Short Span Steel Bridge Systems

Some agencies involved with the application of modular bridge technology in the design and construction of short span steel bridges have developed entire bridge systems for rapid and efficient bridge construction. This section will look into some of these specialized bridge systems for short span modular steel bridges, provide illustrations that display these systems and provide an evaluation.

Amcrete (Inverset™)


Description

The first modular bridge system to be discussed is the Inverset™ system produced by the company Amcrete Products, Inc. In this bridge system, a section, consisting of the superstructure and the decking surface, is cast upside-down suspended from wide flange steel girders. This method causes a prestressing effect in the steel girders, and when the section is turned upright for placement, the deck is already in a compression state. An example of an Inverset™ Bridge system is provided in Figure 39. [21]


Figure 39 Inverset™ Bridge System

Application

Inverset™ Bridge Systems are used as a combination of the superstructure and decking system of the bridge. It is connected to the bridge substructure on-site. [21]

Constructability

The bridge modules are transported to the site completely fabricated. Once on site, the sections are installed onto the substructure. [21]

Evaluation

This system allows for a quick and complete installation of the bridge with less connection required. The system acts as a prestressed system due to being cast inverted. Transportation and installation of these systems is made easier by fewer amount of pieces to assemble on site.

Research Needed


Folded Plate Bridge System


Description

The superstructure of this type of bridge is composed of standard shapes built from bending flat steel plates into inverted tub sections using a break press. This type of standard shape has many advantages for bridge owners and steel fabricators. Given the size of the largest press breaks in use today, a bridge with a maximum span of about 60 feet is able to take advantage of this system. The folds in the plates are uniform while the thickness and the dimensions vary depending on the required span. Bending the steel plates is an easy process and with the main variables being the thickness of the plate and where to bend them, a quick fabrication and delivery is possible. An example of the cross-section of a folded plate girder is provided in Figure 40 and an example of the modular section is provided in Figure 41. [7]


Figure 40 Typical Cross Section for Folded Plate Bridge System [7]


Figure 41 Section of Folded Plate Girder Bridge Ready to be Stacked and Shipped [7]

Application

Folded plate girders can be already attached to deck panels in order to be used as a combination of the superstructure and decking systems. This system would be applied to the bridge substructure on-site. [7]

Constructability

The system can be constructed using Accelerated Bridge Construction methods or traditional bridge construction methods. [7]

Evaluation

The inverted tub shape used in this bridge system eliminates the need of cross frames for either global or local stability. Eliminating the need for this extra steel can noticeably reduce the cost of the bridge project. The shape is also user-friendly as it will accommodate the standard types of formwork used for casting concrete. The width of the top flange (normally between 25 and 35 inches) also provides a safer walking surface than that of the traditional wide flange section. Due to the opening of the tub shape being on the bottom of the element, inspection is an easier feat than it is for standard box or tub girder bridges.

Research Needed

Research on this modular bridge system is being performed at the University of Nebraska-Lincoln. The effects of cold bending may be a good research topic to include for this modular bridge system. [7]

Simple for Dead Load and Continuous for Live Load


Description

This system involves placing simple span steel members across the piers initially but adding the required concrete diaphragm later in construction to create a continuous structural system. This system was developed to keep the ease of assembling simple spans but also have the benefits of a continuous structure for when the live loads of traffic are applied. This system eliminates field splices and simplifies the details for over top of piers (which normally consist of various combinations of anchor bolts, sole plate and often expensive bearing types). An example of the simple for dead load and continuous for live load system is provided in Figure 42. [24]


Figure 42 Simple for Dead Load and Continuous for Live Load System [42]

Application

The simple for dead load and continuous for live load system is a special bridge construction process rather than an application of special bridge elements as is for other systems in this section. This system can be applied to any situation where it is beneficial to have simple spans during initial construction and needing the strength of a continuous span during service. [24]

Constructability

To convert the two simple spans to one continuous span, a concrete diaphragm is applied at the pier. The bottom flanges of the two girders are connected by a partial penetration weld applied before the pouring of wet concrete. The concrete is then poured over the pier creating a reinforced concrete diaphragm consisting of small steel reinforcing bars to prevent longitudinal movement. Before the placement of the diaphragm, a thin layer of foam is applied to the pier to separate the diaphragm from the pier cap. [24]

Evaluation

This system has the benefits of assembling a simple span bridge but also has the benefits of carrying live loads with a continuous system. The assembly process is easier and more cost effective than performing field splices and traditional connections over the piers.

