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Bridge
Engineering
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| 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. |
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| 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 truss structure. |
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 An under-truss or deck truss bridge has the truss structure under the deck. |
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| In addition, there are a variety
of 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. |
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| 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. | ||||
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| 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.” |
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| 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.
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