Early Bridges
Early bridges were constructed of timber and stone because those materials were readily available. Most early stone bridges incorporated arches that transferred the weight of the traffic and the structure into the foundation of the bridge. Timber bridges were the first to incorporate beams that carried the load of traffic from one support to another. These supports are often referred to as piers or bents.
Many bridges were made of a combination of timber and stone; for example, the piers could be constructed of stone while the deck was constructed of timber. Trestles, a form of pier commonly used by railroads to support bridges, can be made of timber, steel or concrete.
As the need for longer spans grew, timber was used to create trusses capable of spanning long distances with a minimum amount of material. Trusses are lattices formed of straight members in triangular patterns. These patterns vary and some have been named for the engineers who created them--examples include the Warren, Pratt, Howe, Fink and Parker trusses.
Covered bridges are examples of wooden trusses. There are many early 20th century covered bridges throughout Oregon. Covers protect the bridges from the elements and extend their useful life.
Modern Bridges
Most modern bridges are constructed of various combinations of steel and concrete. These materials allow for the construction of larger, more durable and stronger structures than those made of timber or stone. Many of the early timber bridges in the Portland area were later replaced with steel and concrete structures. For example, the original Morrison Street Bridge was made of timber in 1886 and was replaced by a steel swing bridge in 1905. That bridge in turn was replaced by the current steel lift bridge in 1958.
Steel
Steel has been used in the construction of bridges for many years. Steel beams can be assembled in a variety of ways to provide the necessary strength and span lengths needed to cross the required gap. The SE Circle Avenue Bridge #1 uses steel beams with a concrete deck to cross a small stream. The Hawthorne Bridge uses steel beams, called stringers, to connect the floor beams under the bridge supporting the deck. Large steel girders (beams created by bolting or welding steel plates together) are used to construct long bridge spans. For example, the approaches to the Marquam Bridge use steel girders.
Some of the earliest steel bridges used trusses as the major members for carrying loads. The Hawthorne Bridge incorporates Parker trusses in its design, while the Broadway Bridge uses a version of the Pennsylvania-Petit truss. These bridges are all examples of thru-trusses, where the roadway passes between the trusses. The old Sellwood Bridge was an example of a deck truss, where the roadway rests on top of the truss.
Steel is also used to create other shapes for bridges. The Fremont Bridge is an example of an arch bridge constructed of steel. The Ross Island Bridge is a steel deck truss bridge that incorporates the form of an arch.
Steel Deck Arch
An arch bridge is a bridge with abutments (supports at each end of the bridge superstructure) that are shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. In the case of a deck arch, the deck, or roadway, rests on top of the supporting arches, which in turn are supported by the abutments and piers in the water.
The area between the arches and the deck is known as the spandrel. If the spandrel is solid, usually the case in older masonry or stone arch bridges, it is called a closed-spandrel arch bridge. If the deck is supported by a number of vertical columns rising from the arch, typical in newer bridges, it is known as an open-spandrel arch bridge. For a steel deck arch bridge, the arches and spandrels are made of steel. The abutments, piers and deck are usually made of concrete.
The new Sellwood Bridge is an example of a steel deck arch.
Concrete
Many small bridges today are constructed using concrete beams. These beams can be composed of:
pure concrete
concrete reinforced with steel rods or mesh
concrete that is pre-tensioned or post-tensioned with steel cables
steel beams that are encased in concrete
These various beam assemblies take advantage of concrete’s ability to handle compression loads and steel’s ability to handle tension loads. The bridge carrying SE Hogan Road over Johnson Creek is an example of pre-stressed concrete slab construction. The bridge which carries NE 207th Avenue over the Union Pacific Railroad tracks is an example of reinforced concrete girder construction.
Larger bridges, such as the Glenn Jackson Bridge carrying I-205 over the Columbia River, use segmental concrete box girder construction to span the large gaps between piers to provide support to the roadway above. Concrete can also be used to create arches as illustrated by the SW Vista Avenue Viaduct and the Stark Street Viaduct.
Suspension
Suspension bridges, whose decks are suspended from cables and towers, are used in locations where it is difficult to place piers. The St. John's Bridge is an impressive example of a steel suspension bridge. The large steel towers support the bundles of cables that span the crossing. The deck is supported by additional cables called suspenders attached to the bundles.
The cable-stayed bridge is a new type of bridge that has been recently gaining popularity throughout the country. These bridges use straight cables to connect towers or pylons to the deck. Unlike suspension bridges whose towers are symmetrical, cable-stayed bridges may have one or multiple towers or varying dimensions. The pedestrian bridge at Oregon Health Sciences University and the new TriMet Tilikum Bridge across the Willamette are local examples of cable-stayed bridges.