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


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™ Press-Brake-Formed Steel Tub Girder Bridge System


Description 

The Con-Struct™ Prefabricated Bridge System incorporates a unique composite element which combines the strength of a press-brake-formed steel tub girder and the corrosion protection of hot-dipped galvanizing with the durability of a precast concrete bridge deck. The steel tub girders are prestressed to induce camber in a special forming system and the concrete deck is precast to them, creating the composite element. When released from the form, the concrete deck goes into compression, eliminating temperature and shrinkage cracks, and thereby providing superior protection to the underlying steel reinforcement. The system's precast concrete deck is the driving surface, eliminating the need for a field-applied, cast-in-place deck and/or wearing surface. Examples of bridges constructed with the Con-Struct™ system can be seen in Figures 51A and 51B, and a diagram of a standard cross-section of a module can be seen in Figure 52. 



Figure 51A - Con-Struct™ Prefabricated Bridge Construction



  Figure 51B - Final Con-Struct™ Prefabricated Bridge Superstructures

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

Application

Con-Struct™ bridge units are installed directly onto the bridge substructure with the concrete driving surface already included. The press-brake-formed steel beam trapezoidal shape requires no welding or cross frames and comes hot-dipped galvanized for 60+ years of corrosion protection. The total superstructure weight of the bridge unit is typically less than that of a prestressed concrete box beam alone, allowing for ease of installation and increased live loading of foundations. 

Constructability

Individual Con-Struct™ bridge prefabricated units are designed as simple span for dead and live load and are applied directly to the bridge substructure. The Con-Struct™ units are prefabricated with the driving surface, eliminating the need to construct the driving surface on-site. Once the individual units are placed on the substructure, they are tied together by a non-shrink, high-performance grout deck joint. View an installation video provided by TEG Engineering at www.constructbridge.com

Evaluation

Thin superstructure depths, lightweight elements, durable driving surfaces, rapid deployment and a 75-year design life are all features of this system. Con-Struct™ is designed in accordance with AASHTO LRFD Specifications. 

Research

The Con-Struct™ Press-Brake-Formed Steel Tub Girder Bridge System has been independently researched and proven by Michigan State University for the Michigan Department of Transportation (MDOT). In addition to the specific research performed on the Con-Struct™ system, press-brake-formed steel tub girders are under continued research by West Virginia University and Marshall University for the Short Span Steel Bridge Alliance (SSSBA). This research has focused primarily on moment capacity, live load distribution factors and fatigue characteristics of the press-brake–formed steel tub girders.