27 February 2013

Kruunusillat bridge design competition: the entries (Part 3)

This is the last of three parts, concluding the entries to the Kruunusillat bridge design competition ... You can find more details on all the entries online, and there's a set of all the videos on YouTube.

Gemma Regalis
Most of the entries seem to have taken the view that the bridge should not compete visually with the relatively low-lying Helsinki skyline. This is the only one to buck the trend, with a single tall cable-stayed mast at its centre. The secondary spans are prestressed concrete on Y-shaped supports. The walkway is on one side of the bridge only, which I think is the best choice.

Recreatio Maritimus
This bridge is a sequence of very different experiences, with different structural forms to suit. At one end, there's a garden walk, with a tree-lined promenade carried on an arch bridge, and at the other the tramway and walkway are separated by a single central truss, supporting canopies on either side. This separation of tramway and walkway seems sensible, but it's far from clear why users would only want shelter at one end of the crossing and be exposed to the elements along the remainder. This design would also presumably require a substantial box girder below deck level to provide torsional stiffness, which detracts from the otherwise slender lines a truss can offer. On some of the shorter spans, an undulating spine girder provides the same separating function.

From an engineering perspective, I find this last entry by far the least convincing. The obvious question is why they went for a suspension bridge, one with twin 550m spans - there seems nothing about the site to suggest that multiple foundations in the water will be an issue for navigation or construction. The resulting design attempts to be slender, but requires additional stay cables and a massive under-deck truss to provide the necessary stiffness. Design of light rail bridges is governed by stiffness, and a suspension bridge is fundamentally ill-suited to satisfying this criterion. The concept reduces the number of bridge bearings and joints to a minimum, but at what cost?


Anonymous said...

"there seems nothing about the site to suggest that multiple foundations in the water will be an issue for navigation or construction"

Regarding Oculus. Neighbors and people into boats might like this one. Many might think it does not block the sea view as much. I haven't seen any sketches how any bridges look from the nearby beach.

The Happy Pontist said...

That has to be a pretty powerful demand to justify choosing a structure which will cost far more to build than some of the alternatives.