e-mosty June 2017: Osman Gazi B. US Suspension. Hålogaland B. e-mosty June 2017: Suspension Bridges | Page 43

North America as a whole has a combined total of
nearly 30% of the world’s suspension bridges. Notable
bridges in the U.S. include Verrazano-Narrows, Golden
Gate, Mackinac, George Washington, East Bay in San
Francisco and Tacoma Narrows – the longest being
Verrazano-Narrows with a main span of 1282m.
Notable bridges in Canada include Lions Gate, Angus
L. Macdonald, and A. Murray MacKay with suspended
spans reaching up to 472m.
The U.S. has the oldest major cable suspension bridge
inventory with an average age of 73 years (Figure 2).
The older of these bridges includes Williamsburg
(1903), Brooklyn (1883) and Roebling (1867) with
Wheeling (1849) being the oldest in the U.S.
As these bridges get older their maintenance
requirements naturally keep increasing with time and
it will become more and more critical to ensure that
adequate funding is provided to ensure that
acceptable levels of safety and service are provided
throughout the life of these bridges.
Cable supported bridges are different from other
bridges in many ways and one significant difference is
that some of the most critical structural elements are
above the heads of the very users of the bridge. This
makes both inspection and maintenance very
challenging and ensuring there is safe access and
effective containment is one of the key elements
required before any inspection or maintenance work
is carried out.
Funding of course is always an issue. Typically long
span suspension bridges in the US are tolled and are
owned by stand- alone public agencies. It is not usual
for them to be owned by the State or Federal
Government. Of course there are exceptions to the
tolling and ownership rule. Brooklyn, Manhattan and
Williamsburg Bridges in NYC are not tolled and are
owned and maintained by the New York City DOT. In
many cases the Bridge Authority will also be
responsible for other facilities and infrastructure as
diverse as subways, ferries, ports and airports.
Figure 2: Distribution of Age of Major US Suspension Bridges
However, since Verrazano Narrows opened in 1964
only three new long span suspension bridges have
been constructed in the US at Carquinez, Tacoma and
the notable self-supporting East Bay Bridge in San
Francisco.
The preferred form of long span bridge in the US is
now the cable stayed bridge. The new Tappan Zee and
Goethals Bridges are both cable stayed bridges.
Although it is worth noting that there are no cable
stayed bridges currently in the US with spans
exceeding 500 metres.
Therefore, because of the relatively young age of the
cable stayed bridge stock in the US and the age of the
older suspension bridge inventory, most of the work
carried out on existing bridges in the cable supported
bridge market in the US is ensuring that the
existing suspension bridges have a service life
equivalent to their design life.
As toll increases are rarely popular, there is always a
challenge for the bridge engineer to ensure that
adequate funds are made available for work to be
carried out on the bridges. This is never easy
especially for preventative maintenance works but it
is one that engineers cannot ignore.
The safety of the traveling public is of the highest
priority and has to be assured. This is achieved by
preserving the long term structural integrity of the
bridge throughout its service life. However, in
addition, the safety of the public and all personnel
working on the bridge has to be of the highest priority
during any inspection, maintenance, preservation or
rehabilitation works. This assurance has to apply also
to operational works including setting out of traffic
restrictions and controls; security patrols; winter
maintenance and routine tasks such as gulley
cleaning.
To achieve these goals, engineers have to be prepared
to have robust and well communicated arguments set
out to ensure that sufficient funds are made available.
2/2017