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  • Writer's pictureRaj Singh

Constructability in Bridge Design #3: A Focus on Site Access

Updated: Mar 14

Introduction

Constructability is a critical element in bridge design, significantly focusing on how constructors access the construction site. This factor requires a thorough assessment of access restrictions and time limitations that could impact the cost and feasibility of the foundations. Consequently, it affects the span arrangement and, in turn, the structural system best suited to the circumstances.


Challenges in Accessing Specific Locations

Construction in areas with difficult access, such as deep valleys, entails a unique set of challenges that require careful planning and creative solutions. These sites necessitate not only intricate logistical planning but also the deployment of specialized equipment and enhanced safety measures. For example, the creation of temporary access ramps or trestles is crucial to facilitate foundation work in these hard-to-reach terrains. Furthermore, it can be complicated to arrange access for larger superstructure elements such as girders at these sites. This includes the provision of heavy lifting equipment and large cranes for their erection. This often requires the utilization of overhead installation techniques like incremental launching or progressive cantilevering, particularly when the use of large cranes is unfeasible due to space limitations or the nature of the terrain.

Consider the Sombrio Bridge, a two-lane, two-span bridge along Highway 14 on Vancouver Island with spans measuring 40m and 82m. The uneven span arrangement was dictated by the terrain and a geotechnical instability that necessitated a longer span to cross a deep ravine. The positioning of the pier at the crest of the steep northern slope necessitated meticulous planning. It was strategically placed to ensure reasonable access from the north abutment, facilitated by a temporary access ramp. The steel girders of the bridge were designed and detailed to facilitate an incremental launch procedure for erection from above.




Addressing Environmental Limitations

Environmental components can impose constraints on construction activities. For instance, 'fish or ice windows' represent specific periods when in-stream work is allowed. This strategy is implemented to reduce disruptions to local aquatic ecosystems or prevent hazardous work conditions due to ice formations. The construction of both the Deh Cho and Fort Nelson River Bridges, situated in northern regions with severe cold weather, involved evaluating the feasibility of erecting the bridge superstructure from an ice bridge or a work bridge. The ice bridge approach necessitates the accumulation of a certain ice thickness to safely support construction equipment. However, it comes with a limited time frame for completion before the ice break-up. In contrast, a work bridge needs to be either designed to endure ice impact during break-up or must be dismantled before the annual ice break-up event. Each scenario presents cost implications, scheduling constraints, and potentially heightened environmental impact on the river. Consequently, on both projects, an incremental launch method was chosen to avoid environmental disruptions.

In Canada, 'fish windows' protect aquatic life by scheduling in-stream bridge construction to minimize ecological disruption. For example, in British Columbia, work may be limited to August to October to safeguard salmon spawning. This can impact construction strategies, possibly necessitating shifts to accelerated techniques or design adaptations. These constraints can increase costs due to rapid methods or extended timelines when work must pause. Therefore, early integration of 'fish windows' into planning and design is essential for strategic planning, appropriate methodology selection, and a smoother, environmentally compliant process.


Impact of Traffic on Bridge Design & Construction

Bridge construction often requires temporary closures of highways or railway lines below, which can significantly impact the project timeline. This situation necessitates detailed planning and rigorous coordination with local authorities. Take the Glen Road Bridge project as an example. The replacement bridge included a multi-use trail with a 4.8m wide pathway and a span arrangement of 30m - 40m - 30m. This three-span frame bridge with inclined legs is located 20m above the busy Rosedale Valley Road. The completion of the structural steel erection had to occur within two weekend closures of the road. This constraint influenced the design of the girder segments, the placement of field splices, and the sequencing of erection stages to allow the road to reopen after the partial structure's erection following the first weekend closure. The girder segments were pre-assembled in the shop and transported to the site as ready-to-lift units, minimizing on-site installation time. The second weekend closure involved the installation of the drop-in girder segment, a challenging task due to the precision required to fit it between two erected cantilevers. This process can get easily delayed without meticulous planning and good communication among all parties involved. Successful execution within the prescribed timeline necessitated substantial coordination among various entities, including the owner, designer, general contractor, erector, fabricator, trucking company, and the erection engineer.


Construction activities for the erection of the structural steel for the Glen Road Pedestrian Bridge during weekend closures of the Rosedale Road underneath
Drop-in segment installation - Glen Road Pedestrian Bridge across Rosedale Road, Toronto

An interesting case study is the design of the 199A Street Viaduct, part of the Golden Ears Bridge Project in Langley which is an off-ramp spanning over the mainline and railway tracks. Refer to blog article here.


Conclusion

To conclude, the discussion underscores the significance of constructability in bridge design, particularly concerning site access. Various challenges are presented by specific locations, environmental limitations, and traffic closures. The examples shared demonstrate that through proactive planning and consideration of these factors during the design phase, potential issues can be mitigated, project schedules can be optimized, and a smoother construction process can be ensured. Moreover, the importance of coordination among various stakeholders and the necessity for innovative solutions to successfully navigate these complexities are highlighted.

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