21 May 2013

Winner announced for Hamburg Southern Elbe crossing

A winner has been announced for the bridge design competition to design a new highway bridge across the Elbe near Hamburg. The bridge will connect the A1 and A7 highways, and is approximately 535m long, with a main span of 350m. The winning entry was selected from 12 competition entrants.

Winner: Schlaich Bergermann und Partner / Dissing + Weitling / WTM Engineers

Centrally located mono-tower cable-stay bridges seem to in fashion. The winning design in Hamburg is on a much smaller scale but bears a certain family resemblance to Hong Kong's Stonecutters Bridge, which is probably no surprise given the involvement of architects Dissing + Weitling in both designs.

Both bridges have twin planes of cables supported from two main towers (140m tall in Hamburg). These carry the inner edge of twin decks, which are connected ladder-like by a series of cross-girders. Both bridges have towers which flare below deck level and which are essentially prismatic for most of their above-deck height. This is a decision which is driven by the geometric requirements of the cable anchorages, to avoid unsightly cable attachments outside the tower envelope.

Below the lowest cable, the tower splits in two, with a similar twin-leg arrangement below deck, echoing the Millau Viaduct, another bridge which follows the mono-tower fashion. The same fashion has taken over in the UK, with Mersey Gateway and the Forth Replacement Crossing both taking the same approach. The reason, I believe, is visual legibility, with the single row of pylons and single plane of cables creating a far clearer silhouette than is achieved with more traditional twin-pylon and portal-frame pylon designs (UK examples include the Kessock Bridge, Second Severn Crossing and Dartford Crossing).

I don't know enough about these designs to comment on the economics - it seems to me that the greater torsional stiffness required in the deck for the mono-tower design, plus the much greater cross-section of the tower itself, will make it more expensive overall, even if the number of foundations and towers is reduced. The Southern Elbe crossing design is certainly reasonably elegant overall, although the second image shows how perspective can lead to a certain awkwardness when horizontal and vertical curvature is combined.

2nd place: Kinkel + Partner / Ges Consulting Engineers

The second placed design strikes me as quintessentially German, although the simple harp-type cable arrays here are those better known from Düsseldorf than from Hamburg. It always seems the most visually elegant arrangement, although rarely if ever the most economic, as the forces in both the deck and tower are greater than for the fan-type arrangement chosen by the winning entry. In the second-placed design, this is doubly true, given the wide spacing of the cables.

Indeed, it's hard to see how this bridge could be built economically. With a large number of cables, individual deck units can be relatively short, and hence straightforward to assemble in place as the bridge deck is progressively cantilevered out from the towers. Assuming the second-placed design has the same span as the winner, each deck segment is up to 40m long between cables, requiring both larger plant for erection and also potentially considerable temporary works to stabilise the main towers against out-of-balance bending.

3rd place: Ponting

Slovenian designers Ponting depart from the symmetrical two-span cable-stay solution with this asymmetric arrangement. Similar to the winning design, the main span is 355m, but with only one main tower, the masts have to be 200m tall to support the deck. It's not clear to me why such an arrangement would be preferred, and building inclined masts of such height requires some justification. I am not sure that the response of the cables to the deck curvature, or the desire to add transverse stability to what is already quite a wide deck, are sufficient.

There seems to be me an element of trying too hard to stand out, which is a frequent problem in bridge design competitions, and one which was also very much true in the Stonecutters Bridge contest some years ago. Ponting's website has several more images not shown here, plus a video.


Paul Mullins said...

Glad you're back after your recent hiatus.

Imre said...

Taking a look at the renderings on Ponting's website, they reveal that the slenderness of the pylon masts rely strongly on the different "unit weights" of the spans. The main span features a steel box girder with orthotropic plates, while the back span consists of a massive reinforced concrete box girder. This helps - along with the anchor piers in the back span - to balance the longitudinal bending monents at the pylon base. This does not only allow for slender masts, but also reduces the dimensions of the foundation considerably.