Substructures

The substructure of the bridge consists of the portion of the bridge that supports the entire structure on the given surrounding soil. The design of this can be varying especially due to the different soil conditions for each bridge-site and the weights of the structures differing for each project. Despite the great variance possibilities, some applications of Modular Bridge Technology have been developed. This section will describe some of these applications, provide illustrations and evaluate the application to short span modular steel bridges.

Precast Concrete Cap Beam


Description

Precast concrete cap beams are the most common prefabricated elements in bridge substructures. These are generally the most difficult elements to construct on site using cast-in-place concrete, where shoring and forming can be extensive. This issue shows the amount of benefit that can come from prefabricating the element off-site and only needing to be concerned with transportation and connection of the element on-site. An example of a precast concrete cap beam can be seen in Figure 2. [12]


Figure 2 Precast Reinforced Concrete Bent Cap [12]

Application

Precast concrete cap beams connect to the tops of the piles/columns of the bridge substructure and support the bridge deck. [12]

Constructability

Due to the tolerance of cast-in-place columns and piers, large blockouts in the pier caps have been used successfully. Another type of connection used for this situation is large grouted pockets to develop semi-moment connections. Simple bolted connections can be used as well as a pinned connection. [12]

Evaluation

Due to the time and difficulty involved in the placement of a cast-in-place concrete cap beam, a prefabricated element should be considered; it is easier and faster to transport and connect the element than it is to cast the element on-site.

Research Needed

There is still research that is required in the way of connection details.

Precast Concrete Integral Abutments


Description

Standard abutment construction has the potential to be a long process; therefore prefabrication can provide an excellent opportunity to reduce the overall construction time of a bridge project. With integral abutments, the structure of the abutment is made integral with the elements of the superstructure. The advantages of the integral abutment include: a reduction in bridge deck joints (a common area of deterioration in bridges) and the forces of the soil are transferred into the bridge superstructure, reducing the need for spread footings or multiple rows of piles. These types of abutments can be separated into two categories: fully-integral abutments and semi-integral abutments. Fully-integral abutments are more common and involve the connection between the abutment and the superstructure being a full moment connection. The connection between the semi-integral abutment structure and the bridge superstructure are pinned connections that allow for rotation at the ends of the superstructure. An example of a precast concrete abutment can be seen in Figure 3 and a diagram of this modular bridge element working with a steel superstructure is shown in Figure 4. [12]


Figure 3 Precast Concrete Abutment Stem [12]


Figure 4 Diagram of Precast Concrete Abutment Stem with Steel Superstructure [48]

Application

This system is installed atop the piles of the bridge substructure to support the ends of the bridge while also supporting the adjacent soil. [12]

Constructability

The connection between the abutment stem and steel piles can be accomplished using anchored steel plates that can be field welded or embedding the piles in large pockets to later be grouted or sealed with concrete. To connect to concrete piles, drilling and grouting for reinforcing bars. Similar to the steel piles, pockets and grouting can be used to connect the stems with concrete piles. To connect the adjacent stems, post-tensioning or small closure pours can be used. [12]

Evaluation

Prefabricating an integral abutment can save a noticeable amount of time in a bridge construction. Using these integral abutments, deck joints can be eliminated preventing problem areas for deterioration. This system can also reduce the need for a spread footing or multiple rows of piles.

Research Needed

Connections between the piling and footing and the connections between the adjacent stem elements are still the subject of ongoing research.

Modular Precast Wall Systems


Description

Prefabricated wall panels can be assembled and connected on-site to create modular precast wall systems. The two common forms of this modular bridge technology include mechanically stabilized earth systems and modular block systems. In the first form, mechanically stabilized earth systems, thin wall panels are placed and anchored to the soil behind them. The devices used to anchor the wall panels engage the soil mass behind the wall panels to create a soil mass gravity wall. The process of setting up this type of wall abutment can progress rapidly because the system is built while the soil is still being filled in behind the wall. In the latter system, modular block system, modular reinforced concrete modules are interconnected to build a soil gravity wall. An example of a mechanically stabilized earth wing wall is shown in Figure 5. [12]


Figure 5 Mechanically Stabilized Earth Wing Wall [27]

Application

Mechanically stabilized earth systems are anchored to the soil to help support the soil and support the bridge superstructure. Similarly, modular block systems are gravity walls placed against the soil to meet the same objectives. [12]

Constructability

Modular block systems interlock with each other as they are constructed into a wall. The mechanically-stabilized earth system panels, are anchored along the soil adjacently to create the full wall. [12]

Evaluation

These wall systems provide an efficient construction process. Either, the wall and its anchorages are placed while the backfill is placed, or the wall is built using reinforced concrete modules while backfill is placed. This system can be constructed faster than geosynthetically confined soil wall abutments. Mechanically stabilized earth walls do have the downside of a failure rate of approximately 2-10%. [17]

Research Needed


Geosynthetically Confined Soil Wall Abutment


Description

Geosynthetically confined soil wall abutments are systems that connect the wall and the soil to create a composite structure. To keep the structure internally stable, fabric sheets are used to connect the wall with the soil behind it in the form of a friction connection. Similar to the mechanically stabilized earth systems, these walls are assembled with fabrics being placed within the soil while the backfill material is placed in layers. An example of a geosynthetically confined soil wall being installed is provided in Figure 6 and an example of a geosynthetically confined soil abutment is provided in Figure 7. [17]