Research Needed

The topic of system design and behavior may be a valid research area.

Pretopped Girder Section


Description

This prefabricated bridge system includes combinations of superstructure elements and decks fabricated together before transporting them to the job-site. This system is good for the rapid time of construction it provides; this is due to the bolt connections on-site and the lack of field welding. Some have the negative perception that these bridges are only useful for temporary bridges or that the span must be right for the prefabricated sections available. Pretopped girder sections can be designed to be permanently installed and are specifically designed for the required span. Different groups have developed different methods of pretopped girder bridges. An example of a Big R Bridge is provided in Figure 43, a bridge installed in Virginia is presented in Figure 44and the bridge designed by SDR Engineering Consults is shown in Figure 45. [43]


Figure 43 Assembly of Pretopped Girder Section Built by Big R Bridge [6]


Figure 44 Unloading Pretopped Girder System for I-95 Bridge in Virginia [52]


Figure 45 Precast Modular System Developed by SDR Engineering Consultants [40]

Application

Pretopped girder sections as sections of preconstructed steel framework with bridge decking already installed can be used on the bridge as both the superstructure and bridge deck. This system can be installed to the bridge substructure on-site. [43]

Constructability

All bridge welds are performed during fabrication and not at the bridge site. Bolted connections are used on site in order to connect the bridge segments during installation. These bolted connections allow for easy and quick construction with small crews and light equipment. [43]

Evaluation

This system provides quality bridges that are constructed quickly. Despite the negative perception of this type of short span steel bridge, they can be designed for permanent use and are normally designed specifically for the bridge site.

Research Needed

Ongoing research on the longitudinal and transverse joints between the sections is being performed.

Modular Steel Girder/Cast-in-Place Deck System


Description

The modular steel girder/cast-in-place deck system was presented in a report developed by SDR Engineering Consultants. This system is similar to the pretopped girder system described before except that the deck is not cast before delivering bridge sections to the bridge site. Cold formed steel plates are attached to the steel girders to act as the formwork for the bridge deck. Wire mesh is welded to the cold formed plates to provide reinforcement for the concrete deck that is poured on site. As the bridge sections are brought to the bridge site and placed adjacently, they are bolted to one another. A diagram displaying the bridge sections is provided in Figure 46. [40]


Figure 46 Modular Steel Girders with Stay-In-Place Formwork Plates [40]

Application

The modular steel girder/cast-in-place deck sections are used as the superstructure of the bridge and provide a means of easily pouring the deck without requiring additional formwork. [40]

Constructability

The modular sections are attached to one another through bolted connections. The reinforcing wire mesh is welded to the steel plates. [40]

Evaluation

While this system does not provide the benefit of saving contruction time with a prefabricated deck, it does provide formwork to easily pour the deck soon after the sections have been installed. Connection of steel sections is easy with on-site bolting.

Research Needed


Acrow Panel Bridging System (700XS® System)


Description

The Acrow Panel Bridging System, also known as the 700XS® System, is a light bridge composed of large orthotropic deck units and tall truss systems. The trusses of this type of bridge are 50% taller than alternate panel bridges which provide the bridge with 50% greater bending strength and 20% greater shear strength. The orthotropic deck units can handle heavy wheel loads such as those in the AASHTO LRFD Bridge Design Specifications. These bridges can be easily transported to the bridge site using standard trucks or standard dry ocean containers. These bridges can be erected quickly and easily. An example of an Acrow Panel Bridge is provided in Figure 47. [3]


Figure 47 Acrow Panel Bridge [19]

Application

Acrow Panel Bridges are composed of both the truss systems and deck panels. This system acts as both the superstructure and decking system of the bridge. This can be brought to the job-site and installed on the bridge substructure. [3]

Constructability

There are several methods to install the Acrow 700 XS® Bridge. The most common method is to slide the bridge into place as a cantilever system from the home bank to the end bank. For this method, a launching nose must be constructed at the front of the bridge with rollers. Counterweights are added to the back end of the structure in order to keep the center of gravity from the being past the launch nose. The other common method of installation is lifting the bridge into place with the use of a crane. This option can be more difficult, but if an adequate sized crane is available, it is a plausible installation method. [3]

Evaluation

This bridge system can be transported and installed quickly and easily. Due to the design of the superstructure, this type of bridge is stronger than alternate panel bridges.

