Civil Engineering Reference
In-Depth Information
Two temporary erection cables, each consisting of 12 No 15.7 mm strands and located
150 mm above the extrados of the deck were draped in place, over saddles on the
piers and attached to temporary anchors on the abutments. 2,073 mm long precast
deck elements were then suspended from these erection cables, working from the piers
outwards. When all the precast elements had been placed, the erection cables were re-
stressed to correct the deck profi le. The permanent prestress was then threaded into
place and ducts were jointed in the 400 mm wide joint troughs at either end of the
precast units, which were then fi lled with cast-in-situ concrete. After 24 hours, two
tendons were stressed to avoid the risk of cracking of the jointing concrete. When this
concrete had attained a strength of 35 MPa, the remainder of the permanent cables
were stressed and the temporary strands removed.
18.6 Steel cable catenary bridges
It is possible to design a footbridge where the essential structure of the deck consists
solely of highly tensioned steel cables anchored on abutments. The cables may be
clad, for instance in precast concrete sections, which provide the walking surface,
and anchorage for the handrail. Although superfi cially similar to stressed ribbons, as
the concrete structure is not longitudinally compressed these bridges have different
characteristics.
When additional load is applied to the catenary, the cables will extend under the
greater tension and their sag will increase. The changes in sag under live loading will be
greater than for a concrete stressed ribbon due to the higher stresses in the steel. When
the temperature rises, the sag will increase and the tension in the cables will drop;
when the temperature falls, the sag will decrease and the tension in the cables will rise.
Thus the cables are subjected to continual variations in stress, and it is necessary to
check their fatigue performance. The changes in sag will also cause angular changes
where the cables pass over piers and at the face of abutments. If the cables are not
protected from these angular changes, by for instance providing a stiffer duct locally
that spreads the angular change over a length of the cable, their fatigue life is likely to
be further reduced.
Such decks have no torsional or bending strength, other than that provided by
the stiffness of the taut cables. Consequently, they are likely to be more susceptible
to vibrations due to pedestrian excitation or to wind. Any such vibrations could be
prejudicial to the fatigue life of the cables.
The transverse joints between precast sections are normally fi lled with a sealant,
which must be suffi ciently fl exible to accept the changes in length and geometry of
the cables. However, this sealant should not be relied upon to protect the cables from
water seeping from the deck. The cables must be provided with corrosion protection
similar to that of cable stays, and it must be possible to inspect and change cables if
necessary.
This structural system is subject to the risks of progressive collapse; if one cable
is out of service, its load will be shed onto the remaining cables. Consequently, there
must be adequate reserves of strength.
In view of all these factors, it would be prudent to limit the maximum tension in the
cables to 45 per cent of their breaking load, as for cable stays, and to make provision
for cables to be inspected and replaced. However, the stress in the cables is likely to be
still lower in order to provide suffi cient steel area to control defl ection.
