Suspension bridge cable evaluation and maintenance

Inspection and maintenance of suspension bridge cables requires specialized expertise. After
the first thirty years of the bridge’s life the main cables should be unwrapped and the interior wires or strands
should be inspected in-depth at selected locations. NCHRP has established guidelines for the inspection and
evaluation of suspension bridge cables. These guidelines include inspection protocols and techniques for the
capacity evaluation of the cables. New products for maintaining the cables and extending the expected service life of the bridge are being utilized by owners of suspension bridges throughout the world. The use of
dehumidification systems to dry out the cables is now in place on numerous suspension cable inspection. This paper will present
details of the techniques and procedures now utilized around the world to inspect and evaluate suspension
bridge cables. Examples of recent projects will be cited.

Since the Brooklyn Bridge was built in New York City in the late 1800’s virtually all main cables of major
suspension bridges have been constructed of high strength galvanized steel wire. Until the mid-twentieth century, all large suspension bridge cables were air spun by pulling one or more pairs of wires at a time from one
anchorage to the other and adjusting each wire to theoretically share the load equally with the others. In 1969,
the Newport Bridge in Rhode Island was constructed using shop-fabricated parallel wire strands (PPWS), the
method that has now gained favor for many new bridges.
Since John A. Roebling pioneered the art of suspension bridge design, the main cables of suspension
bridges have typically been protected by a tight covering of soft wire wrapping bedded in a sealing paste,
usually red-lead (Pb3O4) in linseed oil, and coated with paint. Some exceptions are notable, such as the Newport and Bidwell Bar (Oroville, California, U.S.A., 1965) Bridges where glass-reinforced acrylic was used,
and the William Preston Lane Bridge (Maryland, U.S.A., 1973) where neoprene sheet was used.
Recognizing the advantage of using an impervious covering on the cables, a number of U.S. suspension
bridges have been retrofitted with elastomeric coverings placed over the existing wrapping wire. There are
now a number of bridges in Europe and Japan that use a dry-air injection system in conjunction with an elastomeric wrapping to ensure that no moisture can enter the cables. Some suspension bridges have also been
constructed using twisted strands. The inspection is significantly different for this type of bridges and is
therefore not covered in this paper.

Based on the experience of bridge owners and consultants familiar with this field of expertise, the Federal
Highway Administration (FHWA) developed guidelines for these special inspections that include some basic
descriptions of how to open and wedge cables: Federal Highway Administration Guidelines for “Inspection
of Fracture Critical Bridge Members for Cable Suspension Bridges”, FHWA-IP-86-26, 1986.
An expanded report; NCHRP Report 534, “Guidelines for Inspection and Strength Evaluation of Suspension
Bridge Parallel-Wire Cables” was developed with a concentration on the computational methods used to estimate remaining cable strength of a corroded cable. The NCHRP guidelines were published in 2004 and describe today’s standards for bridge inspection. They include recommendations on when to inspect, where to
inspect, how many samples to extract, etc. to achieve statistical accurate cable strengths from the inspection
results.