July 1, 2001
by Canadian Architect
Vancouver’s original SkyTrain line, constructed in the mid-1980s, comprises continuous 30-metre long pre-cast concrete beams spanning between concrete columns. The length of the beams exceeded the maximum permitted for normal road transportation, so special trucks were needed, and transportation was restricted to nighttime hours. Because of the oversize loads, there was considerable damage to roads and sidewalks during construction.
The Millennium Line is being constructed by a different method that overcomes these difficulties. Instead of continuous beams, the guideway is made up of pre-cast segments that are lifted into place by giant bridge trusses and then post-tensioned to form continuous beams. The system is similar to that used to construct the Confederation bridge between New Brunswick and PEI. The contractor for that project, SAR, was part of the joint venture company that won the contract for the Millennium Line.
Typically, the guideway segments are 2.7 metres in length and wide enough to support both the eastbound and westbound tracks. Most segments weigh about 25 tonnes (although some are as much as 60 tonnes) and are small enough to be transported by conventional flat-bed trucks without need for special permits or time restrictions.
The bridge trusses are crane-like assemblies that creep like giant insects along the guideway as each section is completed. There are two types of bridge truss, the span-by-span truss that erects a conventional beam section before moving to the next column bay, and the balanced cantilever truss that sits on top of a column and erects cantilever beams on either side of it simultaneously.
Using this technology, the erection becomes a fast, reliable and repetitive process. Each segment is uniquely designed for a specific location, according to the radius of curvature and slope of the track. It is brought to the site and then lifted into place on cables suspended from the bridge trusses. Once a complete span has been accurately positioned, steel cables are threaded through the segments and then tensioned with a force of 15,750 kilonewtons, creating a continuous beam. The cables are completely encased in concrete to prevent corrosion. Once a span is complete, the bridge truss crawls along the completed section to the next open span, and the process begins again.