On June 6, 4,000 people walked across the Northumberland Strait from New Brunswick to Prince Edward Island. No, it wasn’t a miracle: it was a celebration of the first anniversary of the 12.9-kilometre Confederation Bridge, one of the longest multi-span bridges in the world.
Impressive though it is, Confederation Bridge is really just a modern version of a series of logs laid on stepping stones across a stream. That, a single log long enough to span a stream by itself (a beam bridge), and twisted vines hung across a stream and tied to tree trunks on either side (a suspension bridge) were the earliest bridges. Next came the masonry arch bridge: tradition states such a bridge was build about 1800 B.C. in Babylon, and the Romans built masonry arch bridges still standing after 2,000 years.
A beam bridge is limited in span by the strength of its main girder; if it’s too long it can’t support its own weight, and breaks. One way to get around this is to add supporting girders, or trusses, above the span. Wooden truss bridges go back to the Renaissance; many old highway bridges are steel truss bridges. Another way to extend the span is to suspend it from cables; the first modern suspension bridges were built in the 19th century.
Concrete, which can’t be easily compressed and can be reinforced with steel to help it withstand flexing, has been the 20th century’s bridge-building material of choice, but even when made of the latest materials, bridges sometimes fail due to faulty steel, wind, earthquakes, floods, being rammed by ships or simple erosion, as the flow of water eats away the rock beneath the supporting piers.
The engineers who built the Confederation Bridge were aware of all these hazards, but their biggest concern was ice: every year, millions of tonnes of floes, driven by tides, winds and currents, race through the strait, sometimes piling up into large masses of compressed ice rubble 20 metres deep under the water and as tall as a two-story building above. Fortunately, the Confederation Bridge’s segments are even more massive: each pier-and-girder unit weighs more than 20,000 tonnes. (They’re not even fastened to the floor of the strait: gravity holds them in place.) The ice never exerts a force of more than 3,000 tonnes. Each pier also features a cone-shaped ice shield, which forces ice floes to ride up, bending them and causing them to crack. The piers are 250 metres apart to let as much ice through as possible.
Winds were another concern. The Northumberland Strait acts like a giant wind tunnel. Barriers more than a metre tall on each side of the roadway cut down on gusts that otherwise could topple semi-trailers. (They also keep drivers from seeing how far above the water they are.) More wind-tunnel tests were done on designs for the Confederation Bridge than for any bridge before it.
The Confederation Bridge is made of a new high-performance concrete, invented specifically for its consturction, that’s easily shaped, but resistant to ice abrasion, the corrosive action of salt water and other elements. The mixture includes cubical stone with a rough, irregular surface (conventional concrete uses flat-edged stones), plus unusual ingredients like fly ash, silica fume, blended cement and a plasticizing agent.
(Future bridges may be built of “smart” concrete. Researchers from the University of Toronto’s Institute for Aerospace Studies have lined the columns of the Leslie Street bridge in northern Toronto with fibre-optic sensors and wrapped them in lightweight advanced composite materials. The wrap protects the bridge, while the sensors track corrosion. The goal is to develop a method of monitoring the health of a bridge by computer from a central location.)
The Confederation Bridge, which has a slight S shape to help keep drivers more alert, soars 60 metres above the water at its highest point, and consists of 22 smaller bridges of two main girders each, joined by 60-metre, fixed drop-in spans. The largest main girders weigh 7,500 tonnes, and are as long as two football fields. Every second span is hinged; that allows for expansion and contraction due to temperature changes and also ensures that if something destroys one section of the bridge, only the damaged section will drop out–it won’t cause the entire bridge to collapse.
In fact, the Confederation Bridge is such a wonderful technological achievement, maybe the fact 4,000 people could walk from New Brunswick to Prince Edward Island is a bit of a miracle after all.
