2 146th Avenue South at Fish Creek Light Rail Transit Station
facing page, bottom: This view looking towards the west has Macleod Trail in the foreground. In the background can be seen the ramp to the parking area for the Light Rail Transit station. Steps from the overpass and ramp to the station platform are under construction behind the scaffolding. Dark lines near the base of the two main piers indicate the location of the turning bearings. The transparent shields projecting from the face of the girders over the railway tracks were custom designed to provide a safe barrier to the LRT power cables
The second pedestrian overpass was a larger project that connected a parking lot to an LRT station, crossing the platform, two LRT tracks, the adjacent Canadian Pacific Railway tracks and Macleod Trail, a busy major highway. As a design-build project, costs had dto be very competitive. The shape of the new girders were critical to the construction cost and a solid tee-girder of varying depth was chosen, a shape that made post-tensioning particularly easy as the tendons had a very flat profile. The top flange of the tees formed the walkway slab and this too was post-tensioned with grouted tendons. At each end of the bridge were long ramps down to ground level. Three turning girders were constructed, essentially the same except for the inclination of the deck surfaces – the overpass is not straight and so some torsional movement joints were accommodated. Because of the small cross section of the structure it was decided to use pairs of cylindrical bearings for these. These custom designed bearings, mounted in pairs, use piston- like stainless sliders in cylindrical PTFE and elastomeric surrounds. (see the photograph on p 31) The connection between piers and girders was complicated by the narrow web of the girders having to transfer bending moments to the piers. There was no room in the girder concrete for hooked reinforcing bars from the piers so the transfer had to take place within the depth of the webs. During the placing of the concrete in one of the girders there was a very heavy rainfall. The girder was sloped and the higher elevation concrete was placed first. As the water collected, it ran down to form a pond in the lower end of the form. The contractor eventually cleaned out the over-saturated concrete and replaced it. During post-tensioning of this girder, jacking was carried out from the high end only. Just at the point when the concrete was receiving near maximum force, before transfer, the lower end anchor concrete exploded sending equipment hurtling off the girder to the ground below. Luckily, the inspector standing on the upper concrete and the men carrying out the stressing were unhurt; the ground havd been previously cordoned off for safety. An investigation found that the girder had excessive cover above the tendons, leading to insufficient bursting reinforcing steel at the anchors. Partly to blame was the City of Calgary requirement that overpasses have a minimum of 70 mm top surface concrete cover as protection against de-icing salts. With the relatively small cross-section of a pedestrian bridge this made it difficult to provide effective bursting reinforcing steel in the area where it was most needed. Because of the wet conditions, the reinforcement may well have dropped a little thus worsening the situation. To repair the girder, the end 1.5 metres of concrete was cut back and the end- block reinforcing steel replaced. Additional SS reinforcing bars were placed in the 70 mm cover area to prevent any repetition of the bursting failure. Again, the turning of the girders across the highway was carried out in early morning. The drop-in girder for the highway span was joined to the other two with small concrete joints and then the whole span post-tensioned to form a long continuous girder. Traffic continued to pass under the structure, but not under the drop-in section while supported by the cranes, during this process.
below: This view towards the NE shows the nearly completed overpass (without handrailing). The drop-in span across Macleod Trail can be seen between the second and third rotated girders. The formwork for the 100 mm wide joints for this can be seen to be still in place. The east ramp is outlined behind the far trees. A Canadian Pacific Railway locomotive and wagon are passing in front of the second rotated pier. The towered building behind the west ramp is the new LRT station.
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