The First Gussetless Truss Bridge in the World

And the first to incorporate cold bending of steel, and the first vertical-lift bridge to tuck its machine rooms under the bridge.  A triply innovative play!

Designed by Theodore P. Zoli, the first structural engineer to win a MacArthur “Genius” Award, this truss bridge eliminates the gusset plates typically used to connect beams and girders to columns and truss members.  Gusset-plate connections have been problematic in truss design:  they tend to collect debris, to corrode and deteriorate, to be difficult to inspect, and to be impossible to remove and replace without shoring from below.  In the picture below, you can see the gusset plates on the Memorial Bridge connecting Portsmouth, N.H. and Kittery, ME.  Their deterioration contributed to the demise of the 1923, historic structure which required its replacement.

Gusset Plates Connecting Vertical, Horizontal and Diagonal Members*

Zoli’s genius was to replace the gussets with splice plates in a design for the new Memorial Bridge that is cleaner and easier to inspect while allowing for partial splice replacement under load.  Additionally, the gussetless design required significantly less bolting which in turn resulted in faster construction and less costly maintenance long-term.

Splice Plates on the Horizontal and Diagonal Members Eliminated Gussets*

The top and the bottom chords were efficiently fabricated as plate girders with pieces that fit together like a puzzle.  Chicago Metal Rolled Products curved 24in wide 1in thick steel plate as well as 30in wide both with eased edges to the specified radii and the specified straight tangent lengths.  The inside radii were either 20in or a tight 10in.  The customer supplied lengths ranging from 8ft to a very long 34ft 7in.  (This last required a considerable under-hook height to fabricate in the shop.)

These bent plates looked like the letter “V” with a rounded bottom or like giant sleds or toboggans.  Assembled, the curved plate sections formed triangular shapes inserted horizontally (like a sleeve) into the flame-cut plate girders.

24in x 1in Plate Curved with Straight Tangents

Another innovation was locating the machinery to operate the vertical-lift bridge underneath and at both ends of the bridge.  Typically, this machinery would be located in a room built above the roadway at mid-span of the lift truss.  The benefits of the new design include increased protection of the machinery, reduced tower height and truss depth, installation of the equipment before bridge construction, and an open view of all three bridge spans.  The bridge is scheduled to open this summer.

Architectural Rendering of the Memorial Bridge Opening Later this Year

*All photographic images courtesy of Denise Goldberg

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