Building the Bridge
Interactive illustration of a suspension bridge
Hover over the structural elements to learn more
Your Suspension Bridge – Up Close and Personal
Our Tacoma Narrows Bridge and soon-to-be parallel bridge do just what the name
indicates: suspend the roadway or deck by large cables. Those cables, in turn,
pass over the towers that are anchored at either end.
The basic parts of a typical suspension bridge fall into two categories - superstructure
and substructure. The superstructure (super = above) is composed of a deck (or
roadway), two towers, and the main suspension cables. The substructure (sub = below) is composed of the caissons (piers,
or tower foundations) in the middle of the span that support the towers, and
the anchorages (anchors) for the cables at each end of the bridge.
The deck is one continuous driving surface. Large anchors, or anchorages, at
both ends of the bridge act as counter weights that hold the ends of the main
cables. In the case of the new (and old) Narrows Bridge, the anchorages are
large masses of concrete.
In the anchorage, the cables splay into separate strands to distribute the
tension load evenly and safely. The main cables stretch from one anchor over
the tops of the tower and attach to the opposite anchorage. The cables are compacted
strands of parallel wires carried back and forth across the water. At the anchorage,
each cable strand wraps around a strand shoe. Each strand shoe connects to a pair of high-strength anchor rods. Concrete is used to 'cement' the anchor rods into the anchorage.
The main cables are attached to the deck by suspender cables, also called "suspender
ropes" and "hanger cables." In a suspension bridge, the main
cables suspend the deck (roadway). Most of the bridge's weight as well as vehicles
on the bridge is suspended from the cables.
By the time spinning is complete, the strong yet flexible cable will be 20½
inches in diameter. The cables’ flexibility – on this or any suspension
bridge – makes the span susceptible to wind forces. But since the mid-1940s
– in large part due to Galloping
Gertie’s collapse – critical advances in structural engineering
go to ensuring aerodynamic stability and minimizing vibration and swaying in
a suspension bridge under heavy winds.
See glossary for more bridge engineering terms.
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