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  • Writer's pictureRaj Singh

Constructability in Bridge Design #5 - Consideration of a Feasible Erection Methodology in Bridge Design

Updated: Mar 15

As bridge design engineers, our responsibilities extend beyond analyzing and designing the final structure. An important part of our scope involves ensuring that there is at least one feasible construction method for the bridge. This requires an understanding of the local construction industry practices and the ability to foresee potential construction challenges.

Progressive cantilever erect of 199A Viaduct mainspan
Progressive cantilever erect of 199A Viaduct mainspan

Outlining how the main components of the structure will be erected and temporarily stabilized can significantly influence design considerations. While this might not be as apparent in girder bridges, which are typically erected using cranes, it becomes essential in structural systems where the erection sequence affects the stress state and geometry of the final structure. If the designer assumes that the final bridge simply appears in its final stage as a fully constructed structure, the calculated dead load stresses may differ from the stress state considering staged erection. For instance, the main-span of the 199A Viaduct, erected in a progressive cantilever, has nearly zero dead load stresses at midspan. However, if analyzed as a completed structure, it would show a positive dead load moment at mid-span which would overestimate the demand at midspan while underestimating the negative moment demand at the piers. Refer to CASE STUDY: Influence of Railway on the Design & Construction of 199A Street Bridge.

The selected construction method doesn't have to be the most cost-effective. However, it should align with the common industry practices in the region. This alignment ensures that the design is compatible with the existing skills and resources of the local construction industry. The contractor may opt for a different erection methodology. In this case, their erection engineer would need to demonstrate that the partially built structure maintains strength and stability throughout the construction stages. They must also ensure that the final stress state and the geometry of the completed structure fall within the designer's specified parameters or stress envelope.

When it comes to the erection of the superstructure, it's important to select a practical approach for each conceptual option. Each approach may have its benefits and drawbacks, and the choice depends on the specific requirements of the project. Some common industry practices for girder bridge erection include:

  • Conventional Crane Erection: Concrete girders are typically lifted individually, often requiring a tandem lift with a crane at each end. Steel girders may be lifted individually, in pairs, or as multi-girder assembled units. Rather than being lifted in full length, steel girders are commonly lifted in segments and then spliced in the air. As a recent example, approximately 2700 tonnes of structural steel were erected across 18 girder lines for the MLX MacKay River Bridge using a 300-ton crawler and an 825-ton mobile crane. Click for information here.

MLX bridge girders erected by cranes
Crane erect of MLX bridge girders

  • Crane Assisted Launch involves using a crane or multiple cranes to help launch or position girders, into their designated locations. This method is often used when the components are too heavy or large to be lifted entirely by the crane. Instead, one end of the component is lifted by the crane, while the other end is maneuvered into position using methods such as being positioned on a truck or pushed along rails by an excavator. As a recent example, on the new Athabasca River Bridge a 200-tonne, 71-meter-long girder pair was launched from the staging area behind the west abutment. Click for more information here.

Schematic of a crane assisted launch
Schematic of a crane assisted launch

  • Incremental launching is a method where bridge superstructure segments, typically girders, are assembled at one end of the bridge and gradually pushed across the spans. This assembly occurs in a staging area known as the Launch Bed, located behind the abutment. The segments are then slowly moved or "launched" over the bridge substructures on temporary supports until the entire span reaches the opposite abutment. This technique is especially beneficial for constructing multi-span bridges across rivers, valleys, or roads, where traditional crane lifting could be problematic. One major advantage of this method is the minimal impact on the environment and surrounding infrastructure.

Schematic of an incremental launch of bridge girders
Schematic of an incremental launch of bridge girders

Assembled girder segments with launch nose for the Athabasca River Bridge at Ft. McMurray
Assembled girder segments with launch nose

If an incremental launch is considered suitable, the design engineer should perform a staged analysis. They must verify that the girders and substructures have the required capacity. They should also ensure the girders are appropriately detailed for this erection method. All assumptions, including the weight and length of the launch nose, the use of intermediate supports, and girder capacity envelopes, should be clearly documented in the design plans.

Another aspect of our role may require maintaining the operation of traffic lanes along the existing roadway and bridge during the construction of the new replacement bridge. This requires careful planning and coordination to determine construction staging options that allow the flow of intended traffic while the bridge is replaced in a phased manner. Finally, it's essential to verify that there is enough room for construction equipment like pile drivers and cranes. This involves identifying right-of-way and property lines, ensuring there's adequate footprint for the working range of the equipment, and checking for overhead clearance to utilities and other structures.

In conclusion, an engineer's role in bridge design is multifaceted and requires a keen eye for assessing the various erection approaches and incorporating the influence of an assumed method in their design. By ensuring a feasible erection methodology, you are not only fulfilling your responsibility but also facilitating smoother construction processes and better project outcomes.

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