Figure 6 Installation of Geosynthetically Confined Soil Wall [17]


Figure 7 Geosynthetically Confined Soil Bridge Abutment [16]

Application

Geosynthetically confined soil bridge abutments attach to the adjacent soil to support the soil and the bridge superstructure. [17]

Constructability

The blocks of the geosynthetically confined soil abutments were stacked in rows while the fabric is applied between the blocks and the soil. This binding to the soil helps connect and stabilize the wall as a whole. [17]

Evaluation

This system is more stable and has a higher safety factor than mechanically stabilized earth systems. The fabric inclusions are lightweight and the installation process is not difficult.

Articles

Deseret News, "UDOT using faster, cheaper technique in bridge building on I-84," by Jed Boal, July 2, 2013. UDOT is the first in the U.S. to use the reinforced soil technique at the abutment of an interstate bridge. Read more here.

Research Needed


T-WALL® Retaining Wall System


Description

The T-WALL® retaining wall system, provided by the Neel Company, combines the design principles of precast concrete modular walls with the gravity wall. The precast concrete, T-shaped wall segments that make up the retaining wall are designed to stack and interlock to create the wall surface. The stems of the "T's" have a friction interaction with the soil backfill placed behind the wall. This method causes the system to act as a stable gravity wall. An image showing the installation of the T-WALL® system is provided in Figure 8 and an image of a bridge where these elements have been installed is provided in Figure 9. [31]


Figure 8 T-WALL® Wall System Installation [31]


Figure 9 Southard Street Bridge, Trenton, NJ [32]

Application

The T-WALL® modules are stacked and arranged to create an earth retaining wall for the substructure of the bridge. [31]

Constructability

The modules are stacked and connected using key elements. The weight of the modules and the friction between the wall stems and the soil hold them in place. [31]

Evaluation

This system combines the ideas behind the modular precast wall and the geosynthetically confined soil wall. Construction of this system is simplified in that only the modules and the backfill need placed sequentially.

Research Needed


Precast Concrete Footing


Description

Few states have worked with Precast Footings in bridge projects. The difficulty in effectively using this application of modular bridge technology is insuring adequate seating on the subgrade. If the seating is inadequate, rocking of the footings and settlement of the foundation are possible results. In consideration of this issue, one can apply flowable concrete or grout under the footing. The grout can either be a flowable fill or a low grade concrete. The strength of the flowable material is not of great importance since the material is simply being used as a filler material. An example of a plan for a precast footing is provided in Figure 10. [12]


Figure 10 Drawing of Precast Footing [12]

Application

Precast footings are installed on the soil to support the substructure and superstructure of the bridge. These are used when soil conditions are adequate to not require piles. [12]

Constructability

For the connection between the precast footing and the subgrade, flowable concrete or grout is used to create adequate seating. One state has used grouted shear key connections to connect adjacent precast footings. A small closure pour can be used as well to connect the footing sections. Due to continuing research, connection between the footing and the piles is specific to the situation. [12]

Evaluation

This system is appropriate when the engineer has confidence in the soil subgrade's ability to support the precast footing. While a filler material can be used, the possibilities of settlement or rocking can be an important issue. This system can work well, but it should only be used when it is safe for the structure.

Research Needed

This prefabricated element is still being researched. Very few states actually have experimented with this technique. More research will take place before Precast Footings are used more frequently. [12]

Precast Concrete Pile


Description

Precast piles are used more commonly than precast footings. Normally, these piles have a square, round or octagonal cross-sectional shape. Precast concrete pile companies have developed standard details for their product. An example of a precast pile is shown in Figure 11. [12]


Figure 11 Precast Concrete Pile [37]

Application

Precast concrete piles are used when soil conditions are not adequate for spread footings. The piles are used to support the rest of the bridge structure upon the soil. [12]

Constructability

The PCI manual "Precast Prestressed Concrete Piles" (BM-20-04) gives details for splicing precast concrete piles. One state has developed a detail for splicing hollow square piles using a reinforced concrete closure pour. [12]

Evaluation

This system provides a driven pile instead of a cast-in-place concrete pile. Cast-in-place piles require more time and preparation.

Research Needed

Research may be needed to investigate the ductility of precast piles with integral abutments.

Driven Steel Piles


Description

Similar to precast concrete piles, driven steel piles have been used to make up the abutments and/or piers of short span modular steel bridges. These piles are driven to the required depth in order to provide support the required loads and a pile bent is installed along the top of the piles to support the bridge superstructure. Examples of these types of piers are provided in Figure 12 and Figure 13. [12]


Figure 12 Driven Steel Piles for Piers/Abutments [11]


Figure 13 Driven Steel Piles for Piers/Abutments [11]

Application

Steel piles are driven to the required depth to support the structure. The portion above ground is braced and topped with a pier cap to create a pile bent that supports the superstructure on. [12]

Constructability

Some states connect the steel piles to the pier cap through welds that connect the tops of the piles to steel plates. Other states have used piles that are hollow with precast pier caps; an anchor system is established between the cap and piles with a closure pour used to finalize the connection. [12]

Evaluation

This system provides a driven pile instead of a cast-in-place concrete pile. Cast-in-place piles require more time and preparation.

