Short Span Steel Bridge Decks

The decking system of a bridge can be defined as the surface which the traversing traffic drives upon. This is the surface which is used as a continuation of the driving surface on either side of the bridge. Commonly, cast-in-place concrete is used as the method of placing a deck on a short span bridge, but due to the time of preparing formwork this can be a very time consuming process. Prefabricated deck systems are some of the most commonly used applications of modular bridge technology. This section will go into greater detail of describing, illustrating and evaluating different methods of prefabricated deck systems for short span modular steel bridges.

Full Depth Precast Deck Slabs


Description

Full depth precast deck slabs are one of the most common prefabricated deck systems. With this decking system, the deck is poured and cast in section before being delivered and installed at the bridge site. The reinforcing in the concrete deck is generally either mild reinforcement or prestressing. An example of a full depth precast deck slab can be seen in Figure 29. [12]


Figure 29 Full Depth Precast Deck Panels [12]

Application

Full depth precast deck panels are used as an alternative to cast-in-place concrete decks. They provide a driving surface for traffic. [12]

Constructability

In the strength direction of the panels, the panels will be connected progressively and small reinforced closure pours can be used. In the distribution direction of the panels, grouted shear key connections are used. [12]

Evaluation

This system has been used by a number of states already and significant research has been performed to improve the technology. A PCI Bridge Technical committee has published design and detailing standards for full depth precast deck slabs making design easier for the engineer.

Research Needed


Open Grid Decks


Description

Open grid decks can be described as small-scale steel framing systems used as a bridge deck. They are made up of transverse sections to distribute the load across main rail members providing strength to the decking system. An example of an open grid deck is provided in Figure 30. [12]


Figure 30 Open Grid Decking Being Placed [35]

Application

Open grid deck panels are used as an alternative to cast-in-place concrete decks. They provide a driving surface for traffic. [12]

Constructability

The connection between grid panels is made up of bolted or welded connections. Due to the possibility of fatigue issues, bolted connections are preferred. Bolted or weld connections can be used to connect the deck panels to the steel frame. Grouted shear connector pockets are another option for this connection detail. Generally, when steel guard rails are to be mounted on the deck panels, bolted connections are used. [12]

Evaluation

These decks are beneficial for situations where lightweight decks are required, such as movable bridges and suspension bridges.

Research Needed


Concrete/Steel Hybrid Decks


Description

Concrete/steel hybrid decks consist of a combination of the open grid deck and the full depth precast deck panel systems. There are two common forms of this decking system: partially filled grid decks and exodermic decks. The partially filled grid decks involve casting concrete for the lower section of the deck and including the open grid. Later the rest of the deck will be poured on site. The exodermic decks involve the same process as the partially filled grid decks except that a majority of the concrete is placed above the steel grid. These systems act as composite members. An example of an exodermic deck is provided in Figure 31. [12]


Figure 31 Exodermic Deck Details courtesy of the D.S. Brown Company [12]

Application

Concrete/steel hybrid deck panels are used as an alternative to cast-in-place concrete decks. They provide a driving surface for traffic. [12]

Constructability

To connect the separate panels to one another, bolted or welded connections are used. Since the deck is very similar to a full depth precast concrete depth, welded stud shear connectors are used to connect the deck panels to the steel framing. [12]

Evaluation

With the partially filled grid decks, the deck can be placed without on-site formwork, which is a time-consuming process in bridge construction. With the exodermic deck, the benefit of not having to prepare formwork is again prevalent. This case also has the benefit of the composite action in the deck increasing the efficiency of the system.

