Situated north of Wanganui where the road runs beside the meandering and flood prone Mangawhero River, the project site is infamous for its subsidence and poor alignment. After careful analysis and review, the New Zealand Transport Agency (formerly Transit) concluded that it was uneconomical to continue to maintain the road in its present location. A scheme to avoid the area of instability and straighten the road emerged as the appropriate way forward provided costs could be contained.

The scheme featured two bridges that carry the realigned route across the river. The 90m North Bridge is curved with a constant cross-fall of 7.6%. The 96.6m South Bridge is straight with a 3% cross-fall about the bridge centreline. Three-span Super ‘T’ pre-cast concrete bridges were detailed in the tender documents. In the conforming design, abutments were to be founded on a pair of 1.5m diameter bored piles, with a single 2.4m diameter pile supporting pier stem columns, with reinforced concrete pier caps at intermediate positions.

Concrete Structures (NZ) Ltd identified a number of key challenges with the conforming design. These included transporting the Super ‘T’ girders to the site; poor access to the bridge locations between steep river banks (the North and South Bridge decks are respectively elevated 15 and 20m above the river); adequate cranage; and the large diameter piles. Although the proposed structures would have provided a robust solution, the tender documents did not preclude alternative designs.

The Managing Director of Concrete Structures, Mike Romanes, instinctively predicted that a steel composite alternative would be more economical. Steel girders could be transported to the site in reasonable lengths as normal loads and spliced on delivery. Being substantially lighter than the Super ‘Ts’, the girders could be erected with a relatively small crane, saving on costs. And since the superstructure would be lighter, pile sizes could be reduced as would the cost of the substructure.

Holmes Consulting Group was subsequently charged with investigating steel options, with assistance from structural steel bridge specialist Eastbridge Ltd in Napier. Eastbridge’s managing director, Bruce Mellsop, comments on the role that the steel constructor plays: “We add value through economical fabrication detailing. This entails optimising members for steel sizes and grades that are available. We also make recommendations about site constructability.”

Phil Gaby and Michael Chan were the primary Holmes personnel involved. Phil Gaby: “Two steel girder systems were evaluated: a traditional steel composite three-girder arrangement in which continuous steel girders support a transverse spanning pre-cast concrete deck; and a ladder deck arrangement in which girders land directly on top of the pier columns, eliminating the need for expensive pier caps. Load sharing between the main girders of the ladder deck bridge is excellent, with each girder taking half the dead load and nearly half the live load. Load sharing in the three-girder bridge is not as even. The ladder deck bridge was the clear winner.”

The new bridges as designed by Holmes Consulting Group each consist of two continuous, fabricated “I” girders, varying in depth from 1.5m at mid-span to 2.4m at the piers. UB transoms span between the girders at 4m centres along the decks. The transoms carry the deck slab, which is made up of longitudinally orientated pre-stressed planks with an in situ concrete topping.

Michael Chan: “Integral abutments and piers eliminate the need for expensive and maintenance-intensive bearings and joints. Each pier consists of pair of relatively slender 12m high 900mm diameter reinforced concrete columns, one under each of the two girders. The columns land on 1.2m diameter bored reinforced concrete piles founded in the rock below. Each abutment is supported by a pair of 900mm diameter bored reinforced concrete piles Additional structural efficiency was also realised as a result of the connectivity between the substructure and the superstructure with this form of construction.”

A single coat of zinc silicate provides protection for the steelwork. The NZ Heavy Engineering Research Association assisted by recommending various coating options and estimating the net present value coating system maintenance regimes. These were key factors in helping the client choose the steel option.

Concrete Structures offered the steel ladder deck bridge as an alternative tender to the conforming design. Being 15% cheaper, even after the re-engineering costs for changing the conforming design into a steel structure, the ladder deck solution was preferred.

A final point deserves to be made regarding the ease of erection of steel compared with the concrete alternative. Bruce Mellsop: “The steel superstructures of the two bridges were delivered and erected in less than four weeks. They were not erected simultaneously but one after the other and by a small site team of six. This is awesome for large span bridges, especially at sites where access is difficult.”