07 November 2017

Canadian Bridges: 8. Port Mann Bridge, Vancouver

This was the final bridge I visited on my September trip to Vancouver. The Port Mann Bridge was completed in 2012 as a 10-lane replacement for a steel arch highway bridge spanning the Fraser River. It is the centrepiece of a much longer highway improvement project.


With a 470m main span it is reportedly the second longest cable-stayed bridge span in North America (behind Mexico's Baluarte Bridge), but it doesn't even dent the top 50 worldwide. It was also briefly the world's widest long-span bridge (67m wide), before being overtaken by the San Francisco Oakland Bay Bridge (79m wide) in 2013.

I can't confess to being a great admirer of the new cable-stayed bridge span.

The choice of single pylons sitting in between twin deck sections seems at first sight to be aesthetically wise (cf. Stonecutters Bridge, Millau Viaduct, Queensferry Crossing). However, due to the bridge's tremendous width, four planes of cable stays are required, leading to quite a dense, confused appearance.

This also directly leads to the over-sized upper pylon sections, which are as large as they are solely to have enough space to accommodate the huge number of cable anchorages. Nonetheless, it is reported that this arrangement is less expensive than the alternative A-frame, H-portal or multiple mast pylons, and cost was the primary driver for the whole project.

Cable-stayed bridges often suffer from an imbalance between tower profile above and below deck, as out-of-balance longitudinal thrusts in the deck are restrained at the towers. The Port Mann Bridge has avoided that particular peril, with well-proportioned lower tower sections. T-shaped crossheads support not only the deck, but also anchor stability cables for the upper towers.

The span arrangement appears odd, but is rational. The south pylon sits well on land, while the north is in the middle of the river. The cause for this is the spacing of various obstacles: a railway yard requiring a significant span at the south end; a requirement for the adjacent tower to be on dry land to prevent any risk of scour to the riverbank protecting the same railway yard; and the position of the main and secondary navigational channels in the river.

The bridge is well known for the troubles it developed shortly after opening, with ice forming on the main cables and falling as "ice bombs" onto terrified car drivers below. This has since been treated with a mixture of hydrophobic coatings and ice-removal collars which can be slipped up and down the cables. I'm only guessing, but perhaps the controversy explains why the main bridge designer TY Lin doesn't feature the bridge on their website project gallery.

My main interest in sharing this bridge, however, is not for the main span but for the northern approach viaduct. I believe this was designed by Californian firm IBT (now part of Systra), and it's a classic post-tensioned box girder viaduct, a form in which IBT have considerable expertise. IBT are certainly happy to have the project on their website.

The arrangement is highly economical and also very beautiful. Three prestressed box girders sit on simple rectangular concrete piers. The girders are trapezoidal in cross-section, with the bottom face lightly arched. The bottom face therefore varies in width, adding interest to what is otherwise very plain.

It doesn't seem in any way unusual, but I think the detailing is excellent, with only a few lighting column and sign supports appearing as an afterthought. The concrete construction is also of excellent quality, and it's a tribute to how attractive a simple and economical engineer-led solution can be when it's delivered with such splendid clarity.



Further information:

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:

05 November 2017

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:

02 November 2017

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:

01 November 2017

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: