design, something of an oddity. It’s a lenticular truss, and that’s a design that was never very common and hasn’t been used much in something over 100 years. I have no idea why the designers – WYX Architecture and Thornton-Tomasetti Engineers – choose this form, and I’m not looking to say they should have used something else. I am interested in discussing the form itself.
The picture above, of the two main spans sitting on the street before being hauled into position and lifted to their bearings, shows the basic form. “Lenticular” means lens-like, and lenticular trusses have curved top and bottom chords, giving them a (usually symmetrical) doubly-curved shape. Since this type of truss is only used in simply-supported spans*, that curved shape (thick in the middle, thin at the ends) coincides pretty well with the moment curve for the span. That has the effect of making the forces in the top and bottom chords more or less constant. In theory, the designer could make the truss curve exactly match the moment curve in a piecemeal-linear fashion, and the forces in the chords would be close to exactly the same end to end.
The chords carry the main loads, but that doesn’t make a truss. There are verticals, and the intersections between the verticals and the chords are where the chords change direction to create the curves. You can have diagonals running in alternating directions in adjacent panels (the Warren truss configuration), in the tension direction (the Pratt truss configuration), in the compression direction** (the Howe truss configuration) or in both directions in each panel. The last configuration is what was used for West Thames Street, with a nice detail to allow the rod diagonals to pass one another freely:
One of the places where lenticular trusses run into trouble in terms of practical design is where you put the bridge deck. In a bowstring truss (curved top, flat bottom) the deck naturally goes at the bottom; in an inverted bowstring*** the deck naturally goes at the top. With lenticular trusses, you can have the deck in the middle of the trusses, in a sort of double-pony**** configuration, suspended below the trusses, or propped above the trusses.
So, it’s a good-looking form but it’s got some issues. It’s optimized for something not very important (keeping the forces in the chords constant) while it doesn’t provide a natural path for the deck. It’s efficient in itself but has difficult geometry. And the efficiency of the trusses is undercut by the need for secondary structure to attach the deck. None of these issues is earthshaking, but they explain the relative unpopularity of the type.
But it is pretty. The Smithfield Street Bridge in Pittsburgh:
The Washington Avenue Bridge in Waterbury, Connecticut:
And the Neshanic Station Bridge in Somerset County, New Jersey:
* By definition. If a lenticular truss were made continuous at a support between two spans (such as the pier in the middle of West Street) then it would no longer have the lens shape and so would no longer be a lenticular truss.
** If you’re a strange person who like to work harder than is necessary.
*** Also called, in one of my favorite names in structural engineering, a fish-belly truss.
**** A pony truss is one where the deck is at or near the bottom chords but the trusses are too short to be braced against one another above, so they’re free-standing on either side of the deck.
You must be logged in to post a comment.