During the years 1931-1934, County bridge inspector Thomas Patrick Blum (1893-1942) traveled throughout King County inspecting and photographing bridges. Licensed as a surveyor in Washington state, Blum served as King County’s bridge inspector from 1924 to 1934.
Blum’s photographs show an artist’s eye for composition and detail. They possess a rich quality showing beauty in even the most mundane of subjects. The photos also illustrate the range of bridge styles and engineering methods of the time. Some of the children in the photos may have been his.
This online exhibit presents a sampling of the more than 500 bridge photographs attributed to Blum. The photographs are part of the King County Road Engineer, Bridge Files: Photographs, 1904-1988, Series 474 held by King County Archives.
The images in this exhibit are organized by bridge type. Click on each photograph to view a larger version and read the caption.
King and Queen Post Truss
This style of bridge dates back to Medieval times. The King post truss is triangular in shape, and the Queen post truss is trapezoidal. This type of bridge was usually made of wood.
Deck, Pony and Through Truss
There are three common placements of travel surfaces in relation to bridge structures: deck, pony and through. On the deck truss, traffic travels on top of the structure. On the pony truss, traffic travels between parallel structures that are not joined together with cross braces over the top. On the through truss, traffic travels through the structure, which is cross braced above and below the traffic.
Simple, Continuous and Cantilever Span
There are three basic types of spans: simple, continuous and cantilever. Simple spans cross from one support to another, continuous spans cross from one side to another, and cantilever spans are supported at the ends or in the middle, and can be held up by tensile suspension.
The trestle bridge has an open, cross-braced, rigid framework that is used to support the spans of the elevated structure. Each supporting frame is referred to as a bent. Many timber trestles were built in the 19th and early 20th centuries for the railroad and are still in use today.
A suspension bridge’s rigid deck is hung from vertical suspenders that are attached to a horizontal cable, rope or other material in tension, and then attached between towers in compression. Suspension bridges are particularly suited for spanning long distances.
A truss bridge is a bridge structure that is supported by trusses. Each truss is composed of connected elements forming triangular sections and may be stressed from tension and/or compression. Early examples were made of wood and were found in the form of the King and Queen posts. Modern examples use metal in their construction, and their forms are often named for their designers, such as Pratt, Howe, etc.
Construction of Kummer Cutoff Bridge, 1931-1933
These images document construction of the Kummer bridge, which crosses the Green River, connecting Enumclaw and Black Diamond.
Reinforced concrete slab bridges were and are popular for their relatively low cost, simplicity, and durability.
Although concrete was used by the Ancient Romans, it wasn’t until the late 19th century when the technology of reinforcing it with steel helped concrete become a dominant material for bridge construction in the United States. During the early 20th century, Midwestern bridge engineers James Marsh and Daniel Luten profoundly influenced bridge design with their arched concrete structures. The Boise Creek Bridge, pictured below, is a Luten Arch bridge.
A drawbridge is a bridge that can be moved to allow tall vessels or vehicles to pass through or to prevent crossing. The bascule bridge, shown below, operates through the use of a counterweight that continuously balances the span throughout its upwards swing.
The following resources were used in creating this exhibit:
- King County Road Engineer, Bridge Files: Photographs, 1904-1988, Series 474. King County Archives, Seattle, Washington.
- A Context for Common Historic Bridge Types, NCHRP Project 25-25, Task 15. Prepared for the National Cooperative Highway Research Program, Transportation Research Council, National Research Council by Parsons Brinckerhoff and Engineering and Industrial Heritage, October 2005.
- Bridges of Washington State: A slideshow premier of technology through time. Historylink.org essay 8860.
- Bridge Basics: A Spotter’s Guide to Bridge Design. Bridges & Tunnels of Allegheny County & Pittsburg, PA.