below: the custom designed movement bearings between sections of the overpass superstructure. Because the superstructure is subject to twist as well as sliding movement (mainly because of temperature changes of the concrete), the twin piston sliders are a good solution. Around the stainless steel shafts, the ends of which are visible in the photograph, are cylindrical PTFE tubes in neoprene housings. [Teflon is the common name for PTFE made by DuPont. PTFE is the short form of polytetrafluoroethylene]
below right: . This photograph was taken shortly after one of the girders suffered an end-block failure. The thick wedge of failed concrete behind the prestressing cables indicates the thickness of the unreinforced concrete cover to the reinforcing system. The lower post-tensioning cables are seen to have survived the implosion. Extension marks painted on these cables show that that concrete is still resisting the post-tensioning forces. The whole end-block area, however, had to be completely rebuilt.
146th Avenue: The main span over the highway is 49 metres and the overall length of the bridge and ramps, 268 metres. The 49 metre span included two 18 metre cantilevers from rotated girders plus the drop-in span of 12.7metres. In spite of the complexity – it included access stairs to the station platforms and another complex set of steps serving the local businesses – this overpass cost about C$1,100,000 or $1,750 per square metre of deck area, 20% more per square metre than the cost of the Sarcee Trail overpass but about 10% less expensive than the next lowest priced proposal which would have been simply supported precast concrete girders.
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Tom Martin
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