Superstructures

The superstructure of a bridge is made up of the portion of the bridge built on top of the substructure and supports the bridge deck. Several materials and structural configurations can be used to make up the superstructure of a bridge, but this report will focus more on short span steel bridges. A publication from the Short Span Steel Bridge Alliance provides different short span bridge superstructures using different steel configurations depending on spans that the bridge must support. This section will describe several different steel superstructures, illustrate the different structure types and evaluate the different systems for short span modular steel bridges.

Corrugated Steel Pipe


Description

Corrugated steel piping is a form of prefabricated steel superstructure that can be installed rapidly. Due to newly developed steel grades with many beneficial properties, a steel superstructure like this can be lightweight, strong and cost efficient. The Short Span Steel Bridge Alliance brochure recommends this type of superstructure for spans under approximately 15 feet. An example of Corrugated Steel Pipe can be seen in Figure 19. [44]


Figure 19 Corrugated Steel Pipe for Bridge Superstructure [13]

Application

The Short Span Steel Bridge Alliance brochure implies that this alternative can be applied to spans under approximately 15 feet to support the bridge deck and applied live loads. [44]

Constructability

The corrugated steel pipe is secured to the adjacent soil through the use of anchor bolts. The sections that make up the pipe are also bolted together. Couplings are used to prohibit soil and water from getting through the sides of the corrugated steel pipe. Reinforcement may be applied to the pipe to provided extra strength. Backfill and an earth retention system is used to make up the rest of the structure that supports the roadway. [30]

Evaluation

Corrugated pipes are available with different levels of coating that can provide service lives of up to 100 years. These pipes also come in a variety of sizes providing a variety of lower-end spans to which they can be applied.

Research Needed


Corrugated Structural Plates


Description

Corrugated structural plates are another prefabricated steel option for a superstructure. These structural plates are formed in such a way to support the rest of the bridge structure and still allow for the traversed travel way to be usable. The Short Span Steel Bridge Alliance brochure recommends this form of steel superstructure for spans between approximately 5 and 60 feet. An example of a bridge using this type of steel superstructure can be seen in Figure 20. [44]


Figure 20 Corrugated Structural Plate as a Bridge Superstructure [15]

Application

The Short Span Steel Bridge Alliance brochure implies that this alternative can be applied to spans between approximately 5 and 60 feet. [44]

Constructability

The plate ends are bolted or anchored to the bridge footing to support the plate. Bolts are also used to connect the sections of the corrugated steel plate and connect the section to the end treatments. Reinforcement is generally added to the plates in order to provide extra strength to the structure. Earth retaining structures and backfill make up the rest of the structure to support the roadway. [30]

Evaluation

These superstructure systems are cost effective and quick to install. There are a wide range of designs that allow for these to be used on a variety of spans.

Research Needed

Of the several different reinforcing ribs being used to stiffen structural plate culverts, only a select few have published composite properties. There is a need for research in the area of the degree of composite action of ribs with structural plate culverts. This research can lead to a more efficient use of the combined strength of the materials and aid in developing more cost efficient designs. [25]

Big R Bridge (Super-Cor®)


Description

The Modular Bridge Company Big R Bridge has developed a unique alternate version of the corrugated structural plate bridge. In the Super-Cor® Bridge, the corrugated plate is replaced by large annular corrugations. These lightweight panels provide more stiffness than a conventional structural plate bridge. The panels are easy to transport and required significantly less bolts than the conventional steel plate. The panels are light enough that they can be assembled next to job-site and then moved into place by relatively light equipment. This system also has the advantage of being adaptable; it can be widened easily by adding more panels and adapting the rest of the structure. An example of one of a Super-Cor® Bridge can be seen in Figure 21. [8]


Figure 21 Double Super-Cor® Bridge by Big R Bridge [15]

Application

Big R Bridge states that the Super-Cor® bridge superstructure can be used for spans exceeding 82 feet. The superstructure supports the deck and applied live loads while allowing for traversing traffic underneath the bridge. [8]

Constructability

The Super-Cor® panels are bolted together and are connected to the footing through either bolts or anchors depending on the footer material. Earth retaining structures and backfill make up the rest of the bridge structure that supports the roadway. [8]

Evaluation

This system can be built quickly and has all of the same benefits as the Corrugated Structural Plates. This system has the added benefit of being easily widened by adding more of the angular plates used to make the initial structure. Also, with the light weight, being able to construct the clearing and then move it to the required location can be beneficial in lessening the time for traffic impact.

Research Needed


Wide Flange Shapes


Description

Wide flange shapes are used as a common superstructure element for bridges between approximately 20 and 90 feet. These elements are aligned parallel to traffic flow under the bridge deck to support the loads of the bridge. Generally the deck is attached to the girders in such a way to make the deck and girders behave cooperatively as composite members. While in longer spans the unit weight of steel used for the bridge can be higher than that of steel plate girders, the unit cost of steel is much lower for rolled members. Transverse stiffeners are not normally required for rolled sections and simple diaphragm details aid in making rolled sections an affordable superstructure. An example of a wide flange rolled steel bridge is provided in Figure 22. [9]


Figure 22 Wide Flange Rolled Steel Shapes as Bridge Superstructure (U.S. Bridge Tour)

Application

The Short Span Steel Bridge Alliance brochure implies that this alternative can be applied to spans between approximately 20 and 90 feet. The superstructure supports the deck and applied live loads and provides clearance for traverse beneath the bridge. [9]

Constructability

Generally, for span lengths less than 200 feet (all bridges considered in this report), girders can be erected with little to no falsework. During erection, pier brackets are often used to provide stability to negative moment sections of the bridge until the positive moment sections are erected. [9]

Evaluation

Rolled steel wide flange sections used as the superstructure of short span bridges can be more cost effective due to not required transverse stiffeners and simple diaphragm assembly. The unit weight of steel for the bridge is higher than that of plate girder bridges, though.

