Architectural Design
II
Project
#1: Form and Function | Pedestrian Bridge
About the Final Design
The two
separate bridges that form my complete bridge are a cable-stayed bridge on the Western
bank and a lift bridge for the Eastern bank. It was important to me that both
bridges remain separated because then instead of splitting apart they would
form together to 'bridge' the gap between the two river banks. Without the
Eastern lift bridge the Western span would be a bridge to nowhere and without
the Western span the Eastern lift bridge would be no more than an over
glorified crane.
Late in the
design process I was inspired by both the concepts of a tower crane and a
gantry crane (but more it's cousin, the overhead crane). I had originally
intended for the support tower to be a gantry crane (but parallel, rather than
perpendicular) and have the whole structure move backwards completely on rails.
But I foresaw problems with balancing the load of the lift bridge so that it
didn't tip forward into the river, since most of the time it would be static. I
also had the idea to turn the whole lift bridge deck out of the way, on top of
a static support tower, but I again foresaw all the design considerations
especially with the mechanical turn mechanism. My hesitation for both concepts
and designs was mostly due to my limited skills in AutoCAD design at the time
and the time constraints of the project. Most of this bridge came together in
the last week and a half before it was due. Once I had my idea for a static
structural tower (which cantilevered over a 45' by 40' opening, allowing a
buffer for the object) with a movable carriage that both lifted and rolled the
bridge deck out of the way, the design just flowed. I also came up with the
idea that all the mechanical function would be housed in the carriage, which
made sense for the lift assembly. I realized that the carriage could move like
a train carriage with all its power fed to it by flexible conduit and the three
motors pulling double duty. It was originally going to be a complicated pulley
assembly with two motors at the base of the back of the tower. I'm glad that I
found a simpler and more elegant solution.
The
cable-stayed tower went through many changes, from a boxy form (much like the
George Washington Bridge, which initially inspired me) into a more open yet
supportive tower. It heavily influenced the lift bridge support tower's design
and compliments it nicely. I originally had the idea to use concrete for the
cable-stayed bridge's support tower, but it was quickly dashed by the
requirement the structure not be hidden (which now I'm glad for). I originally
intended to do the cantilever-spar cable-stayed bridge with the concrete, but
couldn't get it to look right with the steel framing, so I went for the classic
cable-stayed design. The look of it reminds me greatly of the approaches to the
Brooklyn Bridge (another influence) as you walk up to the towers and the cables
seem to rise around you. The observation platform fits in quite well
cantilevered off of the tower for an unobstructed view of both directions down
the river and the lift bridge to keep and eye on its transformation. It puts
some meat on the otherwise light looking tower, which I love.
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