Research Needed


Railroad Flatcar System


Description

One economical bridge superstructure option that has been experimented with is the use of decommissioned railroad flatcars as the superstructure of the bridge. This idea has been applied primarily to short span, low volume county roads. For a single lane road one flatcar can provide the entire superstructure, where multiple flatcars can be placed adjacently for wider bridges. An example of a railroad flatcar trimmed to be used as a bridge superstructure is presented in Figure 48. Pictures of the bridge made from the flat car are presented in Figure 49 and Figure 50. [53]


Figure 48 Decommissioned Railroad Flat Car Trimmed for Use as Bridge Superstructure [53]


Figure 49 Side View of Railroad Flatcar Bridge [53]


Figure 50 End View of Railroad Flatcar Bridge [53]

Application

Railroad flatcars are installed onto the bridge substructure. Concrete is then used to create a flat deck. Guardrails can then be attached to the flatcar to provide more safety to the roadway. [53]

Constructability

The flatcar is attached to the abutment through the use of bolting or welding. On a two lane bridge, the flatcars can be attached using threaded rods through the channel between. Concrete is used to fill the channel while pouring the deck. [53]

Evaluation

This system provides an economical option for short span bridges. The superstructure utilizes recycled materials.

Research Needed


Con-Struct™ Prefabricated Bridge System


Description 

The Con-Struct™ Prefabricated Bridge System offers a sleek, modern profile for a 21st century sustainable design. The system incorporates a unique composite element which uses continuous hot-dipped galvanized steel tub girders with a precast concrete deck. The steel tub girders are stressed into a camber in a special forming system and the concrete deck is precast to them, creating the composite element. When lifted and set, the concrete deck goes into compression, eliminating temperature and shrinkage cracks; thereby providing superior protection to the underlying steel reinforcement. The system's precast concrete deck becomes the driving surface, eliminating the need for a field-applied, cast-in-place deck and/or wearing surface. An example of a bridge made with the Con-StructTM system can be seen in Figure 51, and a diagram of a standard cross-section of a module can be seen in Figure 52. To view a video of the cold-bending process, click here. 

            

Figure 51A Con-Struct Prefabricated Bridge Construction

 

           
Figure 51B Final Con-Struct Prefabricated Bridge Superstructure

 

 Figure 52 Example of 6’ Wide Con-Struct Bridge Unit Cross-Section for 60’ Span

Application

Con-Struct bridge prefabricated units are installed onto the bridge substructure.  The steel beam trapezoidal shape requires no welding or cross frames.  The steel beams are hot dipped galvanized to protect the steel from corrosion.  The total superstructure weight of the Con-Struct Bridge System is typically less than a concrete beam superstructure, allowing for increased live loading of an existing foundation. Future widening can be done quickly and cost effectively with minimal effect to traffic as  fascia beams can be produced with "for future widening" deck details.

Constructability

 Individual Con-Struct beams are designed as simple span for dead and live load and are applied to the bridge substructure on-site.  The Con-Struct beams are pre-fabricated with the final driving surface so that no final driving surface is required to be constructed on-site. Once the individual beams are placed on the substructure, they are then tied together by a welded lateral connector and grouted shear key. View a time-lapse video provided by TEG Engineering.

Evaluation

Thin superstructure depths, light weight elements requiring less foundation, durable driving surfaces, rapid deployment minimizing bridge closure time, and 75 year design life. Vehicular and pedestrian spans to 100' are achievable using AASHTO Standard or LRFD specifications. View a video provided by MDOT ("In Field Report").