Riding the wave with timber

A 1,500 sq m timber deck sitting on top of a steel structure is the centrepiece of a new pedestrian bridge spanning a six-lane motorway on the southern coast of Singapore.

The bridge, called Henderson Waves was opened in May and spans a ravine some 284m above ground and is the largest of its kind in Southeast Asia.

The design concept is based on a folded three-dimensional surface created by means of simple mathematics.

This form bends, undulates, and ascends by 21m in one movement. It also deforms to provide adequate egress, sloping, shelter, and scenic viewing to pedestrians and cyclists.

Kevin Hill, managing director of suppliers Venturer which, in addition to sourcing the tropical hardwood, Yellow Balau, oversaw the fabrication and installation of the timber components, said: “Our involvement began at the early design phase, conceiving the timber details and specifications, through to a combination of modular fabrication and painstaking installation work.”

Below, TiC details the complex design and construction techniques involved.

CURVED PROPORTIONS
The complex, doubly curved proportions of this large expanse of tropical hardwood form a tapestry of 5,000 modular boards, each varying by a single degree every 10m. There is not a single visible fixing on the entire sculpted timber surface.

DECKED OUT

The deck panels were fabricated in a factory using hydraulic presses, and were doubly reverse-fixed with screws and the manufacturer’s own Epoxy products.

The highly curved parts of the deck, namely the backrest and handrail components, were dowel-fixed with stainless pins and Epoxy filling.

DIMENSIONS

The full length of the bridge runs close to 284m, and the difference in elevation between springing and landing is equal to the full height of a seven-storey housing block.

Henderson Waves has seven spans, six of which are of typical size (3.5m high and 24m long, and one of which is larger at 6m high and 57m long).

The upside spans unfold above deck level and house temporary activities, such as seating, lounging and sightseeing.

MODEL STRUCTURE

The primary structure of the bridge features a sequence of steel arches and down-facing arches joined across and resting upon reinforced concrete pylons.

These pylons, the tallest of which reaches 38m, sit between ‘waves’, at the point where the surface-form self-intersects and tapers down to a single beam.

RESPONSIBLE SOURCING

To help establish the legal origins of the chosen essence - a tropical hardwood known as Yellow Balau – the Singapore-based timber supplier and global environmental activist Certisource used their innovative DNA-tracking techniques to identify appropriate individual DNA markers.

The process is still pending as pilot studies of a species are typically carried out from six months to a year before the DNA is relied upon as sufficient evidence for certification.

TIMBER ON STEEL

The entire deck is supported on a steel subframe. The co-ordination of steel and timber became a challenging task. Using its proprietary equations, the architect issued precise numerical descriptions of the surface at regular 500mm intervals, and provided dimensional coordinates that greatly assisted the production of the timber manufacturer’s shop drawings.

To correct any variations between the timber surface and the outer face of the steel caused by the galvanising process, most deck panels were joined with polysulphide mastic.