1 Sarcee Trail Pedestrian Overpass
Sarcee Trail is a divided bypass highway running north-south and lined on both sides by high voltage power lines, critical to the city. Temporarily shutting down power to allow cranes or construction equipment to operate near the conductors was very limited and was considered to be most undesirable. The site for the bridge was chosen where the catenaries of the power lines were high, making it possible to pump concrete into forms while maintaining a safe clearance for the construction equipment from the high voltage cables. At some point during preliminary discussions I remembered the pedestrian bridge over the River Ware at the University of Durham which Ove Arup designed around 1963. It was built by first constructing the piers and girders on the river banks and then rotating them 90º where they met over the river, to be connected with a small mechanical device. A site must lend itself to this strategy, and Sarcee Trail did. The overpass connected children living west of the highway with a school on the east side. We ran an access ramp perpendicular to the highway between the school and an adjacent sports field. The west side was open land leading to an escarpment with housing several metres above the road level – fortuitous circumstances. A through-girder design was used. To keep the geometric centre of the cross-section as high as possible, the concrete sides were made wider at the top and narrower at the base, helping the bridge to span both double lanes of roadway while eliminating a central pier. The two girders were post-tensioned in phases, first individually, before their rotation and then, with a short connection cast between them, after final alignment to form a continuous beam over the two roadside supports with cantilevered lengths either end. These ends were partially supported on short cantilevers from two reinforced concrete ramps built on either side of the main bridge. For the rotation, temporary bearings were installed at the centre of the two main piers: simple circular plates with a 700 mm diameter PTFE (Teflon) surface on highly finished stainless steel. Locating pins (later removed) centred the girders on their piers during the rotation procedure. The shape of the side walls/webs of the through-girders caused problems both during concrete placing and before installation of the permanent bearings under the girder webs. Once the concrete forms, held with conventional form ties and light cross-bracing, were about half-full they started to rotate away from the girder centreline as the downward force on the outer sloping surfaces created an overturning moment about the base. The contractor struggled to provide additional braces across the girder to stop the forms from moving. Although the centre of mass of the concrete was within the base width and the problem would solve itself once the concrete hardened, during the hydraulic stage the system was unstable. Once the individual girders had been post-tensioned and the scaffolding removed a similar problem presented itself while the girder was supported on one central bearing. A small longitudinal crack developed along the top surface of the deck slab on the girder centreline. The engineers had underestimated the bending moments produced by the outer walls remote from the support. To counter this, holes were drilled through the webs just above the slab and temporary high-tensioned bars were stressed across the girder, to be removed after the widely- spaced pairs of permanent bearings had been installed. With hindsight, these two rather elementary problems had arisen because of the unconventional geometry and construction method employed – somewhat obvious after the event but not so obvious before. The actual turning of the girders took place early on two consecutive Sunday mornings with traffic diverted from one of the lanes to the other so that no vehicle passed under a turning girder. It was not known how well balanced the girders might be so weights were suspended with block and tackle from each end of the girders very close to the ground surface with the idea that should a girder tilt, the lower weight would immediately sit on the ground making the light end heavier. In fact this didn’t happen as the bearings were large enough to keep the centre of mass of the girders well within their diameters. The turning of the girders was a wonderful experience – one hundred tons of concrete silently moving, guided by four men with ropes – within an hour the traffic was back to normal with surprised motorists wondering how a bridge could just suddenly appear.
facing page: This picture was taken during the turning of the second girder. The west girder had been turned the week before. In each case four ropes were used, one tied to each corner of the girder. Two ropes were used to pull and two for braking. One man was required for each rope although one person could have probably handled the load quite comfortably. One of the pulling ropes can be seen stretched towards the east ramp between the trees at the right. At the left of the picture diverted traffic can be seen crossing the median strip in the roadway. Other than to slow down, there was no traffic delay during the operation.
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Sarcee Trail: The main span is 36 metres and the overall length of bridge and ramps, 183.5 metres. The whole project took six months and cost just C$600,000, one of the least expensive overpasses in the city.
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