Introduction
Constructing bridges over operational railway lines could pose substantial challenges, especially in the realms of design and project execution. An example of this is the 199A Street Viaduct, an off-ramp to the Golden Ears Bridge mainline in Langley, BC constructed as a part of the $800M project in 2009 by general contractor Bilfinger Berger. The viaduct spans over railway tracks and showcases how active railways can restrict crane placement and dictate the design of the structure. This viaduct serves as an exit ramp from the mainline for traffic originating from Maple Ridge, connecting to 199A Street in Langley. It spans the south approach and railway lines. This continuous, four-span bridge has spans measuring 65m, 82m, 66m, and 55m, respectively, and accommodates two lanes of highway traffic. The bridge's horizontal alignment includes spiral sections with a tight radius of 130m in the main span.
Constructability Obstacles
The main span of the viaduct, at 82m long, crosses existing railway lines and the south approach viaduct. This situation eliminated the possibility of freely accessing the main span with cranes or placing temporary piers. This constraint significantly affected the lifting capabilities of the cranes, as they could only be stationed outside the railway clearance envelope or on the new south approach viaduct. Initially, the viaduct was designed as a composite three steel plate girder system. However, constructability reviews prior to final design unearthed challenges associated with erecting the girders in the main span that crossed active railway lines. The lifting of an assembled three plate girder main-span segment exceeded the crane capacities, and the instability of a curved girder rendered a single girder lift impossible. Given the cranes available on-site, it was not feasible to erect the three plate girder system in the main span. In fact, two 750-tonne cranes were required, which were not readily available in North America at the time.
Modified Bridge Design
In response to these challenges, the team pivoted to a composite single steel box girder superstructure. This alteration enabled the bridge to be divided into multiple segments, facilitating the erection of the structural steel in the main span using the available cranes. The main span was erected using the progressive cantilever method, with cranes strategically positioned on the south approach viaduct and on ground outside the railway clearance envelope. This placement allowed the installation of the segments, each limited to 10 meters in length and 35 tons in weight, without interrupting the railway operations beneath.
The shift to a box girder, with superior torsional efficiency compared to a plate girder system, also resulted in approximately 200 tonnes of structural steel savings. Despite skepticism about potential challenges due to possible misalignment of the cantilever ends, the bridge construction proceeded smoothly. The main span was completed within two weeks.
Conclusion
The project achieved completion within the stipulated timeline besides a 20% saving in structural steel in its final tonnage, validating the effectiveness of the design modification. In addition to advantages in shipping and handling, the lighter superstructure lessened demands on the substructure and foundations under seismic loads. The 199A Street Viaduct case study illuminates how the presence of active traffic, such as highways or railways, can introduce constructability challenges that shape the design of bridge structures. The second part of the 2009 article below from Bridge Design and Engineering Magazine - Issue #56 provides insights on the redesign of the 199A Street Viaduct to facilitate erection of the steel across active railway lines.
Comments