Prefabricated Modular Steel Bridge Solutions for Accelerated Bridge Construction
Many bridge owners today are burdened with narrow design windows, ever-shrinking design and construction budgets, and tight construction timelines. To serve these situations, we, as bridge engineers, often need to put away the finite-element model and instead rely on a classic, tried-and-true engineering design principle: KISS (keep it simple, stupid). Prefabricated modular steel bridges are founded on this principle by using simple materials, simple geometries, simple analysis models, simple erection equipment and simple installation techniques. However, even the simplest bridge designs require comprehensive engineering evaluation and analysis to maximize the structure’s performance at a minimal cost.
What Is a Prefabricated Modular Steel Bridge?
Prefabricated means the bridge modules are fabricated in a plant and delivered to the field ready to install. A Modular Bridge is fabricated in modules that can be installed quickly in the field. A Steel Bridge utilizes a steel superstructure system. Therefore, a Prefabricated Modular Steel Bridge is a bridge that utilizes a steel superstructure, is fabricated into modules and can be quickly installed in the field. These bridges can use a variety of superstructure types such as trusses, plate girders or rolled girders. They also can be designed for a variety of loading requirements such as vehicular or pedestrian loading. For the remainder of this article, we will be focusing on bridges that use a rolled girder superstructure designed for vehicular loading.
Rolled Girder superstructures will use steel girders that are I-shape rolled steel products, usually a W-shape per ASTM A6. The module typically consists of two or three girders with diaphragms shop-attached between girders, and the deck system either shop-installed or ready to install depending on the deck system used. When delivered, the modules can typically be lifted directly off the truck and into the final location with minimal field assembly.
When to Consider Prefabricated Modular Construction
Prefabricated Modular construction is ideal when dealing with Accelerated Bridge Construction (ABC) needs or requirements. As defined by the FHWA, “ABC is a paradigm shift in the project planning and procurement approach where the need to minimize mobility impacts which occur due to onsite construction activities are elevated to a higher priority.” The use of Prefabricated Modular Steel Bridges fits squarely in the procurement approach. Having the bridge delivered in prefabricated modules rather than individual parts and pieces will dramatically reduce the time needed to erect and open the bridge.
Other factors that may lead to the use of a Prefabricated Modular Steel Bridge include site conditions, manpower, safety, environmental impacts, emergency replacement or if the bridge is temporary. Every project is unique and should be evaluated appropriately when considering Prefabricated Modular construction.
Specification Options Common to Prefabricated Modular Construction
In addition to the type of superstructure, other options that need to be considered are the deck type, bridge rail type, steel finish, bearing type and foundation type.
Deck options include cast-in-place concrete, precast concrete, steel bridge planks, open steel grid or wood panels. Cast-in-place concrete will provide the most flexibility in the design and configuration of the bridge but will be less advantageous for ABC. The bridge modules can be fabricated with stay-in-place (SIP) form deck shop-installed to accelerate construction. Deck rebar then will need to be placed; and the concrete will need to be poured, finished and cured prior to opening the bridge. Cast-in-place concrete can easily incorporate the use of composite design, allowing for a lighter steel superstructure.
Precast concrete deck panels offer advantages to cast-in-place concrete for accelerated construction but will be somewhat more restrictive in configuration and will typically be more expensive than cast-in-place concrete. Composite design can still be used, but it will require that the precast panels be built with pockets to accommodate shear studs on the girders, and the pockets and joints will need to be fully grouted to get composite action. If non-composite design is used, the panels can be set and attached in such a way to allow for easy removal in the future if being used on a temporary bridge.
Steel bridge planks are corrugated structural bridge planks that can be shop-welded to the girders, allowing for a complete module with the structural deck to be delivered to the site. Steel bridge planks will require a wearing surface to be placed on them prior to using the bridge. Wearing surface options include gravel, asphalt, concrete, wood or bar grating. If a wood or bar-grating wearing surface is used, the wearing surface can be shop-installed, allowing for a bridge module that’s complete and ready to carry traffic as soon as the modules are set. A gravel wearing surface can be installed quickly after the modules are set, allowing use of the bridge within hours of its installation. Asphalt or concrete wearing surfaces provide a more durable wearing surface, but take more time to place. However, all these options are still much faster than traditional cast-in-place deck construction.
