By Rodger W. Welch, P. Eng. Buckland & Taylor
Golden Ears BridgeTransportation Transportation Infrastructure
The Golden Ears Bridge, which opened to traffic on June 16, 2009, provides a vital new connection over the Fraser River about 30 kilometres east of Vancouver. Set in one of the most beautiful landscap...
The Golden Ears Bridge, which opened to traffic on June 16, 2009, provides a vital new connection over the Fraser River about 30 kilometres east of Vancouver. Set in one of the most beautiful landscapes in the world, the bridge provides a striking new landmark for the surrounding communities and dramatically improves the movement of people and goods through the region.
This main bridge is the centrepiece of a comprehensive network of new roads built to connect Maple Ridge and Pitt Meadows north of the river, to Langley and Surrey to the south. Through a competitive process TransLink, the Greater Vancouver region’s transportation authority, awarded the Golden Crossing General Partnership the $800-million contract to design, build, finance, operate and maintain the Golden Ears Bridge and associated road network ( “Golden Ears Project”) for a 35-year concession period.
The Golden Ears Bridge is the longest multi-span, cable-stayed bridge constructed in North America and one of relatively few in the world. Buckland & Taylor were part of the Golden Crossing’s team and provided the conceptual, preliminary and detailed design of the main bridge and its approaches. They also provided erection engineering for the main bridge.
The Golden Ears Bridge has a total length of 968 metres, with five spans supported by four river piers with heights up to 80 metres.
Hybrid cable-stayed, extradosed form
The bridge is significant for its “hybrid” multi-span composite, cable-stayed form. This innovative bridge form was central to the success of the Golden Crossing Partnership’s competitive proposal because it provided the best solution for the varied constraints of the site. The site constraints included two widely separated navigation channels, high river flows, sensitive environmental areas, poor foundation conditions, and relatively high seismicity.
Buckland & Taylor’s design team worked closely with the construction team on the design solution. More than a dozen preliminary concepts were developed, and these were carefully screened down to three alternatives for detailed consideration: cable-stayed, concrete segmental box girder, and the “hybrid” cable-stayed/extradosed. The final concept was selected for its comparative light weight, seismic performance, constructability and cost effectiveness.
The geometric constraints imposed by the two navigation channels naturally led to an optimal series of three river spans of 242 metres and two 121 metre end spans. These span lengths are well within the range of a true cable- stayed bridge. However the towers would have en- croached on the glide path for the nearby Pitt Meadows Airport. An extradosed bridge characterized by low profile towers would have satisfied this tower height restriction but the span length is beyond the effective range of a true extradosed bridge.
The chosen hybrid bridge form combines attributes of a cable-stayed bridge and an extradosed bridge, taking advantage of benefits from each of these bounding forms of cable-stayed bridges. The final multi-span composite cable-stayed form has the span range, lightness and constructability of a composite cable-stayed bridge, but also uses low-profile towers which are features drawn from an extradosed bridge. The combination of the parallel harped cable-stays and the low profile towers that do not need crossbeams above the deck provides a clean, aesthetically pleasing view.
Flexible piers with both permanent and ductile hinges
The bridge design was influenced by the moderate-to-high seismicity of the site and deep, soft liquefiable deposits in the river. The relatively high seismic demands and poor foundation conditions led to a flexible pier concept, which uses a twin wall pier leg “tuning fork” arrangement. The arrangement provides a ductile substructure that essentially isolates the superstructure from the foundations seismi- cally. The pier legs are designed to perform elastically, with ductile “plastic hinges” at their top and bottom sections. They are designed for the service level design earthquake, which has a 475 year return period, and to perform inelastically for the ultimate level design earthquake, which has a 2500 year return period.
The substructure concept, however, was not without challenges for the design team. The main navigation channel requires approximately 45 metres of height clearance close to the northern bank of the river. As a result there is an asymmetrical roadway profile to the bridge which requires the pier legs at Pier M2 near the southern bank to be significantly shorter and stiffer than the other piers. These legs are disproportionately at risk during seismic events, so an innovative solution was developed to reduce their stiffness. We introduced a permanent steel “hinge” detail that effectively acts as a pin at the bottom of the legs. The hinge is a steel plate which provides the necessary vertical capacity as well as the flexibility to shed seismic force and to accommodate the seismic displacement demands. The hinge plate section (100 mm thick by 3600 mm wide) transmits the entire load from the pier leg to the foundation.
Incorporating “settlements slabs”
In addition, the underlying deep soft silt and clay deposits posed a significant risk related to future settlements. Differential settlements exceeding the design value of 250 mm would threaten the integrity of the bridge. The solution was to incorporate a unique feature –“settlement slabs” –which can be used to raise the bridge towers using hydraulic jacks in the event that excessive foundation settlements occur. The bridge piers are supported on the 3-m deep reinforced concrete settlement slabs, which are in turn locked to the bridge foundation at the pile cap by high strength post-tensioning bars. In the event of excessive settlements the bars can be released, the settlement slab raised off the pile cap to correct the geometry, the gap filled with high-strength grout, and the post-tensioning bars re-stressed to lock the settlement slab back to the pile cap.
An important requirement of TransLink was to create a visual icon for the region by having an overall aesthetic theme for the new route. As a result, the design of the bridge incorporates architectural features reflecting the natural area such as bridgehead lanterns, bridge fencing with an historic “fish trap” theme, golden eagle sculptures, and interpretive panels to narrate the story of the Katzie First Nation and the historic Fraser River.
The final result is a beautiful, functional bridge form that efficiently addresses the unique challenges of this crossing and is a striking work of engineering.
Rodger Welch, P. Eng. is an Executive Engineer and Senior Project Manager with Buckland & Taylor of Vancouver.
Project owner: TransLink
Main bridge design & erection engineer: Buckland & Taylor (Don W. Bergman, P. Eng., Rodger W. Welch, P. Eng., Dusan Radojevic, P. Eng., Hisham Ibrahim, P. Eng.)
DBFO contractor: Golden Crossing General Partnership (led by Bilfinger Berger BOT)
Design-builder: Golden Crossing Constructors -Joint Venture, (Bilfinger Berger (Canada) and CH2M Hill)
Geotechnical consultant: Trow Associates
Roadway design: McElhanney Consulting Services