Research Needed


Fiber Reinforced Polymer Decks


Description

Fiber reinforced polymers (FRP), primarily used in the aerospace industry, have started being applied to the design of bridges. FRP composites are primarily made up of fibers aligned within a resin material in such a way to make a very strong and very customizable material. The most common fiber choices are glass and carbon fibers. In the use of bridge decking, FRP have been molded into cellular panels that can be installed as full-depth deck panels. An example of an FRP deck panel is provided in Figure 32. [12]


Figure 32 FRP Deck Panel Installation [20]

Application

FRP deck panels are used as an alternative to cast-in-place concrete decks. They provide a driving surface for traffic. [12]

Constructability

To connect the panels to one another, the panels are design to interlock with male-female shear keys. Another option for connecting the panels is the use of high quality epoxy adhesives. To connect the panels to the steel framing, pockets are formed over the beams to allow for welded stud shear connectors and non-shrink grout. Bolts can also be used to connect the panels to the steel framing. [12]

Evaluation

FRP products have the benefits of having high strength, low weight, high stiffness to weight ratio, and corrosion resistance. The deck being prepared in panels, transporting the deck to the jobsite and placing the deck panels is efficient.

Research Needed

Research is needed for the durability of the wearing surface of this type of modular bridge element.

Partial Depth Precast Concrete Deck Panels


Description

The partial depth precast concrete deck panels system involves first placing a layer of deck panels on the steel superstructure and then pouring the remainder of the reinforced concrete deck at a later time. This method prevents the need for as much formwork (normally, the most time consuming part of concrete deck placement) as a cast-in-place concrete deck. An example of a partial depth precast concrete deck panel is shown in Figure 33. [12]


Figure 33 Diagram of Bridge Deck Employing Partial Depth Precast Concrete Deck Panel

Application

Partial depth precast deck panels are used as an alternative to cast-in-place concrete decks. They provide a driving surface for traffic. [12]

Constructability

The panels will be connected to one another when the rest of the deck panel depth is being poured. To connect the panels to the steel framing, welded stud shear connectors are used in the gap between adjacent panels. [12]

Evaluation

With the lower portion of the deck being precast, forming is not required in setting up for the deck system. This system is similar to the partially filled grid deck.

Research Needed

Research on the effectiveness of the composite action between the deck and the girders may be necessary.

Timber Deck Panels


Description

Due to a great amount of study by the United States Department of Agriculture Forests Products Laboratory (USDA FPL), there is a significant amount of information about timber panels and beams as well as standard details for timber bridges. Currently, timber bridges are primarily used on low-volume travel-ways, but the same design idea can be applied to larger volume roads as well. Timber deck panels can be applied to superstructures besides timber. Standard details are available for attaching transverse timber panels to longitudinal stringers. The bridges often incorporate crash-tested railings attached directly to the timber deck panels. Generally, an asphalt wear surface is applied to protect the panels. An example of timber deck panels being applied to a steel bridge is provided in Figure 34. [12]


Figure 34 Installation of Transverse Timber Deck onto Steel Stringers [50]

Application

Timber deck panels are used as an alternative to cast-in-place concrete decks. They provide a driving surface for traffic. [12]

Constructability

These deck panels have been connected to one another using steel dowels placed in the depth of the panels. Currently, load transfer beams are placed mid-bay between the stringers to transfer the shear more effectively. To connect the deck panels to the steel framing, bolts and brackets are used. It should be remembered that this does not cause composite action. [12]

Evaluation

Timber decks are generally used for low-volume roads. When attached to beams, composite action does not take place.

Research Needed

Currently, research is being performed on new waterproofing products to be applied to the top of the deck panels to protect the steel from moisture moving through the timber deck panels.

