Bowstring Bridges

Bowstring Bridges are another form of Arch Bridge, albeit where the road crosses the arch rather than sitting on top of it. Obviously, this requires the arch to be built of metal, or a concrete truss rather than the traditional solid stone, and the roadway is suspended from the arch members by cables similar to those used on Suspension Bridges.

Bonar Bridge on the old A9

A good example of a Bowstring Bridge is the bridge spanning the Kyles of Sutherland on the old A9, now A836 at Bonar Bridge in the Scottish Highlands. Here two large steel arch beams span the channel, with a lattice spanning the road to hold the two together. Sufficient height is left clear at either end to allow traffic on and off the bridge - rather essential really! The deck is then suspended from the arches, in this case by 15 rods on either side. The deck itself is of concrete, supported by steel girders.

The Tyne Bridge

Bonar Bridge is a design sometimes referred to as a through-arch bridge. This differentiates it from a half-through arch bridge, where the arch springs from below the road deck as seen at the Tyne Bridge (right) in Newcastle. In both cases the arch needs to be securely anchored at each end to prevent it from spreading. The massive towers of the Tyne Bridge help to counteract the arch, while Bonar Bridge is anchored into the bedrock of either river bank with deep foundations.