Arrow route – bridge design
Peter Blundell Jones
This footbridge unites the two halves of Graz with a figure that incorporates memories of bows and beasts.
Unless a bridge is very short, its form tends to be strongly influenced by its structural operation, its elegance or lack of it conditioned by the way the span is achieved. Yet there is always a choice of structural forms, each with its own supporting theory and method of calculation, which is inevitably reflected in the result. And although structural performance seems the most pressing issue, there are others that turn out to exert a strong influence. The bridge must be stable not only after construction but during it, and it may have asymmetrical end conditions that demand response. It may need to be quite flat or be allowed to rise and fall. The joint with the abutments must be made. A surprisingly difficult problem can be the integration of balustrade and handrail with the rest of the structural design.
Domenig, Eisenkock and Egger’s new footbridge across the Mur in Graz links two central areas of the city close to the Schlossberg, and was the winner of a competition. It is remarkable for the way in which all the design problems are integrated in one neat solution, at once structure and sign. The main idea for the 56m span is a structure like a drawn bow with steel cables as its string and a single vertical strut as its arrow, clearly dividing the compression and tension elements. The bow is made of a triangular box section placed one side down to rest its corners on the concrete legs that support it on either bank. To allow it to arch upwards, the footway rises slightly towards the middle, but the box section also tapers losing depth. In plan, this taper effects the transition, from two legs at the edge to the single strut in the middle, and the triangulation of the cables provides lateral stability. The footway is supported on top of the bow by a series of steel ribs that taper with diminishing load towards their edges, where they also support the uprights for the glazed balustrade. These ribs cantilever increasingly as the triangular box section of the bow diminishes.
The description so far treats the bridge as a general type, a pure symmetrical object, but in Graz the site conditions are asymmetrical, the main problem being that the west bank is 2m higher than the east. This condition is accommodated by keeping the structure horizontal but allowing the footway to drop across the eastern half, letting the triangular structure of the bow emerge progressively to divide the footway into two halves and swelling the overall width accordingly. This leaves space at the end for a central staircase to descend to the bank, but the triangular form of the bow end, which has fully emerged from the footway, also needs to be terminated. Beyond its supports, it turns into a projecting spike and becomes a canopy to the central steps that is, for users, the most memorable element of the bridge. It is also the most controversial. Domenig justifies it as a sign announcing the presence of the bridge, but Cedric Price criticised it as being too solid for a sign. If a sign be needed, he argued, it would better be done with a hologram.(*)
This ignores the fact that the heavy triangular section, once exposed, needs some conclusion. The asymmetry of the bridge is tantalisingly appropriate, and a welcome signal that it is no mere technical object but a space of transition between two territories whose differences need to be acknowledged.
* From a verbal session at the Haus der Architektur during one of Price’s periodic visits to Graz in the summer of 1994.
COPYRIGHT 1995 EMAP Architecture
COPYRIGHT 2004 Gale Group