The Design of Modern Steel Bridges - TEDI

24 The Design of Modern Steel Bridges
which have become popular for the deck structure of suspension bridges since
the building of Severn Suspension Bridge in 1966, would require a depth of
about 10 m for the deck structure, as the required depth increases with span.
Such a deck would increase the weight of the deck and the cables and the
towers to such an extent that the feasibility of the project would be threatened.
An alternative solution of ‘slotted’ deck is being investigated, whereby the
deck will have voided longitudinal strips through which wind passing underneath
the deck escapes upwards through the voids, reducing the lifting forces
on the deck. It is proposed that one central aerofoil-shaped box deck will carry
twin rail track, and flanking this box on either side, two aerofoil-shaped box
decks will each carry three lanes of road. The three boxes will be only 2.25 m
deep, will be separated by two 8.0 m wide grillage and will be inter-connected
by cross girders of 4.5 m depth spanning the whole width of the bridge between
two rows of suspension cables. It is hoped that this solution will significantly
reduce the weight of the deck structure and hence of the suspension cables and
the towers.
In cable-stayed bridges the cables are virtually straight between their top at
the tower and their bottom end at the deck where they support the deck
superstructure. Thus, unlike suspension bridge cables, their tension is uniform
along their length and, in this respect at least, they are more efficient. Elimination
of substantial anchorages in the ground is another advantage. This type of
bridge construction has become the favourite in the span range of 150–500 m,
replacing suspension bridges in the higher part of this range.
Cable-stayed bridges are statically indeterminate for structural analysis;
each cable stay represents one redundancy. Thus for a three-span bridge, with
one pair of cables supported from each tower top and two vertical cable planes,
there will be eight redundancies for the eight cable supports, in addition to the
two represented by the intermediate piers. Historically, several bridges were
built in the first half of the nineteenth century, with inclined cable stays
supporting the bridge span. These cables were made from bars and chains and
were not initially tensioned; this allowed large deflections of the deck under
loading. This shortcoming led to the concept of combining main suspension
cables of a suspension bridge with a system of inclined cable stays fixed
between the deck and the towers.
Arnodin in France was a pioneer of a system in which the central portion of
the span was supported by suspension cables, but the end portions near the
towers were held by cable stays radiating from the towers. The Franz Joseph
Bridge in Prague (1868), the Albert Bridge over the Thames in London (1873),
the Ohio River Bridge at Cincinnati (1867), and the Niagara (1855) and
Brooklyn (1883) Bridges by Roebling were examples of the concept of
combined suspension and cable stay system. The cable stays not only took a
substantial portion of the vertical dead and live loading, but also provided the
crucial aerodynamic stability. The Lezardrieux Bridge over the Trieaux River
in France built in 1925 is the first known example of the modern elegant cable-