Research Needed


Modular Steel Piers


>Description

Modular steel piers are prefabricated braced frame structures based on systems developed initially for offshore platforms. These piers resist lateral forces more efficiently that concrete piers. Installation of this type of pier can be performed in days instead of months required for cast-in-place concrete piers. An example of modular steel piers used in a bridge structure is provided in Figure 14. [38]


Figure 14 Modular Steel Piers [38]

Application

Modular steel piers are used to support the bridge superstructure at intermediate support points along the bridge. [38]

Constructability


Evaluation

This system is more structurally efficient than concrete piers. Elements being prefabricated, installation can be completed at a faster schedule helping reduce the time of traffic impact and costs. The impact to the environment is also minimized.

Research Needed


Precast Pier Box Cofferdam


Description

Constructing pier footings on piles is one of the more difficult processes in the construction of piers over water. Complicated sheeting systems and cofferdams can be involved in this type of construction. Precast concrete pier boxes have been used to dewater areas where drilled shafts connect to bridge footings. These can be used to reduce the need for complicated dewatering systems and deep cofferdams. An example of a bridge pier box can be seen in Figure 15. [12]


Figure 15 Bridge Pier Box (Photo courtesy of Cardi Corporation) [12]

Application

Precast pier box cofferdams are applied as an alternative to sheeting systems and cofferdams that are normally used to dewater areas for the connection of pier footings to piles installed into underwater drilled shafts. [12]

Constructability

In cases, the precast cofferdam has been placed over the pile and sealed with a small tremie pour around the shaft. [12]

Evaluation

In preparation for bridge footings in water, a precast pier box can greatly ease the process of dewatering and connection.

Research Needed


Sheet Pile Wall Abutments


Description

Sheet pile wall abutments are constructed from hot-rolled structural shapes with interlocks on the flange tips. These interlocks permit individual sections to be connected to form a continuous steel wall. Steel sheet piles are characterized by their profile which includes Z-profiles, U-profiles, and straight-profiles. The majority of design involved in using a sheet pile wall abutment comes in determining what type of sheet, vertical and horizontal forces are taken by the sheet piling in this structure, how deep to drive it and determine if and where anchorage devices are needed. Examples of sheet pile wall abutments can be seen in Figure 16 and Figure 17. [14] [41]


Figure 16 Steel Sheet Pile Wall Abutment [34]


Figure 17 Steel Sheet Pile Wall Abutment [33]

Application

Sheet pile walls are used as a bridge abutment alternative. This system supports the soil adjacent to the bridge approach. [14]

Constructability

The plates of the steel sheet piles walls are designed to interlock along the edges while the sheets are being driven into place. [14]

Evaluation

Hot-rolled steel sheet piles are cost effective solution for a piled foundation is required to support a bridge or where speed of construction is critical. Abutments formed from sheet piling are able to act as both foundation and abutment and can be driven in a single operation, requiring a minimum of space and time for construction. The material is lighter and easier to transport than precast concrete panels and sheet piling is produced to meet one of several applicable ASTM specifications. The interlocking steel sheet piling provides a water tight structure and the site does not need to be dewatered before installation is performed. [18]

Abutment structures have their own unique set of exposure conditions, design requirements, service life, aesthetic goals and economic requirements. While some projects benefit from some supplemental corrosion protection i.e., coatings, sacrificial steel, alternate materials, cathodic protection, in many applications steel sheet piling does not require any additional protection. When supplemental corrosion protection is required, there is a wide variety of protection alternatives to ensure the steel sheet piling meets the project requirements. The need for corrosion protection is a function of both the exposure, which determines the projected loss of steel due to corrosion, and the design life of the structure. Local experience with corrosion in similar structures can be a valuable guide in this decision. [18]

Research Needed


SuperSill® Abutments and Back Walls


Description

Developed and implemented by Roscoe Bridge, Supersill® Abutments and Back Walls are another application of modular bridge technology. This system uses a steel spread footing casing that is filled with cast-in-place concrete and a steel soil retaining wall. The system is designed so the bridge assembly can continue even if the concrete truck has not yet arrived to fill the footing casing. The empty casing is lightweight and easier to unload and install than precast concrete footings. An example of the SuperSill® Abutment can be seen in Figure 18. [46]


Figure 18 SuperSill® Abutment and Back Wall by Roscoe Bridge [46]

Application

The SuperSill® Abutments and Back Walls are applied specifically to the ends of Roscoe modular bridges. This system supports the bridge superstructure while also supporting the adjacent soil. [46]

Constructability

The Supersill® Abutment box is placed on top of the piles. Inside of the box is a support system that connects with the piles. The concrete poured into the box, solidifies the system. [46]

Evaluation

This system is easy to transport and install. It considers the variation of cast-in-place concrete arrival. This system also provides the bridge with a modular steel back wall.

Research Needed