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Bridge Engineering


abutments: heavy supports at the ends of a bridge, which transfer the thrust
                   from an arch or strut to the bedrock or earth below
beam: a horizontal structural element spanning two supports
compression: the “pushing” force
deck: the bridge surface on which traffic moves
girder: a large beam, usually created by bolting or welding together steel                    plates
pier: a heavy column or pillar which holds up a bridge
tension: the “pulling” or “stretching” force

Common bridge types

Suspension bridge

Suspension bridges involve a continuous girder hung from cables supported by towers erected on piers. They are a very old bridge form, although, in their modern incarnation,
they are widely used where very long spans are needed. The introduction of steel wire cables in the 1830s greatly extended the potential of suspension bridges. At both ends of the bridge, large anchors are placed (usually underground) to hold the ends of the main cables. The main cables are stretched from anchor to anchor over the tops of the towers, passing over a saddle which allows the cable to slide as loads move on the bridge. Smaller hanger cables are used to hang the girder from the main cables. While other bridge types are supported by piers or abutments, here the girder and bridge deck hang suspended from the main cables. Due to the flexibility of the cables, long suspension bridges are prone to vibrate in high winds, much like the string of a guitar or violin.

Examples in the Minneapolis Riverfront District

Existing bridges:
•Hennepin Avenue Bridge

Lost bridges:
•First and second Hennepin Avenue bridges
  Arch bridge 
An arch bridge is based on the ancient concept of spanning an opening with a
curved structural member. The arch transmits the load from the bridge deck to the abutments on both sides of the span and thus to the ground below. Early arch bridges were built of stone blocks wedged together to form the arch. Short modern arch bridges may use wood or concrete, while longer arch spans are built of steel. Since the arch requires no central support, it can be used to bridge long open spans. The arch can be either above or below the bridge deck. The arch pushes downward and outward against its abutments, which must be heavy to resist the thrust. Since the abutments transfer both horizontal and vertical forces from the bridge deck, arch bridges can only be used where the ground or foundation is solid and stable. The curved arch structure offers a high resistance to bending forces.

Construction of an arch bridge is difficult, since the structure is unstable until the two sides of the arch are joined. Until recently, most arch bridges were built with elaborate wooden falsework as temporary supports below each span.

Closed spandrel deck arch bridge
Arch bridges can be constructed with the deck above the arch (a deck arch bridge), or the deck can be hung from a segment of the arch which rises above
the deck (a through arch or tied arch bridge). In a deck arch bridge, the space between the bottom of the arch and the deck can be solid (a closed spandrel deck arch) or open with supporting vertical members (an open spandrel deck arch, such as that illustrated at the beginning of this section).

Examples in the Minneapolis Riverfront District
Existing bridges:
Stone Arch Bridge (St. Paul, Minneapolis, and Manitoba Railway Bridge)
Third Avenue Bridge

Lost bridges:
Third Hennepin Avenue main channel and east channel bridges
  Truss bridge

A truss is a simple, rigid skeletal structure, usually based on a triangle-shaped frame. A truss bridge uses a series of triangles in some sort of superstructure to transfer the load from the deck to the piers. Since the elements of a truss are subject only to tension (stretching) or compression (pushing) forces and cannot handle bending forces, truss bridges are typically best used for straight alignments. A truss bridge can support heavy weights and span long distances, but it requires a fair amount of vertical room to accommodate the truss structure.

There are a number of types of truss bridge. First, the bridge deck can pass either over or under the skeletal truss structure. In a through-truss bridge, the deck is located under (or through) the

Through-truss bridge
truss structure.

An under-truss or deck truss bridge has the truss structure under the deck.

Under-truss (deck truss) bridge
In addition, there are a variety of specific truss structures, some of which have been used in the Minneapolis Riverfront District.

Pratt truss: a simple truss structure with vertical compression members and diagonal tension members. Except for the end sections, the diagonal members all slant down toward the center of the span. Since these members are subject to tension forces only, they can be thinner, allowing for a more economical design. Patented by Caleb and Thomas Pratt in 1844, this was among the most common American bridge types for the ensuing decades and was built in both wood and metal. There are many variations on the basic Pratt truss.

Pratt truss bridge
Howe truss: this truss structure is the reverse of the Pratt truss. Here, the diagonal members all slant toward the closest bridge end, so they are subject to compressive forces. This design necessitates large steel members, rendering it an uneconomical choice for steel construction. This truss type was patented by WIlliam Howe in 1840.

Howe truss bridge
Examples in the Minneapolis Riverfront District
Existing bridges:
Boom-Nicollet Island pedestrian bridge (Wisconsin Central Railroad bridge)
Merriam Street bridge
Portion of current Nicollet Island (Burlington Northern Santa Fe) Railroad bridge crossing main river channel

Lost bridges:
Minneapolis Western Railroad Bridge
Lower (10th Avenue South) bridge
Third Hennepin Island – east bank bridge
First and second Plymouth Avenue (Upper) bridges
Second Hennepin Avenue east channel bridge
First Nicollet Island (St. Paul and Pacific) Railroad bridge
Beam bridge (or girder bridge)

In its simplest form, this is a beam (such as a board or log) laid across supports or piers. The beam must be able to take the heaviest load which may be placed on it, and the weight of the beam (plus its load) pushes straight down on the piers. The load on the beam causes the upper edge of the beam to be pushed together (compression) and the lower edge to be stretched (tension). Many modern beam bridges are composed of beam girders (typically I-beams or box girders) supported on piers. I-beams are simpler and less expensive to fabricate, but box girders (which are literally a long, box-shaped member) are better suited to handling twisting forces (such as would be found in curved bridges) and longer spans. Other modern beam bridges use pre-stressed concrete beams, which combine steel’s ability to handle tension with concrete’s strength under compression. Since the strength of a beam bridge depends largely on the close spacing of piers, this bridge type is generally ill-suited to long spans, unless many beam bridges are linked end-to-end in a “continuous span.”

Examples in the Minneapolis Riverfront District
Existing bridges:
Hennepin Avenue east channel bridge
Nicollet Island (Burlington Northern Santa Fe) Railroad bridge

Lost bridges
Minneapolis and St. Louis Railroad Bridge
First Street South Bridge
First and second Hennepin Island-east bank bridges
First Hennepin Avenue east channel bridge

  Concrete box girder bridge

This modern bridge type uses box girders (see above) made not of steel plates but of concrete reinforced with steel bars. Beams of this type take advantage of the ability of concrete to handle compression loads and the ability of steel to handle tension loads. The box girder segments allow long gaps to be spanned between supporting piers.

Examples in the Minneapolis Riverfront District
Existing bridges:
Plymouth Avenue Bridge

Online bridge resources