15 October 2013

Rotterdam Bridges: 1. Erasmus Bridge

I managed to find time on a recent trip to Rotterdam to visit a few of the city’s most spectacular bridges, which I’ll discuss over this and the next few posts.

As with many modern city bridges, the Erasmus Bridge serves both a practical and a symbolic purpose. Prior to its construction, transport connections across the river were limited, and the new bridge made generous provision: two single-lane highways, two tramways, two cycleways, and two pedestrian walkways. However, it was also required to satisfy the needs of civic pride, to be the most visible symbol of Rotterdam’s post-war reconstruction and growing economic success. It had to be a landmark both in purely visible terms, signposting the city centre from far and wide, and also as a technological achievement.

The bridge design was proposed by an architect, Ben van Berkel of UN Studio, directly inspired by Calatrava’s Alamillo Bridge in Seville. It was initially hoped that the bridge could be built in the same manner, without back-stays. The backwards-leaning inclination of the inverted-Y-shaped pylon allows the tower to act as a counterweight to the main deck. With a main span of 284m and a tower height of 139m, it proved impossible to make the bridge work without backstays, with even a balance of dead load alone requiring a significantly more substantial pylon.

The cranked arrangement of the pylon is the bridge’s signature feature, seldom replicated (I can think of one obvious example). It apparently provoked controversy amongst engineers not used to being subservient to architects and unhappy with the significant increases in cost required to build such a structure.

It was put to me by a friend that the bridge is not structurally logical, but this is unfair. The inclined pylon does to some extent balance the main span, reducing loads on the back stays significantly, and the crank is a coherent response to the vertical cluster of anchorages high on the mast. The only obvious enhancement in terms of how it distributes its forces would be for the upper part of the pylon to be curved, a way of reducing tower bending under spread cable forces which has been adopted elsewhere by Calatrava.

Certainly, the temporary propping required during bridge construction will have added to its cost, but I really find it hard to imagine that a more conventional vertical pylon would have looked as satisfactory.

A key element which makes less structural sense is the treatment of the deck girders. In the main span, the steel plate deck sits on transverse ladder beams, which span between and cantilever beyond two primary steel box girders. This allows the deck to appear reasonably slender in elevation, and is probably less expensive overall than allowing the entire deck to span between massive edge girders.

The back span is very different, as it does span between two massive edge girders, these girders forming the distinctive “legs” to the tower, and giving it a kind of abstract resemblance to a person kneeling (I am not sure if this was a conscious intention).

This arrangement has no structural rationale, as it would be more logical to continue the recessed girders. The result is the need to transfer the axial forces in the front span girders into the back-span girders (and pylon foundations) via substantial transfer steelwork. The “legs” are also far larger, at a maximum of 12m deep, than is required to actually carry the back span, with the result that they are largely comprised of fascia elements, with the actual structural girder being much shallower.

Behind the legs, there is a bascule span, designed to provide a shipping clearance of 50m. This is claimed to be the longest single bascule leaf in the world, making it an important structure in its own right, although it is only a small part of the overall structure.

Shortly after the bridge opened, the cables were observed to vibrate under certain combinations of rain and wind. The problem has since been solved by the provision of a hydraulic damper at the foot of each cable.

Close up, the pylon has been detailed largely for architectural effect, with a variety of kinked plate surfaces presumably intended to catch the light and avoid the plainness present on many other cable-stayed bridges. I think it looks good from pretty much almost every angle. There is a slightly odd "lipped" element on the main span face of the pylon, but I think it does need this sort of treatment to avoid looking quite dull.

One issue with cable-stayed bridges at this scale is that the cable sag can get quite significant. This is irrelevant from most perspectives, but disconcerting when you actually look directly along the cables, where the sag leads to the impression that the cables aren't properly tensioned. However, I suspect hardly anyone other than the curious bridge engineer will ever notice this.

Although the deck, cables and pylon are the primary expressive elements of the Erasmus Bridge, I was particularly impressed by the treatment of the substructure, the piers, abutments, and associated ramps and staircases at the north end of the bridge in particular. These take the shape-making desire visible in the steelwork even further in concrete.

There are a couple of twisted pier columns, and the north river pier is in the form of a monolithic "V" with some attractive chiselling. The ramps and staircase which lead from below the bridge up to deck level are also very nicely sculpted. These are all areas where the attention to detail of the architect has paid dividends.

On the whole, I find it hard to see why there's much controversy about this bridge. The design is well-detailed and visually appropriate, given everything the bridge was asked to do. I think it's a fine bridge that deserves to be more highly regarded.

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