e-mosty June 2017: Osman Gazi B. US Suspension. Hålogaland B. e-mosty June 2017: Suspension Bridges | Page 47

5.3 Fracture Mechanics of Cable Wires
Cable wires are usually about 0.196 inch in diameter
and in the US have a range of tensile strength from
1000 to 1600 Mpa. The wires are usually galvanized,
pulled across the spans, and then compacted together
to form a near circular cable cross-section, comprised
of thousands of individual wires, or tens-of-thousands
in longer-span bridges.
Figure 9. Cable dehumidification process
The premise of cable dehumidification is to protect
the individual cable wires through the control of
humidity within the cable. This is a long-proven
technique dating back to the first half of the twentieth
century, where work was carried out by W.H.J.
Vernon and later by H.H. Uhlig of the MIT Corrosion
Laboratory. The results of corrosion studies indicate
that if the relative humidity (RH) is kept below 60%,
the corrosion rate dramatically decreases, and below
40% corrosion practically ceases (Figure 10).
There are many contributing factors to cable
corrosion and loss of strength. However, it is the
initiation and propagation of cracks that ultimately
cause cable wires to break. This process starts with
the water that has collected within the cable reacting
with atmospheric pollutants leading to zinc depletion
– the degradation of the galvanized coating on the
cable wires. Once the zinc protection is depleted,
corrosion pitting will occur; some of the pits develop
cracks, which then grow into the cable wire cross-
section. The very high strength of the steel used in
cable wire makes it more brittle in nature and more
susceptible to hydrogen embrittlement and
associated cracking. Hydrogen embrittlement is a
result of hydrogen at the subatomic level migrating
into the steel matrix, causing the cable wire to
become brittle and prone to wire cracking and
fracture at normal levels of working stress.
Historical data on existing bridge cables that have
been retrofitted with a dehumidification system
demonstrate a marked reduction and near-cessation
of wire breaks over time (Figure 11), illustrating the
effectiveness of main cable dehumidification on the
overall health of the cables.
Figure 10: Rate of corrosion vs. relative humidity
(background: 4 stages of cable wire corrosion)
The implementation of a series of injection and
exhaust sleeves along the length of the cable,
supported by a system of dehumidified air maintained
below the threshold 40% RH, supplies a continuous
lifeline of dried-air to combat corrosion. This can be
contrasted with the original passive system of
protection for most main cables, which typically
included galvanized cable wires with a layer of red
lead or zinc paste below a soft-annealed galvanized
wire wrapping and paint system.
Dehumidification also contrasts the historic approach
to retrofitting cables with the added protection of
oiling – an expensive process of unwrapping and
wedging the cable wires apart and pouring in specially
formulated oil in the hopes of protecting the cable
against continued corrosion.
Figure 11. Reduction in cumulative wires breaks
on dehumified cables
5.4 Monitoring the Vital Signs of the Cable
Assessing the condition of the cables by visual and
hands-on inspections can be supplemented by
acoustic monitoring. Depending on the condition of
the cable, internal cable inspections typically occur
but once a decade and focus on just a few locations
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