Civil Engineering Reference
In-Depth Information
double-deck cable-stayed bridge. Ten lanes were to be carried on the 42 m wide top
fl ange and six lanes on a 26 m wide lower deck. The webs of the deck consist of
prestressed concrete trusses. A truss arrangement was chosen both for economy and
to provide natural ventilation to the lower traffi c level. Clearance was left to install jet
fans to ventilate the lower deck if this proved necessary. The total deck area of 48,000
m
2
carried by this bridge would have made it among the very largest cable-stayed
structures in the world.
a) Articulation
The two 120 m high reinforced concrete towers are split longitudinally into two leaves.
This arrangement gives the fl exibility which allows the deck to be built into the towers,
and provides a stable base to resist wind forces during the cantilever erection of the
deck. The remote ends of the side spans are carried on fl exible reinforced concrete
piers built into the deck. These piers also carry the holding down tensions. Thus the
bridge required no bearings either at the main towers or at the abutments.
b) Cable-stayed deck
The deck structure consists of a reinforced concrete box 7.15 m deep and 26.8 m
wide. The box has 7.4 m wide propped side cantilevers extending to the cable planes.
The top fl ange of the box is a 200 mm thick reinforced concrete slab thickening to
300 mm at the webs. Outboard of the webs the cantilevers remain at a thickness of
300 mm until they further thicken to 500 mm adjacent to the stay anchors, to control
the shear stresses due to the transfer of the stay forces into the deck. The bottom slab
of the box is also a 200 mm reinforced concrete slab thickening to 300 mm adjacent
to the webs.
The top and bottom slabs are supported on steel stringer beams at 5 m centres that
run longitudinally. As the slab spans transversally, it retains all its strength to resist
the longitudinal compressions. The stringers are carried by transverse steel beams at
7 m centres. The upper transverse steel beam is 850 mm deep and is supported by a
central steel stanchion, resting on the lower fi sh belly beam. This lower beam spans
25 m between the box webs and is 2.5 m deep at mid-span. Its end reaction, together
with the end reaction of the upper steel beam transmitted down the webs, is picked
up by inclined prestressed concrete ties which transfer the load to the cable stays. The
prestress of the inclined ties is continued across the bridge, along the bottom fl ange of
the lower transverse steel beam, effectively prestressing this beam.
The deck is prestressed longitudinally by internal grouted tendons housed in the
haunches of the upper and lower slabs and anchored in blisters beneath the slabs.
The webs of the box consist of concrete 'N' trusses, Figure 18.20. The thickness of
the truss members is 800 mm , which is adequate to resist accidental vehicle impact.
The pitch of the truss is 7 m, while the height between the points of conjunction
is 6.45 m. Both the diagonal and vertical members are 700 mm wide. The vertical
tension members are prestressed, and the cables are given transverse eccentricities to
carry the bending moments in the webs derived from the continuity with the upper
and lower cross-beams. In areas near mid-span where the intensity of the 'reversed'
shear increases, the diagonals are also prestressed. For the fi rst 37.5 m of the deck
