A cantilever bridge is a bridge built using cantilevers, structures that project horizontally
into space, supported on only one end.
For small footbridges, the cantilevers may be simple beams; however, large cantilever
bridges designed to handle road or rail traffic use trusses built from structural steel, or
box girders built from prestressed concrete.
The steel truss cantilever bridge was a major engineering breakthrough when first put into
practice, as it can span distances of over 1,500 feet, and can be more easily constructed
at difficult crossings by virtue of using little or no falsework.
Origins Engineers in the nineteenth century understood
that a bridge that was continuous across multiple supports would distribute the loads among
them.
This would result in lower stresses in the girder or truss and meant that longer spans
could be built.
Several nineteenth century engineers patented continuous bridges with hinge points mid-span.
The use of a hinge in the multi-span system presented the advantages of a statically determinate
system and of a bridge that could handle differential settlement of the foundations.
Engineers could more easily calculate the forces and stresses with a hinge in the girder.
Heinrich Gerber was one of the engineers to obtain a patent for a hinged girder and is
recognized as the first to build one.
The Hassfurt Bridge over the Main river in Germany with a central span of 124 feet was
completed in 1867 and is recognized as the first modern cantilever bridge.
The High Bridge of Kentucky by C. Shaler Smith, the Niagara Cantilever Bridge by Charles Conrad
Schneider and the Poughkeepsie Bridge by John Francis O'Rourke and Pomeroy P. Dickinson
were all important early uses of the cantilever design.
The Kentucky River Bridge spanned a gorge that was 275 feet deep and took full advantage
of the fact that falsework, or temporary support, is not needed for the main span of a cantilever
bridge.
The most famous early cantilever bridge is the Forth Bridge.
This bridge held the record for longest span in the world for seventeen years, until it
was surpassed by the Quebec Bridge.
Benjamin Baker illustrated the structural principles of the suspended span cantilever
in the photo on the left.
The suspended span, where Kaichi Watanabe sits, is seen in the center.
The need to resist compression of the lower chord is seen in the use of wooden poles while
the tension of the upper chord is shown by the outstretched arms.
The action of the outer foundations as anchors for the cantilever is visible in the placement
of the counterweights.
Function
Cantilever Bridge.—A structure at least one portion of which acts as an anchorage
for sustaining another portion which extends beyond the supporting pier.
— John Alexander Low Waddell, Bridge Engineering
A simple cantilever span is formed by two cantilever arms extending from opposite sides
of an obstacle to be crossed, meeting at the center.
In a common variant, the suspended span, the cantilever arms do not meet in the center;
instead, they support a central truss bridge which rests on the ends of the cantilever
arms.
The suspended span may be built off-site and lifted into place, or constructed in place
using special traveling supports.
A common way to construct steel truss and prestressed concrete cantilever spans is to
counterbalance each cantilever arm with another cantilever arm projecting the opposite direction,
forming a balanced cantilever; when they attach to a solid foundation, the counterbalancing
arms are called anchor arms.
Thus, in a bridge built on two foundation piers, there are four cantilever arms: two
which span the obstacle, and two anchor arms which extend away from the obstacle.
Because of the need for more strength at the balanced cantilever's supports, the bridge
superstructure often takes the form of towers above the foundation piers.
The Commodore Barry Bridge is an example of this type of cantilever bridge.
Steel truss cantilevers support loads by tension of the upper members and compression of the
lower ones.
Commonly, the structure distributes the tension via the anchor arms to the outermost supports,
while the compression is carried to the foundations beneath the central towers.
Many truss cantilever bridges use pinned joints and are therefore statically determinate with
no members carrying mixed loads.
Prestressed concrete balanced cantilever bridges are often built using segmental construction.
Construction methods Some steel arch bridges are built using pure
cantilever spans from each side, with neither falsework below nor temporary supporting towers
and cables above.
These are then joined with a pin, usually after forcing the union point apart, and when
jacks are removed and the bridge decking is added the bridge becomes a truss arch bridge.
Such unsupported construction is only possible where appropriate rock is available to support
the tension in the upper chord of the span during construction, usually limiting this
method to the spanning of narrow canyons.
List by length
World's longest cantilever bridges: Quebec Bridge 1,800 feet
Forth Bridge 2 x 1,710 feet Minato Bridge 1,673 feet
Commodore Barry Bridge 1,644 feet Crescent City Connection 1,575 feet
Howrah Bridge 1,500 feet Gramercy Bridge 1,460 feet
San Francisco – Oakland Bay Bridge 1,400 feet
Horace Wilkinson Bridge 1,235 feet Tappan Zee Bridge 1,212 feet
Examples
References
External links "Cantilever Bridge" by Sándor Kabai, The
Wolfram Demonstrations Project, 2007.
Bridges – Their Structure and Function, Brantacan
Biggest of Finished Girders Go Traveling: six giants of 70 tons gave engineers a hard
nut to crack, Popular Science monthly, February 1919, page 79, Scanned by Google Books: http:books.google.com/books?id=7igDAAAAMBAJ&pg=PA79