Bridges of Washington State: A Slideshow Primer of Technology Through Time

  • By Priscilla Long
  • Posted 10/29/2010
  • Essay 8860

In Washington, bridges are ubiquitous. As of August 4, 2010, there were 9,415 bridges on the Washington State Department of Transportation (WSDOT) inventory. These include all bridges 10 feet and longer, all bridges owned by state and local agencies, and docks and transfer facilities owned by Washington State Ferries. The inventory includes only a few federally owned bridges and it only includes railroad bridges that cross public roadways. Bridges are added and removed from the inventory every year at the rate of about 100 a year. Forest roads and hiking trails also incorporate bridges. This slideshow offers a brief overview of Washington bridges and bridge technology as it evolved over time.

Bridge design is always evolving. Even so, there are only a few basic types:

  • trestle bridge -- the span is supported by a row of simple towers with vertical supports (called bents) leaning toward each other and steadied by cross pieces.
  • beam bridge -- the simplest type is a log placed across a stream
  • cantilever -- a development of the beam bridge. You have two anchor spans, supported by piers. Then two arms, the cantilever spans, extend out from the anchor spans. Some of these bridges have a center span hung between the two cantilever spans.
  • truss bridge -- the supporting truss looks like a Tinker Toy structure: The connecting members or struts (straight pieces) come in different arrangements, such as triangles or an N shape.
  • arch bridge -- favored by ancient Roman engineers using stone and cement
  • moveable bridges -- these include swing bridges, lift bridges, and bascule bridges (or drawbridges) whose spans are raised and lowered like a see saw.
  • suspension bridge -- the deck is supported by cables suspended from towers. The cables run from tower to tower, and the deck is hung from vertical suspenders that hang from the cables.
  • cable-stayed bridge -- A different type of suspension bridge. The cables run from towers directly to the deck, forming an A shape.
  • floating bridge or pontoon bridge -- the original floating bridges may have been canoes tied together to reach across a river.

There are a few terms and concepts that bridge engineers and bridge aficianados and buffs commonly use.

The dead load is the weight of the bridge itself.

The live load is the weight of the traffic on the bridge, whether autos or pedestrians or sheep.

Bridges work by tension (pulling apart) and compression (pushing together). Take a rope or a towel in your two hands and pull it apart as hard as you can. That is tension. Put your hands together as if you are praying and push them together as hard as you can. That is compression. The struts in trusses are in tension and compression.

As you walk, bicycle, drive, or take the train whether in your home town or on a trip, being able to recognize bridge types and understand something of how they are put together will increase your enjoyment of these monumental icons of our built environment. All bridges must stand up and carry their load. Some are ordinary, even homely. Others in their grace and great beauty attain something approaching a work of art. 

Sources: Craig Holstine and Richard Hobbs, Spanning Washington: Historic Highway Bridges of the Evergreen State (Pullman:WSU Press, 2005); Paul Dorpat and Genevieve McCoy, Building Washington: A History of Washington State Public Works (Seattle: Tartu Publications, 1998); Eric DeLony, Landmark American Bridges (New York: American Society of Civil Engineers, 1993); Lisa Soderberg, "Historic Bridges and Tunnels in Washington State," typescript, Washington State office of Archaeology and Historic Preservation, Olympia, 1980, available online (; Richard L. Cleary, Bridges (New York: W, W, Norton with Library of Congress, 2007); Craig Holstine to Stephen Emerson and Priscilla Long, August 4, 2010, email in possession of Priscilla Long.

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