Feet first

Two nearly identical footbridges straddling State Highway 20 have brought cable-stay bridging technology to Auckland for the first time.   By Hugh de Lacy

Built at a cost of $1.2 million each to re-unite Manukau communities split by extensions to the Western Ring Route expressway, they were completed in February and March last year by Fulton Hogan for the New Zealand Transport Agency, and named after local body identities Keith Hay and Ernie Pinches. In February of this year a cable-stayed road bridge joined them in Manukau, and will be reviewed in next month’s issue of Contractor.

Cable-stayed bridges represent modern developments of construction methods pioneered by a German carpenter,

CT Loescher, in the late 18th Century, and bear some similarities to swing-bridges which are common in New Zealand. Like swing-bridges they feature towers or pylons that take the live load through cables, but the key difference is that cable-stayed bridges can require only one tower – as is the case with the Manukau footbridges.

And where the main cables are the load-bearing component in swing-bridges, it’s the tower – or sequence of towers – that take the weight in cable-stayed structures. The advantages of the cable-stayed bridge over the swing-bridge are the much greater stiffness of the deck and the absence of the need for large in-ground anchorages for the main cables.

Cable-stayed bridges come in two main types, the fan and the harp. In the harp design the cables from the tower to the decking are virtually parallel because the height of the attachment point on the tower is the same as the distance to the attachment point along the decking. In the fan design, adopted for the Manukau footbridges, the cables all connect to or pass over the top of the tower.

The technology is applicable to a wide range of bridging requirements, and probably the world’s most famous cable-stayed bridge – and arguably the most beautiful of any design – is France’s seven-pylon Millau Viaduct, 270 metres above the Tarn River. At 2460 metres it is the longest bridge of its type, while its piers, at 341 metres, are the tallest in the world.

The first of the Manukau foot-bridges to be built was the Keith Hay, which connects the park of the same name to three schools, Mount Roskill Primary, Intermediate and Grammar, across not only State Highway 20 but also the suburban railway line and a local street, Somerset Road.

The Ernie Pinches bridge spans only the motorway from Richardson Road, providing access for residents and for pupils of the Wesley Primary and Intermediate Schools. Both bridges are 170 metres long, contain 13 pre-cast concrete decking slabs, and with piers that are 600mm square at the base spreading to a ‘Y’ shape at the top.

The Ernie Pinches bridge has two piers supporting the footway approaches compared to 10, at heights of up to 5.5 metres, for the Keith Hay bridge, because the latter has a spiral walkway on the side connecting it to the park of the same name.

The single steel pylon supporting the cables on each bridge stands 25 metres above ground, while the height of the decking above the motorway is about six metres. Each pylon supports 40 metres of decking on either side through six pairs of 36mm stress-bars. Each bar has a 500 tonne support capacity.

While the cable-stayed design was something of novelty for consultant URS and Fulton Hogan project engineer Neill Dunlop, the only real problems with building the bridges arose from the challenging ground conditions. Proof drilling revealed a layer of peat lying on top of a basalt flow that varied in thickness from one metre to six metres above the underlying Waitemata sandstone series.

This meant that each of the piles for the 12 approach piers had to be individually designed, with most of them being socketed into the underlying sandstone. The piers or columns

themselves were pre-cast on site. To ensure the levels were correct, the length of the pre-cast unit was compared with the difference in height between the deck and the pile cap.

A steel frame was pre-fabricated and welded to the steel of the column. When the column was placed, plastic shims were used to get the top of the column within tolerance of the deck. The column was then supported by props fixed into Kelly blocks, and once the pile lap was boxed and the level checked again, the concrete was poured.

Once a few of the columns were in place, temporary support was erected to receive the deck sections, each of which was pre-cast on site and weighed bout 35 tonne. Neill Dunlop told Contractor it was a bit tricky establishing the correct levels for the columns on either side of the steel pylon, but the erection of the pylons themselves was “pretty much straight-forward.”

Placement of the deck sections was likewise free of problems except for the two on the part of the Keith Hay bridge that spanned Somerset Road. Installing a column in the middle of the road left too little space for traffic diversion, there were restrictions on constructing an extra lane under the drip-line of the trees, and it was also considered risky to place a temporary support tower between two lanes of live traffic.

The only alternative was to pre-cast both deck sections and stitch them together on the ground, then the road was closed for one night while the two sections were installed as one.

Special care had to be taken where a cable anchor, which required precise orientation, had to be incorporated in the mass of steel protruding from two connecting deck sections and their support column. This was achieved with the help of a pair of binoculars and a small laser.

More or less routine was the stressing of the cables: the point at which the load applied, the deflection on the pylon and the movement at each stitch joint had to be monitored to the design engineer’s satisfaction before the staging could be removed.

Taken as a proportion of the multi-million dollar cost of Auckland’s Western Ring Route, the two Manukau footbridges over State Highway 20 are a minor item. But in the context both of their being a novel approach to Auckland bridge-building, and of their significance to the communities otherwise split in half by the highway, they comprise a landmark development.