e-mosty March 2019 Long Span and Multiple Span Bridges | Page 49
Part of the cable stiffness problem is due to the
longitudinal movement of the cable, the same
problem occurs on the single span suspension
bridge at the mid span - with about half of the
span loaded.
The optimum mix of stays and suspension
structure is with stays covering 25% to 30% of the
span from each tower.
Hybrid stay-suspension bridges have been
proposed (Figure 10), particularly for long spans
where the stays relieve the cables of a significant
proportion of the dead load.
This is often solved by clamping the cable to the
deck at mid-span (Figure 8f and Figure 9), thus
limiting the longitudinal movement.
Most modern suspension bridges since the 1930’s
have been pure suspension structures. Recently
the 1600m span Sultan Selim Bridge (3 rd Bosphorus
Bridge), using a hybrid cable stay-suspension form,
was completed 4 .
Cable clamps were used on the 3-tower Taizhou
Bridge (Figure 1).
Negative stays clamped to the main cables at
intervals along its length and anchored to the
tower at deck level (Figure 8g) serve a similar
purpose to the cable clamp.
Another cable stiffening system is the use of
inclined hangers to form a truss arrangement
(Figure 8i) or a cable net if the hangers cross.
The inclined hangers working with the deck and
cables to form a composite system carrying
imposed loads and limiting deflection.
The amount of load that can be carried depends
on the load in the hangers; the additional load
should not be such that the hangers become slack.
These inclined hangers often have increased
fatigue issues.
From the analysis of multi-span suspension bridges
large movements of the towers are associated with
the reduced stiffness.
Figure 9: Cable clamp on the Runyang Yantze Bridge
3
Conventional stays radiating from the tower top
(Figure 8h) were often used to stiffen early
suspension bridges.
Limiting the tower movement is key to stiffening a
multi-span suspension bridge. Providing a more
rigid tower (Figure 8j) significantly influences
deflections.
A-shape towers or wide braced towers have been
proposed. The A-shape tower is effective in
limiting deflections.
Roebling’s bridges such as the Brooklyn Bridge are
particularly noted for the use of additional stays.
However, research in China for the Taizhou and
Maashaan bridges has shown the stiffness of the
The stays cannot extend far into the span unless
the towers are made taller.
Figure 10: A long span, deep-water, hybrid multi span suspension bridge proposal
1/2019