Steel Orthotropic Decks


Description

Several bridges in the world with long spans have utilized orthotropic steel deck systems in their superstructures. Even though these types of decks have been used extensively in Europe, Asia and South America; the concept has not yet fully been accepted in the United States. With the growing trend towards quicker construction with an overall longer bridge life, the steel orthotropic deck may be an economic solution. If the decks are designed integral with the girders as a common flange, cost savings on designing these components can be realized. This method can lead to a completely steel superstructure which has the potential to provide a long service life. The other leading benefits of this bridge decking system are the minimization of dead load in the design and the rapid construction that will lessen the impact on traffic. An example of a steel orthotropic deck is provided in Figure 35. [23]


Figure 35 Underside of Steel Orthotropic Deck [23]

There have been past problems with steel orthotropic decks especially in the area of fatigue cracking in the weld connections. Welding details are being developed to try and minimize this type of cracking. By their nature, steel orthotropic decks are inherently redundant in their design causing many of these fatigue cracks to arrest themselves. [23]

Application

Steel orthotropic decks can either be added as a decking system to the steel superstructure frame on site or can be prefabricated with steel girders and installed on site as part of a modular bridge system. [23]

Constructability

The multiple elements that make up steel orthotropic deck systems are fabricated off-site to make bridge deck modules that will be assembled and field welded at the bridge site. The sections are generally light enough to place safely with a single crane. [23]

Evaluation

Steel orthotropic decks have the potential to be a great solution for modular steel bridges. Their rapid construction, minimization of dead load and long service life are great benefits that could really help the infrastructure of the United States. Once research provides more efficient means of fatigue crack control in these deck systems and more success stories of this system in U.S. bridge applications accumulate, a trend in the use of this system is likely to develop.

Research Needed

Research on fatigue cracking in steel orthotropic decks is being performed at the ATLSS Engineering Research Center at Lehigh University. [39]

Sandwich Panel Modular Steel Bridge Deck


Description

This bridge decking system is composed of two layers of steel plates attached by welds to an inner layer of HSS steel members. The deck is transported to the bridge-site in 8 foot wide panel sections. The top plate of the "sandwich" is generally a 5/8" steel plate to resist wheel loads and ensure the performance of the wearing surface; the bottom plate of the "sandwich" is generally a 3/16" plate to accommodate for the weld of the sandwich materials. The panels are field welded on-site to remove the bridge joints on the top of the deck and powder actuated fasteners are used to attach panels on the bottom plates. Precast Jersey barriers can then be bolted onto the deck and finally the wearing surface is applied. A diagram of the sandwich panel modular steel bridge deck assembly is provided in Figure 36 and an example of the panel assembly is provided in Figure 37. [54]


Figure 36 Sandwich Panel Modular Steel Bridge Deck System [54]


Figure 37 Sample "Sandwich" Composition [54]

Application

Steel "Sandwich" Panels are used as an alternative to cast-in-place concrete decks. They provide a driving surface for traffic. [54]

Constructability

The panels are field welded on-site to remove the bridge joints on the top of the deck and powder actuated fasteners are used on the bottom plates to attach the panels to one another. The panels can be attached to the steel framing using bolting or grouting. [54]

Evaluation

This system is approximately half the weight of a concrete deck. It is suitable for automated mass production. The deck provides the structure flange bracing eliminating the need for cross frames. The construction time of this type of deck is approximately two weeks.

Research Needed


CANAM (Steel Orthotropic Deck Product)


Description

Orthotropic decks were initially used as a cost-effective and rapid system in the replacement of bridges destroyed in Germany during the Second World War. The technology has grown over the years, especially in Europe and Asia, and has recently begun to be applied to bridges in North America. The steel orthotropic deck product recently developed by CANAM is fabricated into long panels that facilitate efficient transportation and field assembly with a minimum amount of field welding. An example of their steel orthotropic decking panels is provided in Figure 38. [4]


Figure 38 CANAM Steel Orthotropic Deck Panel [4]

Application

CANAM Steel Orthotropic Deck Panels are used as an alternative to cast-in-place concrete decks. They provide a driving surface for traffic. [4]

Constructability

Inverted Ts (as seen in Figure 38) are installed along the longitudinal axis of the bridge to transfer shear and generate composite action between the steel framing and the orthotropic decks. [4]

Evaluation

This type of decking has a service life of up to 75 years. Being fabricated in long panels, transportation and assembly is efficient.

Research Needed