Open steel grid deck systems can be shop or field- installed by welding directly to the girders or by using a bolted system. This deck system will allow immediate use of the bridge but is typically more expensive than steel bridge planks.
Wood panel deck systems are typically shipped loose and field-installed by use of a bolting system. These decks provide a more aesthetic bridge system but take some time to install and are not quite as durable long-term as some of the other systems.
Bridge Rail Systems
Bridge rails are typically categorized into two types: crash tested or non-crash tested. Crash-tested rail systems have been physically crash tested to AASHTO MASH or NCHRP 350 testing requirements. A crash-tested rail used on a bridge must match the crash-tested configuration, therefore the deck system must be the same as used on the crash test. The majority of crash-tested systems use concrete decks and a few use glue-laminated wood decks, but there’s currently no crash-tested system available for steel bridge plank or open grid deck.
When steel bridge plank or open grid deck systems are used, the bridge rail must be a non-crash-tested system. Non-crash-tested systems can be designed to meet the loading requirements of AASHTO for equivalent crash-tested systems, but they can only be considered as crash-rated and not crash tested. Some agencies such as the U.S. Forest Service have allowed the use of crash-rated rails in place of crash-tested rails to save time and money on projects. It’s important to know the project requirements when choosing a rail and deck system.
Structural steel allows for multiple finish options depending on site conditions. Self-weathering steel is typically the least-cost option when it’s appropriate. Self-weathering steel requires no coatings over the steel surface but will develop a weathering patina over time and with wet-dry cycles. The patina is tightly adhered to the surface and will help prevent further corrosion of the steel. Weathering steel will perform well in appropriate environments, but if used in a more-aggressive environment, it may lead to accelerated deterioration of the steel.
If weathering steel isn’t a good option, the steel can be painted, galvanized or metalized to provide a protective coating over the surface. Painting typically is less expensive upfront than galvanizing or metallizing but will require more maintenance over time. Galvanizing will provide the best protection for the steel but is more expensive, and the bridge sections will need to be sized to fit in a galvanizing tank. If the bridge is too large for galvanizing, then metallizing becomes an attractive option. Metallizing is like galvanizing, except the molten zinc is thermally sprayed onto the surface of the steel.
Bearing types will be determined by the loading and movement requirements at the bearing. Typically, plain or layered elastomeric bearing pads are used on rolled girder bridges.
Foundation systems are similar to conventional construction, but prefabricated foundation components may be used to accelerate bridge construction.
The design of Prefabricated Modular Steel Bridges is very similar to conventional bridge design. Designs should follow the AASHTO LRFD Bridge Design Specifications. For rolled girder bridges, a typical line girder analysis is almost always adequate. Typically, all rolled girders should use the same member even if the interior and exterior girders experience different loading. This straightforward design technique will simplify the work needed to perform a Load Rating analysis per the AASHTO Manual for Bridge Evaluation and per any state-specific requirements, when needed.
Dead loads should include the weight of the steel superstructure, deck system, rail system, any wearing surface and any utilities or miscellaneous loads. When using steel bridge plank decking, it’s important to make sure the wearing surface is included in the design. Typically, around 80 psf of wearing surface load is adequate to cover almost any wearing surface.
When calculating the wearing surface load, you must include the weight of the material inside the corrugations as well as the weight of material above the top of the corrugations. When using a wood or bar-grating wearing surface, it’s always good to include some load for material inside the corrugations, as dirt and debris will accumulate in the corrugations over time. If the deck system includes sidewalks, the design should include the weight of the sidewalk. Dead loads shall be distributed to girders by use of an equivalent section analysis, where the area of deck and wearing surface above the girder is applied to that girder. A moment distribution can be used, assuming a very stiff deck, but it won’t add much value to the design.
