3 John Laurie Boulevard Pedestrian Overpass
right top: the north offset pier bearing being prepared for assembly. The teflon disc in the foreground is eccentric to the circular ring of the pier. The teflon has a dimpled surface and is coated with a special paste. The top mating disc is seen behind; it is a flat steel disc to which a thin highly polished stainless steel plate has been welded. Central holes in these discs are for the locating pin which ensures that the top and bottom parts of the pier are properly aligned. The stainless steel/teflon interface provides a very low friction coefficient allowing rotation of the very heavy girder with a quite low force. The length of the girder also provides a lot of leverage for the rope handlers. The bottom part of the pier is visible with its steel circumferential ring in place. The top part of the pier will have a similar ring which, after rotation, enables the top and bottom parts to be welded together to provide a rigid connection. right middle: the drop-in girder is being set on the ends of the two inner arms of the main girders. The road was completely closed to traffic during this operation. A temporary tower was built on the roadway median to support the girder during adjustment allowing the two rather expensive heavy cranes to be removed. Once the drop-in girder was aligned the two post-tensioning tendons were threaded through and the joints made. Finally these tendons were post-tensioned and the ducts grouted. The result was a monolithic superstructure with movement joints only at the two abutments. right: The finished structure from the east. It is just possible to see that the superstructure fits into a notch in the abutment wall. Sliding bearings are fitted above and below the superstructure at these notches. Because the centre span is very long relative to the end spans, the reaction from the superstructure is largely upwards unlike most bridges which sit down on their end supports. In addition, under temperature changes the curved superstructure tends to twist hence the upper bearings that balance these forces. The abutments were only built after the superstructure was finally post-tensioned because of the impossibility of being able to anticipate the final shape with real precision.
The pedestrian overpass here connects a small neighbourhood park on the south side of a busy road with Nose Hill, a major Calgary park, on the north side. The roadway cuts into the hill with a high embankment to the north and a lower one on the south. The road descends from east to west so to gain the most advantage from the site, a skewed crossing from northwest to southeast was chosen. Because of the undulations of the little park, a curved bridge with a built- up southeast embankment was optimal. Using all the experience gained from the two earlier bridges, circular piers with a turning joint at a convenient height for welding were chosen. The north girder was shorter than the south – to balance this the piers were made proportional to the spans with the north pier a smaller diameter. This time a relatively long drop-in span was required to bridge the gap between the two turned girders. A box girder has superior torsional strength and a classic style. The outer sides of the webs are vertical avoiding a compound curve for the formwork surface and thus extra construction costs. The architect suggested that a handrail be designed similar to one he had seen somewhere in Europe with only top and bottom horizontal torsion members with an offset handrail at about two-thirds of the height. The vertical members are thin plates viewed from their narrow sides – a very light-looking handrail on the strong lines of the concrete. Because of the curvature, the centre of mass for the girders was not central to the piers so the bearings were offset to compensate. Also because of the curvature of the superstructure, temperature changes cause the ends to twist. Twin bearings were used on each side of the deck: one below and the other above the deck to resist these torsional moments. Small concrete bridges carry mainly their own weight and, because of construction code requirements for web and flange thicknesses and concrete cover for protection, they are difficult to build economically. By varying girder depth and using post-tensioning, an elegant structure is possible. One advantage of building unusual structures or structures with difficult shapes is that the workers on site take great interest in doing a good job. Admiration for neat and accurate formwork often results in even better work to follow. In this case the formwork, inevitably discarded at the end of the day, represented some very fine workmanship. The quality of the curved surfaces rival precast and no great premium was paid for this achievement. The construction sequence was to build the lower parts of the two piers with the turning bearings properly located. The upper parts of the bearings were then installed and the top parts of the piers cast in their correct final positions. The piers were then rotated to the offset positions for construction of the two girders which were then built integrally with these upper pier parts, given an initial post-tensioning and then rotated to their permanent alignment. The drop-in span was supported in place, connected and the whole finally post-tensioned to provide continuity. The construction proceeded without incident until the north girder was turned. The column section above the bearing tilted slightly and developed a small east/west offset. It was decided not to try to correct the misalignment which was not very noticeable. This, however, turned out to be a mistake as the alignment of the deck connection to the drop-in girder was, of course, similarly affected. The slight offset did not matter as it was adjustable but the tilt could not be adjusted. The tendon ducts could be connected but the slight step at the edges of the concrete, in spite of a small repair, could not be totally concealed. It is thought that the bearing might not have been perfectly horizontal and that after the upper part of the pier had been completed and turned, the contractor noticed a slight lean and adjusted it. Of course, when the pier was turned back to its original alignment, it tilted because of this adjustment and created the cross slope of the girder. The abutment walls above ground were built after the girder had been made continuous in the final post-tensioning phase to ensure that they perfectly matched the ends of the girder. Until this, the ends of the girder were tied down with steel cables to balance the long centre span. This method was probably unwise as a far safer method would have been to weigh down the girder ends with water tanks. Fortunately these temporary cables and anchors held under considerable and rather unpredictable forces. ~
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I would like to note my appreciation particularly to the following persons who made exceptional contributions to the design and/or construction of these three bridges: Colleague Glen Norlander, P.Eng. Architect Chris Roberts MAAA Landscape Architect Gary Browning Client City of Calgary: Alex Broda and Jadwiga Kroman, P.Eng. Contractor Graham Construction and Engineering Inc. John Connolly and Daren Mickler
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