A twisted bridge? Really?
Yes. All bridges may be created equal but not all bridges are built level. It may sound ridiculous, but in 1955, the Angus L Macdonald Bridge in Halifax, NS, was intentionally built lopsided.
When the iconic 2500 ft. suspension was designed, one side was heavier than the other because there was a sidewalk on only one side. To counter this effect, the bridge designers cleverly designed the bridge with one cable higher than the other, so that when the bridge was completed both sides would be at the same elevation.
It had been intended to add a 30 inch (760 mm) water pipe under the bridge at a later date that would be offset to the south of centre by about 692 mm. This would even up the loads on the cables somewhat, but it would result in the deck now being lower on the south side. As it turned out, it was a 24 inch (610 mm) pipe that was added, not a 30 inch pipe, so the effect was less than expected.
However, the situation grew more complicated (from an engineering point of view) when the roadway was widened and a new sidewalk and bikeway were added during the Third Lane Project in 1999-2000. The new configuration eliminated the original imbalance in weight of the deck, but it kept the 24 inch water pipe that was offset from the centre of the bridge, so that now the south side was the more heavily loaded, and the bridge was lower on its south side than on its north.
The result? A lopsided bridge. With the exception of where each end of the suspension bridge meets the road, one side of the bridge is higher than the other side.
Does this matter?
It depends. To passing motorists? No, not really. To the engineers who are currently undertaking the major redecking project? Yes. Very much so.
The new roadway deck is to be level, with both sides of the road at the same elevation. However, this poses a problem when the new deck temporarily joins the old (sloping) deck as each new segment is fixed into placed. Theoretically, if the old and new structures joined at the centre of the road, there would be an abrupt “step” between them at the edges. To prevent this from occurring, both old and new structures must be twisted towards each other so that there is a smooth running surface across the temporary joint.
This is achieved by controlling the tensions in the hangers (the wire ropes that loop over the main cables and support the deck structure). Delicate control of these tensions is required to ensure that, at no time, any part of the structure overstressed.
Finally, when all 46 of the deck segments have been replaced, the roadway will be level, and there will be no residual twisting forces… but one cable will still be slightly higher than the other.
A final interesting wrinkle concerns the towers…
When the weight on the bridge increases, not only do the cables sag more, but also the towers lean towards each other. Due to a lack of records detailing the original construction, we are unsure if the towers were constructed to counteract this. It is possible that one tower leg was pulled back more than the other leg, or one leg of each tower may have been built closer to the centre of the bridge than the other.
Unfortunately, surveying the positions of the tops of the towers was inconclusive. The engineers tested the redecking design for both of these possibilities and fortunately, it is possible and safe to untwist the bridge either way.
COWI North America checked the condition of the bridge, pioneered the replacement concept, performed detailed design, analysed the bridge at every step of reconstruction, designed key parts of the contractor’s erection equipment, prepared technical specifications, and is providing technical assistance on site.