We are nearly there! This is the penultimate episode in my series on anniversary structures, and we have arrived at those celebrating their 20th birthday in 2017. Hopefully, like me, you have learned a few interesting snippets during the series.

As a designer, I acknowledge the important role of inspiration. A designer's work is influenced by past experience and by lessons learnt through observation. A good designer will develop ideas by examining existing structures, learning what works well and what doesn't, and capturing the best aspects for their current project. This is not to copy, but to learn from experience and evolve into something new. I hope that in some small way this series may help you to build a habit of actively seeking design inspiration from the plethora of existing structures, if you don't already do this.

The 20 year-old structures this month are mostly very familiar and many of you will already know a good deal about them. Indeed, several of you have been involved in their design and construction (as I have!) so if you wish to add any interesting details in response to this article please do. There are many more interesting structures celebrating their 20th birthday this year than just the seven illustrated here, like the Sky Tower in Auckland and the Hoga Kusten bridge in Sweden, but space and time doesn't permit me to write about all of them. 

1. Tsing Ma Bridge, Hong Kong

At the time of its completion, Tsing Ma Bridge was the second longest suspension bridge in the world, with a main span of 1377m, and the longest carrying road and rail traffic. The double level bridge carries vehicle traffic on the top deck and the Airport Railway on the enclosed deck below. If you have ever taken the railway into Hong Kong from Chep Lap Kok airport you will have travelled across the bridge, but possibly without knowing it because the lower deck is enclosed and feels much like being in a tunnel. In addition, there are emergency traffic lanes on the enclosed lower level for use in the event that a typhoon closes the top deck.
 
Tsing Ma Bridge(Image source:Wikimedia Commons by mageba sa)

The bridge was designed by Mott MacDonald, and the main contractor was a joint venture between Costain, Mitsui Corporation and Trafalgar House. The steel superstructure was supplied by Cleveland Bridge & Engineering in Darlington. Mitsui also supplied some of the steelwork. The independent design check was carried out by Flint & Neill (now COWI) and I have many happy memories of working on the project.

The concrete towers were constructed by slipforming and reach a height of 206m. As the legs climbed skywards, achieving about 4m per day in the slipforming process, the steel trusses contained within the portals were assembled at deck level and lifted up. Once complete, the massive steel saddles were lifted to the top using a special derrick.

The main cables were constructed by aerial spinning. This is a process whereby two or more individual strands of wire (about 5mm in diameter) are looped over a wheel which carries them under controlled tension from one anchorage to the other, backwards and forwards in a continuous process taking many months. Each time the wheel reaches the end of its run, the loops of wire are transferred to the special shoes which form the anchorage. There is a careful process to ensure the sag profile of the wires is controlled so that the cable ends up with the correct geometry. Once all 70,000 or so of the wires are spun, they are compacted into a circular shape and the cable bands which attach the hangers are clamped on.

Tsing Ma Bridge
Left: Aerial spinning of the main cables - placing wires in the tower saddle. Right: Lifting a deck segment from the barge. (Images source: Kvaerner Cleveland Bridge - from COWI UK library).

The bridge is a steel vierendeel girder, with four longitudinal girders and transverse cross-frames at 4.5m centres, although at first glance it looks like a box girder due to the cladding which encloses the lower deck. The girder was constructed in 18m long segments, each weighing about 480 tonnes, and lifted up into place from a barge using a gantry crane running on the main cables. Importantly, the deck is vented along the centreline, with an open grille in the top and bottom decks. This vented deck allows pressure equalisation between the top and bottom surfaces of the girder and contributes to the aerodynamic stability of the span. This is similar to the rationale behind the twin box girder of the nearby Stonecutters Bridge or the triple box design for the super-long span Messina Strait Bridge. This arrangement also ensures ventilation of the lower, enclosed deck.

2 Kap Shui Mun Bridge, Hong Kong

This cable stayed bridge is part of the Lantau Fixed Crossing along with the Tsing Ma Bridge (you can see it in the background of the image above) and the Ma Wan Viaduct which connects the two. So it carries the same highway and railway traffic in the same cross sectional arrangement. Although considerably smaller than its mighty neighbour, with a main span of only 430m, it is nevertheless an interesting structure, and one of the longest cable stayed spans carrying road and rail, which presented some interesting challenges in the design and construction.

kap Shui Bridge (Image: COWI UK)

Mott MacDonald prepared the outline design on behalf of the Hong Kong Highways Department, but the project was then procured under a Design and Construct contract, with the detailed design being carried out by Greiner in the USA and Leonhardt Andrä und Partner in Germany. Independent design checking was again carried out by Flint & Neill (COWI), and Harris & Sutherland for the approaches. The main contractor was a predominantly Japanese consortium including Kumagai Gumi, Maeda, Yokogawa and Hitachi Zosen.

