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:

• Google maps
• Official website
• 5 awesome suspension bridges near Vancouver, British Columbia

Canadian Bridges: 6. Treetop Walkway, Capilano, Vancouver

The third attraction I visited at Capilano Suspension Bridge Park, of relevance here, is the "Treetops Adventure", an elevated walkway formed of seven suspension bridges slung between eight 250-year old Douglas Fir trees.

It was opened in 2004, and allows visitors to experience the forest from up to 33m above ground level.

The structures were designed by McElhanney Consulting Services to have as little permanent impact on the trees as possible. Platforms around the tree trunks, and the bridge spans themselves, are supported from collars, connected without any nails or bolts. These act only in compression, tightened so that friction is sufficient to hold them in place. Out of balance forces on trees carried by braided polyester guy ropes.

The collars are adjustable and moveable, so that they can accommodate continuous growth of the trees, and are regularly inspected by the engineers to see whether adjustments are necessary. The pressure on the tree bark is reported to be about 20 psi, which in metric is a piffling 0.14 N/mm².

Unlike the neighbouring Capilano Suspension Bridge, the support cables are below the deck, rather than being at handrail level. This is necessary to allow visitors to exit each span and walk around the tree trunks without having to climb above bridge handrail level. It also offers the benefit of giving unimpeded views from each span, although these are marred by chunky brown-painted handrail posts.

As you'd expect, there's a fair degree of movement on each span as you walk across it, but not to an extent which will un-nerve many visitors.

I can't say I entirely admire the appearance of this walkway, which is clunky, with way too much of that brown-painted metalwork. However, I think the way it has been engineered is highly admirable.

There can't be many sites blessed with such mighty trees that they can support a structure in this way, but that's just one reason why this one is worth visiting.

Further information:

• Google maps
• Official website
• Capilano Treetop Adventures - Forest Canopy Walkway Bridges (Williams and LaRose, Annual Conference of the Transportation Association of Canada, 2006)

Canadian Bridges: 5. Cliffwalk, Capilano, Vancouver

There are three main attractions at the Capilano Suspension Bridge Park. The Cliffwalk can be found a short distance to one side of the main bridge.

The Cliffwalk was opened in 2011. It was designed and built by Morrison Hershfield. The walkway is supported from the upper section of a 90m tall granite cliff face. It is some 213m long, with a 30m long cable-supported section, but it is claimed to have an environmental footprint of just 11 square metres. This is the amount of contact made with the cliff-face, which was carefully surveyed to identify suitable locations of rock anchors to be installed.

Over its length the cliff walk descends around 9m, before rising nearly twice as much, with a series of staircases. The main walkway is 0.5m wide at floor level, and 0.8m between handrails. Parapets are 1.2m high, which I found adequately reassuring. The parapet infill is a stainless steel mesh.

The narrow walkway is generally suitable only for passage single file, which requires a degree of patience when the crowds are visiting, but there are balconies and platforms at intervals to allow people to pass or stand for longer to admire the views.

The structure generally comprises galvanised steel, with stainless steel handrails. The floor is timber except for a couple of platforms with glass and grille flooring, deliberately inserted to make the whole experience more of a "thrill".

The whole assembly sits typically 3m off the cliff-face, intended to minimise the risk of damage due to rockfall.

The entire structure is reported to be able to carry 45 tonnes of load, which equates to a live loading of 4kPa. This seems ample given the restricted space provided.

Selected parts of the bridge were fully load tested to this level using water barrels, before the bridge was opened to the public. There's no mention of any analysis or testing for vibration in the paper published regarding the bridge's design and construction, but I didn't detect any significant movement.

The Capilano Cliffwalk is a well-designed and built structure, which strikes a good balance between a sense of fear and a sense of security. The detailing is pragmatic yet attractive, and it has certainly enhanced the value to be gained from a visit to the Capilano Suspension Bridge Park.

Further information:

• Google maps
• Official website
• Design and Construction of the Capilano Cliffwalk (LaRose, Loptson, Wyllie and Masterson, Annual Conference of the Transporation Association of Canada, 2011)
• 2012 Canadian Consulting Engineering Awards Submission

Canadian Bridges: 4. Capilano Suspension Bridge, Vancouver

I mentioned previously that Lion's Gate Bridge is Vancouver's best known bridge, but the Capilano Suspension Bridge must run it close. Spanning 137m long, 70m above the Capilano River, it's also one of Vancouver's major tourist attractions.