Research Needed


Plate Girders


Description

Steel plate girders are one of the most common steel superstructure elements. When used in a bridge structure, the plate girders are installed parallel with the direction of traffic. Floorbeams are placed transversely under the deck to distribute the bridge loads. Similar to rolled steel wide flange members, the deck is placed causing the deck and girders to act as composite members. The shape of steel plate girders differ from rolled sections in that rolled sections are doubly-symmetric "I-shaped" sections and steel plate girders can be detailed to be more efficient and are generally only singularly-symettric. These customizing options cause steel plate girders to have a lighter unit weight. The more difficult diaphragm details and the need for transverse stiffeners lead to this choice not always being as cost-efficient as rolled sections for a wide range of short span situations. An example of a bridge using steel plate girders is provided in Figure 23. [9]


Figure 23 Steel Plate Girders as Bridge Superstructure [9]

Application

The Short Span Steel Bridge Alliance brochure implies that this alternative can only be applied to spans between approximately 60 and 140 feet. The superstructure supports the deck and applied live loads and provides clearance for traverse beneath the bridge. [9]

Constructability

Generally, for span lengths less than 200 feet (all bridges considered in this report), girders can be erected with little to no falsework. During erection, pier brackets are often used to provide stability to negative moment sections of the bridge until the positive moment sections are erected. [9]

Evaluation

This system is more efficient in steel weight per unit length than a rolled steel girder system but is not always as cost effective. Similar to rolled steel sections, this system acts as a composite section with the deck.

Research Needed


Steel Truss Bridge


Cambridge Steel Truss Bridge


Description

The superstructure of a Cambridge Steel Truss Bridges is made up of the two truss structures on the sides of the bridge. Despite the trusses being composed of discrete members (arranged to form triangles) that are subjected primarily to axial loads, the two trusses generally react like two large support beams. Floorbeams are attached to the truss and run perpendicular to the flow of traffic to support the bridge loads that are distributed by stringers that run parallel with the flow of traffic. The top and bottom members of the truss system, chords, are often attached laterally to provide stiffness and resistance to wind loads. For the Cambridge Steel Truss Bridge, the top chords are generally arched. An example of a Cambridge Steel Truss Bridge is provided in Figure 24. [9]


Figure 24 Cambridge Steel Truss Bridge [45]

Application

This type of Truss System is installed along the sides of the bridge deck with floorbeams connecting the bottom chords to support the deck. This type of superstructure can support bridges of varying spans. [9]

Constructability

The members to be assembled are lighter for a truss system than those used for rolled steel girders and plate steel girders. There are of course several more members to be assembled in a truss system than in other superstructure methods. Because of the lighter member size, smaller cranes can be used in the construction process. The elements are connected to one another using bolted connections. For simple span trusses, falsework towers are usually required to facilitate erection. For continuous trusses, a cantilever erection can be used using falsework towers near the interior piers. [9]

Evaluation

Cambridge Steel Truss Bridges are considered highly aesthetically pleasing. The erection process can be much more complicated than that of steel plate girder bridges. Some companies are transporting the trusses as prefabricated elements to the bridge site, quickening the bridge construction process.

Research Needed


Warren Steel Truss Bridge


Description

This superstructure system is similar to the Cambridge Steel Truss Bridge system in that it consists of two trusses acting continuously between the abutments of the bridge. Again, the trusses are made up of top and bottom chords with axially loaded discrete members between them. This truss system differs from the Cambridge system in that the top and bottom chords are parallel and all of the discrete sections are arranged in a way to create inverted alternating equilateral triangles. An example of a Warren Truss Bridge is provided in Figure 25 and a view of a typical section of a Warren Truss Bridge is provided in Figure 26. [22]


Figure 25 Plan of a Warren Truss Bridge [22]


Figure 26 Typical Section of Warren Truss Bridge [22]

Application

This type of truss system can be applied to the sides (similar to the Cambridge Truss) or underneath (as shown in Figure 26). [22]

Constructability

For the Warren Truss Bridge shown in Figure 26, the truss members are prefabricated in sections. The diagonals are welded to the top and bottom chords. The truss sections are delivered to the job-site by truck to be assembled. During erection the sections are supported by permanent pier or temporary support. The trusses will be used to support the falsework to be used for the deck placement. [22]

Evaluation

Similar to the Cambridge Steel Truss Bridges, Warren Steel Truss Bridges are considered aesthetically pleasing. Also these bridges can be more complex to construct unless set as a modular system.

Research Needed


Steel Space Truss Bridge


Description

Where the last two truss systems involved planar trusses, steel space trusses are constructed to be three-dimensional. For this truss scenario, the truss is composed of one chords connected in three planes by the axial members to form a triangular shape. These superstructure elements can be difficult to use for bridge construction unless they are installed as modular sections. An example of a steel space truss bridge is provided in Figure 27 and a view of a typical section of a steel space truss bridge is provided in Figure 28. [22]


Figure 27 Elevation View of Space Truss Bridge [22]


Figure 28 Typical Section of Space Truss Bridge [22]

Application

The space truss system is applied to the bottom side of the bridge deck (as shown in Figure 28). [22]

Constructability

For the space truss bridge shown in Figure 28, the truss is prefabricated in the form of modular units. These sections are transported to the bridge site by truck. The modulated units are installed using erection beams or temporary falsework. Erection beams would be installed between abutments and piers to support the modular sections and lessening traffic disruption. The deck can then be installed atop the superstructure. [22]

Evaluation

As other steel bridge truss systems, they are considered aesthetically pleasing. Due to the three-dimensional truss system, these can be difficult to construct on site unless the elements are installed as modular sections.

Research Needed