Live loads typically will be an HL-93 vehicle as defined in AASHTO. The HL-93 vehicle load will include a HS-20 truck and a 640 plf lane load applied simultaneously and is the only standard design vehicle per AASHTO. Vehicle lanes are typically 12 feet wide; however, bridges that have a clear travel width of 20-24 feet will have two lanes. Pedestrian Live Loading in accordance with AASHTO should be applied to any sidewalk areas. When using a consistent girder size throughout the bridge, the pedestrian load rarely governs the girder design. Owner-specified vehicles also can be considered in the design. Vehicles will be distributed to the girders by use of AASHTO simplified equations when appropriate. AASHTO simplified equations do not cover all deck types or configurations; so, as needed, the Lever Rule, C18.104.22.168.1 from AASHTO, and using a procedure similar to the conventional approximation for loads on piles—otherwise known as the Rigid Cross-Section analysis (see C22.214.171.124d from AASHTO)—should be used.
Live load deflection criteria is an optional requirement per AASHTO. If your bridge is in a sensitive location and user comfort is a concern, then specifying a live load deflection limitation may be appropriate. Length/800 or length/1,000 are the traditional live load deflection criteria, but if the bridge isn’t in a location where live load deflection is a concern, using length/500 is a reasonable limitation.
Other loads such as wind and fatigue loading should be considered in the design of the bridge.
Average Daily Truck Traffic (ADTT) should be identified and will impact the fatigue design of the bridge. The ADTT is the average number of design trucks that will cross the bridge each day during its entire design life. Therefore, if a bridge has a design life of 75 years, and the ADTT is 1,000; then the total number of design trucks anticipated to go over the bridge in 75 years is 1,000 x 75 x 365 = 27,375,000. Note that the ADTT is not the maximum anticipated but the average over the lifespan of the bridge. Often on private or rural bridges, the ADTT may not be known. If that’s the case, the bridge design can be performed to optimize the structure and provide the maximum allowable ADTT. Either way, this number should be documented.
Fabrication of a Prefabricated Modular Steel Bridge should be done in a quality fabrication facility using certified welders. This is typically specified as a shop with a quality certification by AISC as Certified Bridge Fabrication—Advanced (Major) with Fracture Critical Endorsement. If the bridge is to be painted, the shop shall hold a Sophisticated Paint Endorsement as well. The bridge fabricator also should employ a Certified Weld Inspector (CWI) with endorsement by AWS QC1. The CWI should be present during the complete fabrication of the bridge and provide written documentation that the bridge has been fabricated in accordance with the plans and specifications.
The bridge engineer or manufacturer should provide a written procedure for lifting, splicing and setting the bridge. These procedures do not necessarily need to be project specific, but will need to provide enough information for the contractor to determine the site-specific installation procedures. Unless the engineer or manufacturer wishes to take on the full responsibility of the bridge installation, they should leave the specific methods, equipment and sequences of erection used up to the contractor. An experienced bridge contractor will be the best person to decide the site-specific installation procedures, as they’re going to be familiar with their specific equipment and abilities. Since the bridge will be shipped in multi-beam modules, the bridge erection typically requires no additional shoring or bracing, and the modules can be lifted right from the truck and set onto the foundations. The deck systems are either pre-installed or ready to install once the bridge is set.
Steel bridges have been successfully utilized on countless projects throughout the world for centuries. Prefabricated modular steel bridges are about as simple as bridge construction becomes, and yet complex enough to be designed, detailed and fabricated to meet nearly any design requirement or owner-specific need imaginable. The quick bridge installation and reduced effort of installing the deck system will accelerate the construction timeframe on the job, fitting well into ABC construction techniques. Due to these qualities, this simple steel bridge solution has proven time and time again to be an economical option for nearly any crossing application.
Sean L. Johnson, P.E., P.Eng.; and Mike Hemann, P.E., S.E., are with Contech Engineered Solutions: email: firstname.lastname@example.org and email@example.com, respectively.