Scottish civil engineer George Grant MacKay bought 6000 acres of wilderness from the Crown for 1 dollar per acre. In 1889 he worked with local First Nations people to connect both halves of his property with a suspension bridge. An information board at the site states his assistants to have been August Jack Khatsahlano and his brother Willie, but Wikipedia indicates August Jack to have been aged only 12 at the time of bridge construction. The new span was built from cedar wood and hemp rope, secured to trees on one side of the deep gorge, and to a buried tree on the other.

Engineer William Farrell replaced the original span with a bridge using wire cables in 1903. It was always a tourist destination, although in the early part of its life, it was a considerable trip from the main part of Vancouver via a ferry and a six-mile hike. Increasing visitor numbers meant that additional support cables were added in 1914.

The bridge that can be seen today is the result of a complete replacement in 1956, on the instruction of the owner at that time, Douglas McRae Mitchell. It was reportedly designed by Art Williams, the engineer, to carry 1,333 persons. Steel cables were anchored into 13 tons of concrete at each end, which I have to say doesn't sound like very much, and an information board at the bridge notes that the current anchors are 24 tons, which is a little more reassuring.

Today the entire site is a tourist mecca, with plenty to do beyond just walking across the bridge. It crosses a scenic canyon where reportedly bears and eagles can sometimes be seen, although I was not so lucky. It's surrounded on all sides by huge cedar and fir trees, including several very impressive forest giants.

The structural form is a simple suspended span, with the main cables secured into anchorages hidden out of sight below timber platforms. Short hanger cables connect to timber cross-beams which carry the walkway.

The views from the bridge are spectacular. It doesn't feel unsafe under moderate loading as seen in the photos, but it does sway quite noticeably. I've provided some videos of the sway below, seen from different angles.

I found it awkward to walk on, but not to the extent of feeling unsafe, although clearly anyone with vertigo or balance problems may not be able to cross the bridge (the whole tourist attraction is best entirely avoided in this circumstance!)

The Capilano bridge is small beer when compared to other long-span pedestrian bridges, including the new record-breaker near Zermatt, but I found it very impressive.

The suspension bridge park also features a treetop walkway, and a suspended cliffwalk, both of which I'll cover in my next two posts. The cliffwalk can be seen in the first photograph below.

  

Further information:

• Google maps
• Wikipedia
• Structurae
• Capilano Suspension Bridge Park
• Capilano Suspension Bridge Park (McQuarrie, 2017)

Canadian Bridges: 3. Granville Bridge, Vancouver

Granville Bridge is the next bridge east along False Creek in Vancouver after the Burrard Street Bridge.

Completed in 1954, the 1.1 km long bridge carries eight lanes of highway traffic and two footways between Downtown Vancouver and Granville Island (and beyond).

It is the third bridge on this site, with a timber trestle bridge erected in 1889, replaced by a steel truss bridge in 1909. Both structures incorporated swing spans.

The current bridge was designed by engineer John Robert Grant to sit at a significantly higher level than its predecessors, eliminating the opening span and thus accommodating much higher traffic levels. Pressure for change had no doubt been partially addressed when the Burrard Street Bridge was completed in 1932 (also designed by Grant).

Granville Street Bridge lacks the architectural attention that was devoted to its Burrard street neighbour. The main span is a conventional steel cantilever truss, notable only for the main span truss being significantly less deep than the approach span trusses, which didn't have to respect the same shipping clearance. This looks more than a little odd, but it's understandable given the need to reduce the overall length of the structure. The side spans are seen less clearly, with a background of buildings, so I imagine the odd relationship simply isn't apparent to any casual observer.

Although the design is pretty much entirely engineering-led, it's not unattractive or inappropriate to its setting. It is rational, orderly and from most viewpoints, uncomplicated.

Proposals have recently been made to add an elevator where the bridge passes above Granville Island.

Further information:

• Google maps
• Wikipedia
• Structurae
• 1954 film about the bridge construction

Canadian Bridges: 2. Burrard Street Bridge, Vancouver

The next two bridges that I'll feature from Vancouver both span False Creek, a significant water inlet which separates Downtown Vancouver from parts of the city to the south and south west.

I saw both bridges on an evening dinner cruise, so my photos are quite limited.