The bridge girder in the main span here is a composite section, with concrete upper and lower decks acting compositely with inclined steel webs on the outside where the stays are attached. Vierendeel cross frames are used once again. Considerable research went into the detailing of the non-typical composite connections.  The approach spans are in concrete and were constructed by incremental launching on the Ma Wan side.

In October 2015, the bridge was struck by an over-height floating crane. The damage was only minor, but the strike was detected by the bridge's monitoring system which initiated an emergency bridge closure, cutting off access to the airport at Chep Lap Kok for about 2½ hours. The Hong Kong Government's contingency ferry arrangements failed to kick-in quickly enough and many passengers missed their flights. Hong Kong residents will be glad when the new tunnel crossing to the airport from Tuen Mun opens soon to provide a much-needed alternative route to the airport.

3 Hong Kong Convention and Exhibition Centre

The original Hong Kong Convention and Exhibition Centre was built in 1988. Then, starting in 1994, it was extended to form what has now become an familiar iconic feature of Hong Kong Harbour. The enormous building has a distinctive multi curved wing-roof profile, with an area of some 40,000m². It is considered by some critics to resemble a giant turtle. The extension, which juts out into the harbour on reclaimed land, was completed in 1997 and used as the venue for the official handover of the territory to the People's Repuyblic of China on 30 June 1997.

The building was designed by Skidmore Owings and Merrill and the main contractor was a joint venture of Hip Hing Construction with Dragages et Travaux Publics. 

To achieve the wing-shaped roof, steel roof trusses of various shapes were used, supporting the aluminium tiles. Space limitation was a key factor in the roof construction, so the six pairs of roof trusses measuring up to 81m long were fabricated off site and transported to the site by barges.

Steelwork was by Cleveland Bridge and Engineering and erection was by Dorman Long Technology.
The extension, completed within a tight 26-month schedule, houses three exhibition halls, a convention hall, a foyer, 26 meeting rooms and restaurants. Sitting on the reclaimed land, the extension was connected to the original building by a 110m long multi-level atrium.
 
Hong Kong Convention Centre
(Image: Wikimedia Commons by Tksteven)

Around the front of building is a 30m tall glass façade offering a 180 degree view of Kowloon and the Victoria Harbour – one of the biggest curtain walls anywhere.

4. Campo Volantin Footbridge, Bilbao, Spain

The Campo Volantin Footbridge in Bilbao, Spain, is another striking bridge designed by Santiago Calatrava. This is a tied arch whose profile is significantly sharper than the typical second order parabola, and the arch is offset transversely to the deck centerline. At the ends, the arch is perched on the top of the cantilevering approach ramp structures, seemingly just to demonstrate that it is a tied arch and there is no net horizontal thrust at this point. 
   
Campo Volantin(Images source: Wikimedia Commons - Left, Eduardo Right: Daniel Lobo)

I have to admit to finding this bridge rather awkward. The radiating hangers and the profile of the arch seem all wrong to me. All the curvature, and thus the zone of hanger attachments, is concentrated in the middle, with the outer parts of the arch being effectively straight struts – arguably not a very efficient structural form! The end hangers align effectively parallel with the arch, which looks strange to my eye.

I see from Wikipedia that I am not alone, as the entry for this bridge includes the following quote from Matthew Wells: "Regrettably, the balance of light steel superstructure on the cantilevered abutment ends is strained, like a sculptural toy, and the bridge touches the ground uncomfortably." I have to agree with Matthew on this point. I also find the soffit unnecessarily cluttered with lots of varying length transverse ribs connecting to the spine which acts as the tie to the arch.  

The bridge is a curved steel deck with glass block walkway, which has given rise to some problems because the glass gets slippery when wet (no surprises there). There has also been a long running argument between Calatrava and the city of Bilbao over modifications carried out by the city to make the bridge more usable. Nevertheless, it seems to be reasonably popular with the public and has certainly become something of an icon in the city.

