06 November 2017

Canadian Bridges: 7. Sky Pilot Suspension Bridge, Squamish

From Vancouver, I travelled north on the Sea to Sky Highway towards Whistler. Along the way, near Squamish, I stopped at the Sea to Sky Gondola, which takes visitors from the highway 885m up into the mountains, admiring fantastic views along the way and again at the top.

There are various hiking trails for visitors to enjoy, but you don't want to hear about the beautiful natural scenery or the facilities for daytrippers. You want to hear about the Sky Pilot Suspension Bridge.

The name comes from nearby Sky Pilot Mountain, which is clearly visible from the Gondola station and adjacent bridge - you can see it in the background of some of my photos.

The bridge was designed and built by ISL Engineering, working with Macdonald and Lawrence Timber Framing. It is an 86m suspended span anchored into granite mountainsides at both ends, and crossing a valley which spills downhill immediately beside the upper Gondola platform.

The basic structural design of simple suspended bridges like this is not especially complex: the force in the cable is readily derived from the load (self-weight, pedestrians, and/or snow in this instance), the span, and the cable sag. However, the Sky Pilot Suspension Bridge is very well executed and has a number of interesting features.

Compare it to Vancouver's Capilano Suspension Bridge. The Capilano span relies entirely on main cables at handrail level, from which everything else is hung. It is reasonably heavy, yet prone to considerable movement.

The Sky Pilot bridge has a significantly lighter appearance, particularly in the walkway floor, and would be prone to unacceptable movement if it were not for the incorporation of reverse catenary cables, curving downwards to either side of the bridge. These significantly stiffen the span both vertically and horizontally, and make for a much more comfortable crossing.

The counter-curved cables are attached to edge cables at walkway level, and these cables also appear to be tensioned, being connected to the same supports as the handrail cables.

The bridge supports are interesting, short steel posts at the end of the deck, with the handrail and walkway cables attached top and bottom. These posts are then each attached to a single rod about a quarter to a third of the way up, which carries the full tension force from the bridge into the ground. It looks like these posts are having to work very hard in order to make sure the tension loads are carried away below walkway level.

The detailing of the bridge is excellent. There is nothing extraneous, everything is purely functional, but very crisply assembled. The connections of the vertical hanger wires, the horizontal parapet wires, the arrangement of the cross-members below the wooden deck - it has a very Swiss or German appearance, if that's not too much of a stereotype. I don't think I found anything on the bridge to criticise.

It would be interesting to compare this span against other lightweight modern suspension footbridges - feel free to comment on this post if you've visited other comparable structures.

Further information:

1 comment:

crisb said...

The Passerellle du Drac is an interesting simple suspension bridge (220m span) in France if you ever get a chance to visit. Unlike many bridges of this type it feels very secure, and has a wide walkway. Some of the details look a bit unusual but do seem to do the job. The details on Sky Pilot look admirable, but I guess the end posts might be difficult to get to work on a larger bridge. The Swiss type bridges are more rudimentory and economical, but are generally in the mountains and probably only for the intrepid to cross.