Burrard Street Bridge (a.k.a. Burrard Bridge) is the oldest surviving bridge across False Creek, completed in 1932 to a design by architect George Lister Thornton Sharp and engineer John Robert Grant. Grant was a structural engineer with experience in both the USA and Canada, who had first proposed a bridge at this site in 1910, and who later went on to design the nearby Granville Street Bridge. Sculptor Charles Marega also worked on the bridge, and would later design the lion sculptures that gave Lion's Gate Bridge its name.

The bridge supports a five-lane highway, a cycleway and a pedestrian walkway. The centre truss span is 90m long. The most prominent feature is the towers, designed in an art deco style in exposed concrete.

These have an architectural significance well beyond their simple role in holding up the main span. They act as giant bookends, distracting the eye from the very different span forms on either side: a curved truss above deck in the centre, and deep, flat trusses below deck on the approaches. See also Sydney Harbour Bridge, completed in the same year, for a similar solution.

It isn't clear from the river view, but the towers also form wide portals, with windowed internal galleries. As well as acting as gateways, the architect specifically intended these to block views of the overhead truss bracing, clearly considering it unsightly.

An extensive programme of repairs, strengthening, modifications to the non-vehicle lanes, and installation of anti-suicide fencing has just been completed in October 2017. From my photographs, you barely see the fencing, so it hasn't marred the bridge's appearance too much.

It's a nice bridge. The art deco style has generally worn well, with a degree of regularity and restraint that doesn't look too ridiculous even in the modern day.

Further information:

• Google maps
• Wikipedia
• Structurae
• History of Vancouver
• Burrard Bridge Heritage Study (Donald Luxton and Associates, 2001)

Canadian Bridges: 1. Lion's Gate Bridge, Vancouver

September was a busy month for the Pontist. As well as a trip to the Footbridge 2017 conference in Berlin, I attended the IABSE Symposium in Vancouver. As you might expect, I found time to visit a few bridges while there.

The single most unmissable bridge in Vancouver is the Lion's Gate Bridge, which spans 472m across the First Narrows, connecting North Vancouver and West Vancouver to the main city centre. It was built in 1938 to a design by Philip Louis Pratley, who also designed the very similar Angus L. MacDonald Bridge in Halifax, Nova Scotia.

Lion's Gate Bridge was completed two years ahead of the ill-fated Tacoma Narrows Bridge, escaping that bridge's unfortunate fate by virtue of very different deck construction, with open trusses supporting the deck rather than plate girders.

Relative to its span, the bridge is notable for its height above water. The steel towers are cross-braced above and below deck level, with a Vierendeel portal directly above the highway and an arched crown element which is as much decorative as structural. The vertical taper to the towers, the open appearance, and the slenderness of the bracing are all quite attractive in appearance, although the ornamental portal and crown are very much "of their time".

The anchorages are well hidden for a bridge of such height, allowing the northern approach viaduct to share the main towers' open appearance.

The bridge was extensively altered in 2000-2001 to accommodate modern traffic loads, with the complete replacement of the three-lane deck, and moving the pedestrian and cycle walkways to the outside of the cable hangers.

Options were considered for complete replacement of the structure, but deck renewal was the preferred option. The concrete deck was replaced with a steel orthotropic deck, while the original edge trusses (above deck level) were replaced with new trusses below deck level.

Triangular bracing can be seen connecting the main cables to the deck near the anchorages and at midspan. This looks like it was added in the refurbishment rather than being original, and by restraining the longitudinal sway of the cables relative to the deck, it significantly enhances overall stiffness of the structure.

Many North American suspension bridges have a liking for a kind of "ornamental" style that I think has not aged very well. Lion's Gate definitely has some of this quality to its towers, but I think generally its clean lines are attractive, and the designers did well to keep such a tall bridge so elegant.

Further information:

• Google maps
• Wikipedia
• Structurae
• Bridgemeister
• Canada's Historic Places
• The Choice of Alternatives in the Replacement of the Lion's Gate Bridge (Belyea, thesis, 1995)
• The Lion's Gate Bridge - Seismic Retrofit of north approach viaduct foundations (Yan and Sy, VGS Symposium, 2002)
• Bridges / Puentes (Arcilla, 2003)
• Lion's Gate: Contributions to Suspension Bridge Engineering (Buckland, The Structural Engineer, 2003)
• The Lion's Gate Bridge Suspended Span Replacement (Matson, Annual Conference of the Transportation Association of Canada, 2009)