e-mosty June 2017: Osman Gazi B. US Suspension. Hålogaland B. e-mosty June 2017: Suspension Bridges | Page 51
Figure 2: Helical strand cable, Eastbound Bridge (left); PPWS cable, Westbound Bridge (right)
Following internal cable inspections and a thorough
evaluation of options, the Maryland Transportation
Authority adopted a proactive approach to the
preservation of the main cables, by deciding to
implement a main cable dehumidification strategy.
Although cable dehumidification had been
implemented in Asia and Europe, this would become
the first application of main cable dehumidification in
the United States and North America.
2. MAIN CABLE DEHUMIDIFICATION
The Maryland Transportation Authority engaged
AECOM for the design and development of the
contract documents for the main cable
dehumidification, based on our international cable
dehumidification expertise.
AECOM had previously performed the cable
investigations, design, and supervision for the
installation of main cable dehumidification systems on
three signature bridges in the United Kingdom
including Forth Road, M48 Severn and Humber
(Figure 3).
All three bridges had parallel galvanized wire cables
and a conventional corrosion protection system of red
lead paste, galvanized wrapping wire and paint.
The extent of corrosion discovered was greater than
originally anticipated, and it became evident that
intervention was required.
It was determined that the exposure of the main
cables in high levels of humidity and corrosive
contaminants such as chlorides, carbon dioxide and
noxious gases was resulting in a continuous
oxidization process of the main cables, and the
traditional protection systems adopted during their
construction were not as effective as originally
tho ught.
The main cable dehumidification system on Forth
Road Bridge was installed and fully operational in
2009, while the systems on M48 Severn and Humber
Bridges became fully operational in 2009 and 2010
respectively (Cocksedge & Bulmer, 2009; Cocksedge
et al, 2011; Christodoulou et al. 2011).
The premise of cable dehumidification is to reduce
the relative humidity (RH) inside the cables below 60%
where corrosion becomes thermodynamically
negligible (Figure 4). The relationship between
corrosion rate of ferrous steel and humidity was
investigated by Vernon in the 1930s (Vernon, 1935).
AECOM performed the internal cable inspections and
strength evaluation of the main cables using the
NCHRP 534 Guidelines (TRB 2004) for all three of the
bridges between 2007 and 2010.
2/2017