5. The Guggenheim Museum, Bilbao, Spain

While we are in Bilbao, we couldn't leave without mentioning the Guggenheim, also completed in 1997. This extraordinary building, designed by the architect Frank Gehry, and has been said by the architect Philip Johnson to be "the greatest building of our time" – an impressive accolade!  

Indeed, most architects, critics and even members of the general public seem to agree that this is one of the most important buildings of recent years. It certainly exhibits Gehry's famous and inimitable style.
 
Guggenheim(Image source: Wikimedia Commons by Sergio S.C from Zaragoza)

Everyone talks about the architecture and the work of genius that this building undoubtedly is, but few stop to think about the structural engineering challenges. The geometry and arrangement of the interior spaces made for some complex engineering, which was handled by Skidmore Owings and Merrill, together with Srinivasa "Hal" Iyengar. I have tried to find more details of the engineering solutions online but so far without much success. Maybe someone out there can enlighten me, but I am sure it can't have been straightforward! Nowadays, with modern 3D modelling tools, it would not be too challenging to derive the geometry of the steel skeleton structure, but I expect it was quite a challenge 20 years ago. I take my hat off to them.

Gehry chose titanium for the outer cladding after ruling out other materials and testing a sample of titanium pinned outside his office. Approximately 33,000 extremely thin titanium sheets are used, and the finish provides a rough and organic effect, adding to the material's colour changes depending on the weather and light conditions. Together with the complex geometry, the building creates a visual impact that has become a real icon for the city throughout the world, clearly putting Bilbao on the map for many people.

6. The Hulme Arch, Manchester, UK

The Hulme arch in Manchester is a relatively modest scale but striking landmark road bridge connecting two parts of the Hulme community over a busy arterial road into the city. It was opened by Sir Alex Ferguson in 1997.

Designed by Wilkinson Eyre Architects and Arup, the bridge has won multiple awards. The arch crosses diagonally so is seen as an arch in both directions in views along both roads – clearly a "gateway" structure and symbolic of the regeneration of the area.
 
Hulme Arch(Image source: Wikimedia Commons by Mikey from Wythenshawe)

The bridge has a 50m single span supported by 22 high-strength steel, spiral strand hangers - 11 on each side, originating from both sides of a 25m-high steel arch in an unusual crossing arrangement necessitated by the diagonal alignment. The arch has a trapezoidal section varying from 1.6m wide by 1.5m deep at the base to 3m wide by 0.7m deep at the top. This variation not only suits the desired visual appearance in elevation (ie. slender at the top and more stocky at the base where some visual mass is needed to give the sense of stability and strength) but also the structural demand on the section which experiences significant lateral bending effects due to the arrangement of the hangers and the diagonal alignment. Under vertical loads, the whole bridge tries to rotate about a central vertical axis due to the diagonal arrangement, but this is easily resisted by the lateral supports at the abutments. 

7. Bukit Jalil National Stadium, Kuala Lumpur

The Malaysian national stadium in Kuala Lumpur is a reinforced concrete structure with a steel cable and membrane roof structure. It was constructed for the 1998 Commonwealth Games and was officially finished on the 1 January 1998, but the actual construction was completed in 1997.

It is an all-seater multi-purpose stadium and the home ground of the Malaysian national football team. Available information on capacity is contradictory. Some say it is 87,400, which makes it the largest in Southeast Asia, but some claim it seats 100,200 making it the third largest football stadium in the world (larger than Camp Nou in Barcelona).
 
Bukit Jalil(Image source: Wikimedia Commons, Alfianamy)

The roof structure is a classic oval tension and compression ring roof system. Vertical props stand on the main tension ring and support to inner edge of the roof membrane. Radial ties from the base of the props support the tension ring, and thus the props and roof, by taking the loads back to the compression ring around the top of the stands.

The tops of the props are tied together and stabilised by another, lighter, tension ring, and by smaller radial ties which also connect back to the compression ring. This is the classical bicycle-wheel-on-its-side approach to stadium roof design, with the props effectively acting as the hub. It places the inner edge of the roof at a higher level than the outer edge, so rainwater is easily collected outside the stands.

Interestingly, as can be seen from the photo, the designers didn't think it necessary to incorporate a permanent maintenance access walkway around the lower tension ring, as would commonly be done today, so I would be interested to know how anyone is able to inspect all those potentially critical cable clamps and attachments.

 

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